Orgo Lab Final

1)to characterize a known compound(if you had to choose between several known compounds)
2)to record a physical characteristic for a new compound
3)to establish purity of a known compound (if an experiment reliably forms this known compound or if you are taking a melting point of a known compound)
Why are melting points carried out?

A pure solid will melt reproducibly over a narrow range of temperatures, usually less than 1 degree C
Definition of the melting point of a pure compound.

Impure compounds will melt over a wider range of temperatures and at a lower melting point than the original
The solution has a lower vapor pressure than the vapor pressure of the pure components, which results in the lower melting point
What is the criteria for impurity?

Larger hydrocarbons have greater London or Van der Waals forces = higher melting point
Molecules with dipoles will have elevated melting points (stronger than London forces)
Hydrogen bonding increases melting points (even stronger intermolecular force)
Ionic forces will have very high melting points, they often decompose before melting
R and S isomers alone will each have the same melting point, but together one acts as an impurity on the other and the melting point changes
What forces affect melting point? (Also, consider the effect on the melting point of larger molecules, symmetrical molecules and R,S isomers, and hydrogen bonding molecules)

Eutectic temperature – the lowest temperature at which a mixture at a certain concentration will start melting
Eutectic point –
Definition with regard to melting point composition diagram – lowest point on the melting point composition diagram where a mixture has a very narrow melting range
Definition with regard to 2 solids and their liquid solutions – point at which the 2 solids and their liquid solutions are in equilibrium
Define eutectic point (two ways mentioned in notes) and eutectic temperature

1)sample must be dry because moisture from solvents like water and other solvents will depress the melting point like an impurity
2)sample must be finely powdered because if the sample is in chunks, air pockets in the sample will not allow the sample to be packed properly, use the flat portion of the spatula on a watch glass
3)sample must be tightly packed by rapping hardly on a hard surface
4)sample must not be more than 2-3mm high because if there is too much in the capillary, there will be a difference in temperature throughout the sample
Mixed melting points experiment
How sample is prepared? Why sample should be prepared in a specific manner?

by rapping sharply on a hard surface
Criteria for sample before packing and how it should be packed.

Slowly to give enough time for heat to be transferred equally from heating block to sample to the thermometer
How should sample be heated and why? What may happen if you heat the sample too quickly?

When the 1st drop of liquid is observed to the temperature when the sample is completely liquefied
Definition of melting range

Because of crystalline structure changes
Why does sample shrink before melting

There will be a difference in temperature throughout the sample
What happens if you have too much sample

If the sample is in chunks, air pockets in the sample will not allow sample to be packed properly
What happens if sample is coarse (not powdered)

Difficult when determined in a capillary because it represents the point at which an infinitesimal amount of the solid mixture has begun to melt
Why is eutectic point difficult to detect

To purify a solid
Recrystallization
Reason for carrying out

Polar solvents dissolve polar solutes, nonpolar solvents dissolve nonpolar solutes
Picking solvent: like dissolves like

With respect to solute:
-dissolves the solute completely when solvent is hot(at boiling point of solvent)
-precipitate solute completely when solvent is cold (room temperature or colder)
-does not react with the solute
With respect to impurities:
-does not dissolve impurities at all; so impurities can be filtered off from hot solvent
-OR, dissolves impurities very well (even when solvent is cold); so impurities along with cold solvent can be removed from crystals via filtration
Requirements for solvents with respect to solute and impurities

-both solvents have to be miscible in each other
-one solvent has to be a poorer solvent for solute than the other
Requirement for solvents for solvent pairs

used when no one solvent has ideal properties with respect to the solute
Why solvent pairs are used

water – cheap, nonflammable, nontoxic, will dissolve a large variety of polar organic molecules,
Its high boiling point and high heat of vaporization make it difficult to remove from crystals
Ethanol – high boiling point makes it a better solvent for less polar molecules than methanol, it evaporates readily from crystals
Ethyl acetate – right combination of a moderately high boiling point and the volatility needed to remove it from crystals
Acetone – low boiling point means there is not much difference in the solubility of a compound at its boiling point compared to 22 degrees C
Desirable and undesirable qualities of water, ethanol and ethyl acetate and acetone (common crystallization solvents) (Table pg. 63) These 4 are all common and good solvents

Wash the crystals with ice-cold solvent to minimize crystals dissolving during the washing process
Crystals will be lost because some of it will adhere to the filter paper at the collecting step and some crystals will dissolve in the solvent during the washing process (Washing is done to remove adhering impurities)
How to minimize loss of product in crystallization, collecting and washing steps – why does product get lost in these steps

Cool slowly, if cooled too fast, solid will come crashing out of the solution as powder, precipitating impurities along with it, wrap the test tube in paper towels to insulate and allow slow cooling(do not move or disturb the crystallizing solution, doing so will form many nuclei for crystallization)
The solution must be saturated before crystallizing
Requirements when cooling

Supersaturation – when saturated solution cools and no crystals form
Fix this by inducing crystallization using seed crystals or scratching the surface of the test tube with a glass rod
Unsaturated solution – when the solvent is not holding its maximum capacity of solute
Fix this by evaporating solvent to the point of saturation
Remedies for unsaturated solution, supersaturation

Superheating – when you heat a solution above the boiling point without boiling occurring, when boiling does occur, it happens with explosive violence (bumping)
Boiling chips and sticks prevent this by releasing air and forms the nuclei on which even boiling can occur
Definition of superheating and prevention – how boiling sticks and stones achieve their purpose.

Oiling out – when some saturated solutions are cooled, especially those containing water, deposit not crystals but small droplets referred to as oils
Oiling out occurs when: (p.86)
-Saturated solution temperature OR boiling point of solvent is higher than the melting point of the solute
-If melting point of solute is depressed to a point such that a low-melting eutectic mixture of the solute and the solvent comes out of solution
Prevent oiling out by lowering the temperature at which the solution becomes saturated with the solute by simply adding more solvent, in extreme cases, it may be necessary to lower the temperature below 22 degrees C by cooling the solution with dry ice, you can also just use a different solvent
Why crystals oil out – remedies and prevention

This occurs because the solution was moved or disturbed during recrystallization, large crystals are preferred because it is easier to filter and wash
Small powdery crystals – why they form and how to prevent, why large crystals are preferred

It is used to removed high molecular weight byproducts that are highly colored by boiling the solution with this charcoal
The impurities can be adsorbed onto the large surface area of the charcoal
Activated charcoal – purpose and how it works

so crystals do not form on the boiling stick
Why is it important that boiling stick is removed during cooling process?

To purify a liquid
Distillation
Reason for carrying out

Wrap the apparatus/column with foil
Insulation of apparatus

To ensure that the vapor condenses into the sidearm
Wrapping with a wet paper towel – why?

Must be below the sidearm to accurately read the temperature of the vapor distilling through the sidearm
Position of the thermometer

Inside the receiver vial to minimize release of vapors into the hood
Position of end of sidearm

Between 2 miscible liquids (homogeneous) – boiling point is between the boiling points of each of the pure components (for simple and fractional distillations, must be homogeneous)
Between 2 immiscible liquids (heterogeneous) – boiling point is below the boiling point of each pure component
Boiling point of mixture of a homogeneous mixture and a heterogeneous mixture

Simple – Between 2 miscible liquids, boiling point difference between the 2 liquids is large, when one of the liquid’s composition is less than 10%
Fractional – Between 2 miscible liquids, boiling point difference between the 2 liquids is small; the 2 liquid components are substantial
Uses of simple and fractional distillation – what kinds of mixtures are suitable for each kind of distillation

The temperature will continue to rise as the vapor composition changes
Behavior of temperature during a simple or fractional distillation of a mixture.

Minimum boiling azeotrope – below pure boiling points of both liquids
Maximum boiling azeotrope – above pure boiling points of both liquids
Definition of azeotrope (minimum and maximum) – recognize azeotrope on boiling point composition graph

The column packing has a very large surface area and will absorb much of the material being distilled, this holdup is unrecoverable distillate that wets the column packing
Definition of hold up

Too dense – pressure changes may take place within the column to cause non-equilibrium conditions
Too large surface area – holdup/absorb much of the material being distilled
Use of a packing material and criteria for choosing, why should the packing material not be too dense or have too large a surface area

Distillation carried out under reduced pressure for miscible liquids, used when neither ordinary distillation nor steam distillation is practical
Purpose of vacuum distillation

For compounds that decompose at the boiling points or is sensitive to oxidation
What kind of compounds need to be purified by vacuum distillation

Usually done with immiscible liquids (one of the liquids is usually water), boiling point of the mixture is below the boiling point of each pure component, vacuum is done with miscible liquids
Advantages – volatilizing liquids and solids below their boiling points, good for high boiling substances that decompose before boiling point is reached, only works if compound is volatile
Steam distillation – uses, what kind of mixture used in steam distillation – difference between vacuum distillation and steam distillation

As long as adequate amounts of both water and the organic component are present to saturate the vapor space
What requirement is necessary for boiling point in steam distillation to be constant?

The dry residue of the liquids could be explosive
Precautions about distilling to dryness, distilling in closed system.

loose connectors will cause vapor to escape, temperature variations due to the airflow in the hoods, the large surface of the microscale apparatus makes it easy to lose heat to the environment
Sources of error in a distillation – what happens and why it happens

Spotting – use capillary tubes, try to make spots small and compact by lightly touch TLC plate being careful not to flake off any adsorbant, the sample should not be too concentrated, overspotting leads to poor separation and smeary spots
Developing –
-make sure the solvent is below the spotting line so that the solvent does not dissolve samples on the spotting line
-make sure top is placed back on so solvent does not evaporate, evaporation of solvent during this stage causes the development of the TLC plates to be messy and ineffective
-make sure solvent does not rise up past the solvent front 5mm line, if it does, spots in the visualization stage will be difficult to evaluate (Rf values would be too high)
-make sure the filter paper is in the developing chamber to saturate the developing chamber with solvent vapors
-use tweezers to place the plate in the chamber, finger oils can smear or ruin the TLC
Visualizing –
Use UV light for conjugated compounds
Use iodine vapor for most other compounds
TLC
Precautions to take when spotting, developing and visualizing

So liquids that are draw in do not flow out to form a drop
Why should bores of capillaries be small

Polar for polar samples
Nonpolar for nonpolar samples
Like dissolves like
What are typical characteristics for solvents used as eluents

Low boiling points and low viscosities allow them to be easily evaporated and allow them to migrate rapidly
Characteristics of solvents that allow them to migrate rapidly

Silica – for very nonpolar compounds – silica is not as polar as alumina, compound will travel with solvent front
Alumina – for very polar compounds – alumina is very polar/active, very polar compounds will not move
Table 8.2 (Order of solute migration)
More polar substances require polar eluents
More nonpolar substances require nonpolar eluents
Adsorbent materials and specific uses (what kind of adsorbents would be better for what kind of substances)

1. To determine the number of components in a mixture
2. To determine the identity of 2 substances
3. To monitor the progress of a reaction
4. To determine the effectiveness of a purification
5. To determine the appropriate conditions for a column chromatographic separation
6. To monitor column chromatography
Uses of TLC and how it is used- all 6 (p.173)

Too polar – all spots will move to the top of the TLC plate
Too nonpolar – all spots will stay towards the bottom of the TLC plate
Effects of using too polar or too nonpolar a solvent

Used if the spots are not visible to the naked eye
UV light for conjugated compounds
I2 vapor for most other compounds, alkanes, alcohols, ethers do not absorb UV light very well
Visualization techniques – why certain kind of visualization technique was used for certain types of compounds

Graphite is inert and will not chromatograph
Why is a pencil always used to mark a plate?

Ink is organic and will chromatograph to give flawed results
Why is ink never used to mark the plate?

To isolate compounds from natural products or to isolate reaction products
Extraction
Reason for carrying out

Use base to react with an organic acid to form an ionic compound (water soluble)
Use acid to react with an organic base to form an ionic compound (water soluble)
How acid base reactions work

Add weak base to extract strong acid, add strong base to extract weak acid, add acid to extract base, dry neutral compound, add acid to the extracted acids, add base to the extracted base
Flowchart on how to separate strong acids, weak acids, bases and neutral compounds(p.148)

Using a strong base first would extract both the strong acid and the weak acid into the aqueous layer
Why extractions are carried out in the sequence they are (weak base then strong base)

The base being used to extract an acid must have a conjugate acid that is weaker(lower pKa) than the acid that it is trying to extract
pKa requirements for conjugate acids of bases used to extract organic acids.

Should readily dissolve the substance to be extracted at room temperature, should have a low boiling point so that it can be removed readily, should not react with the solute or the other solvent, should not be miscible with water, should not be highly flammable or toxic, should be relatively inexpensive
Requirements/Criteria for extraction solvents

Emulsion – a suspension of one liquid in another as droplets, caused by detergent like molecules that stabilize emulsions, remedied by shaking the solution to be extracted very gently until you see that the 2 layers will separate readily or add NaCl to make the aqueous layer highly ionic (p.141)
Emulsions – definition, causes, and remedies

Add NaCl to make the aqueous layer highly ionic
How to decrease solubility of organic substance in aqueous layer

To remove traces of water, we used CaCl2 pellets, Wet organic layers are cloudy, clear when dry
Why drying agent was used? What observations indicate that an organic layer is dry? You must know what substance(s) you have used as a drying agent in the experiments you have carried out.

Must not react with solute being extracted, the must remove water quickly, the efficiency of the process, easy to recover solute from drying agent
Criteria for choosing drying agent

It clumps together when excess water is present (easy to tell how much drying agent to add – add until it does not clump), does not disintegrate into a fine powder, fast, effective (p.142)
Advantages of using calcium chloride as drying agent when compared to other drying agents in this experiment

By using a Pasteur pipette and pulling up the solution from the bottom of the test tube when clumping occurs and individual particles begin to snow/slowly settle
How do you ensure all product is transferred from drying agent

MTBE is safer, less volatile, and has a higher boiling point
Diethyl ether is highly flammable and easily forms peroxides
Advantages of t-butylmethyl ether and disadvantages of diethyl ether

The solute is never completely transferred to the organic layer
Why don’t you get 100% recovery?

Temperature increases both SN1 and SN2 because it increases the kinetic energy of the molecules
Better (weaker bases[I-]) leaving groups increase SN1 and SN2 rate because it forms better stable ions
SN1 needs a polar protic(ethanol/water) solvent because it stabilizes the transition state and lowers the activation energy of the reaction
SN1’s rate determining step is the formation of the carbocation, this only needs the alkyl halide to be present, only the increasing of the alkyl halide will increase the rate, increasing the nucleophile will do nothing
SN2 needs a polar aprotic(acetone) solvent because hydrogens will form hydrogen bonds to the nucleophile and solvate it and get in the way of the nucleophile
SN2 rate increases with both nucleophile and alkyl halide because everything happens in one step and an increase in any of the reactants would increase the rate
Factors that affect SN1 and SN2 (general and specific to experiments carried out)
-structure of alkyl halides, temperatures, solvents (types of and amounts), leaving groups, concentration of alkyl halides and nucleophiles, etc. (also know why trends are the way they are)

SN1: 3>allyl~benzyl~2>1>CH3~vinyl, the formation of carbocation is favored by carbons that can be stabilized by hyperconjugation from neighboring substituents on neighboring carbons
SN2: CH3>1>2>3, there is the least steric hindrance in the methyl group, tertiary carbons are too sterically hindered, the nucleophile cannot get close to the carbon
– relative rates of reactions and the reason for those relative rates.

For SN2, the amount of nucleophile and alkyl halide affects the rate and would add another variable, for SN1, the amount of alkyl halide would affect the rate of reaction
– the importance of using the exact same amount of alkyl halide and nucleophile in each test tube

The alkyl halide can act as a solvent in which the nucleophile is insoluble
– possibility of a false positive in a reaction

For SN1, the carbocation is difficult to form, in SN2, electrons in the nearby double bonds repel the nucleophile
– why vinyl and aryl halides don’t undergo SN1 or SN2

If SN2, product shows inversion of configuration, if SN1, product is a racemic mixture
– products when starting alkyl halide is chiral and why.

NaCl and NaBr are insoluble in acetone and precipitate out of the solution
– why do the Br- and Cl- not attack the alkyl iodides that form in the SN2 reaction carried out?

Iodine is a weak base and a good nucleophile, the acetone is a polar aprotic solvent that won’t solvate the nucleophile and SN2 will occur
What factors promote SN2 in the NaI in acetone reaction?

Ethanol is a polar protic solvent that will help the alkyl halide’s transition state to form a carbocation,
Because the solvent is protic, the proton can solvate the nucleophile and hinder its attack
What factors hinder SN2 in the silver nitrate in ethanol reaction?

Carbocations can go through methyl or hydride shifts to form more stable carbocations
3>allyl~benzyl~2>1>CH3~vinyl
Recognizing alkyl halides that form carbocations that rearrange in SN1

When a carbocation is formed, both SN1 and E1 can occur
Recognizing alkenes that may form as side products from the competing E1 reactions

Iodine is a weak base and a good nucleophile, a strong base is required to pull off an H from the adjacent carbon for E2 to occur
Why E2 will not occur in the sodium iodide in acetone reaction?

Ethanol is a polar protic solvent that will help the alkyl halide’s transition state to form a carbocation, because a carbocation can form, both E1 and SN1 can occur
Why E1 will occur in the silver nitrate in ethanol reaction?

-Use excess of the cheaper reagent
-Remove product water by drying with molecular sieves(most other drying agents will not remove water at the temperature used to make esters) or by azeotropic distillation
Esterification
How can Le Chatelier principle be used for this esterification (all methods)

We don’t use excess starting material because boiling points of acetic acid, 1-butanol, and butyl acetate are very close, an excess of starting material would be a significant impurity to the product and it is also very difficult to separate product from starting material by simple distillation
This is why we remove water.
– why was one used and not the other?

Dry with molecular sieves or by azeotropic distillation
How water could be removed? (both methods)

most other drying agents will not remove water at the temperature used to make esters
-why can you only use molecular sieves instead of other drying agents in the alternate method of removing water (the one not used in the experiment)?

The wet paper towel was used to ensure that the vapors condense, the angle is so that the condensed liquids can run down into there and separate, the water layer goes to the bottom and the ester and alcohol layer goes to the top, the ester and alcohol can run back down into the reaction flask
Why was apparatus set up the way it was? (on an angle, wrapping the condensor with wet paper towel etc)

Shows the presence and absence of functional groups, if strong characteristic peaks show up in the IR, that means the functional group is present in the molecule
Purpose of IR and interpretation of IR spectrum

Since the 2 reactants and products are not very conjugated, iodine vapor is the best visualization technique, TLC can be carried out to analyze the purity of the product
How TLC could have been used (although it was not carried out) – what kind of visualization techniques would have been used and why

It is an acid catalyst that is easily filtered off
Role of resin and advantages

If H2SO4 was used, you would need to dilute it with ether (as an extraction solvent), use sodium bicarbonate to neutralize the acid, use saturated NaCl for the same reasons as that in extraction, dry the ether layer with CaCl2 pellets, and then evaporate the ether
Disadvantages of using traditional acid catalyst and purpose for each of the steps needed to be carried out if traditional acid catalyst is used

Resin – speeds up reactions by lowing activation energy, never used up and is regenerated at the end of the reaction
Catalytic acid

An OH peak showing up in the IR can mean water, 1-butanol, and acetic acid impurities, also other % yield errors
Sources of error

You can never shift equilibrium completely toward product
Reasons reaction may not go to completion

Heating of a reaction mixture to the solvent’s boiling point for an extended period without evaporation
Definition of reflux

An increase in bonds to H or a decrease in bonds to an electronegative atom
Reduction
How reduction is characterized in organic chemistry

An decrease in bonds to H or a increase in bonds to an electronegative atom
How oxidation is characterized in organic chemistry

Substitution – If the change does not involve an increase or decrease of bonds to electronegative atoms or hydrogen
How to recognize reduction, oxidation and substitution

NaBH4 is milder, more selective in reducing aldehydes and ketones, and does not react violently when in contact with H2O or alcohols
Why NaBH4 used instead of LiAlH4

To ensure the reaction goes to completion, account for NaBH4 reacting with solvent, account for decomposition of NaBH4
Why was excess NaBH4 used in the reaction

R,R and S,S hydrobenzoin form because the carbonyl C is sp2 hybridized – it is trigonal planar and flat and H can attack from the top or the bottom
R and S benzoin form because the reaction did not go to completion and some molecules are only reduced once
Specific side products and why they form

First addition is to hydrolyze the borate ester, the 2nd was to saturate the solution to start crystallization
Role of each addition of water

To control the reaction rate (the reaction was exothermic)
Role of ice bath

To separate the product from its diastereomers (R,R, S,S hydrobenzoin) and also to separate it from the completely different side product of R and S benzoin
Why recrystallization carried out

To determine to purity of the final product
Why melting point was carried out

4th priority on top or bottom – R is clockwise, S is counterclockwise
4th priority on left or right – R is counterclockwise, S is clockwise
Sources of error
How IR (what peaks you expect to see to differentiate between main product and one side product), TLC (how Rf’s would differentiate between main product and one side product)
Must be able to identify R and S stereochemistry on major and side products

benzalacetone is from the incomplete reaction of the initial products, only 1 aldol condensation occurred instead of 2
4-hydroxy-4-methyl-2-pentanone is from the enolate of acetone reacting with another acetone instead of the benzaldehyde
Dibenzalacetone synthesis
Specific side products and why they form

-use a reactant with no alpha hydrogens
-use excess of reactant with no alpha hydrogens
-use a more reactive aldehyde in the presence of a less reactive ketone
-deprotonating one carbonyl with a strong base (such as LDA) to completely convert it to the enolate, then add the 2nd carbonyl slowly to be reacted with the enolate
Mixed aldol – drawback and how to minimize mixture of products

to provide the OH catalyst that carries out the initial enolate formation
Role of sodium hydroxide

from the initial deprotonaton of the alpha carbon by OH, water provides the H for the O- on the B carbon, also forms another OH that carries out the dehydration step
Role of water and its source

to remove the OH catalyst before recrystallization, if not done, it may cause an oily product during recrystallization
Reason for washing crystals with water, what may happen if you don’t

to purify the desired product and separate it from the side products of benzalacetone and 4-hydroxy-4-methyl-2-pentanone
Why recrystallization carried out

because of the resulting stabilizing conjugation
Ease of water leaving – why

addition – 2 aldehydes or ketones react by a base catalyst(OH) to form a B-hydroxyaldehyde or aldol, the aldehyde that forms the enolate must have at least 1 alpha H
condensation – the aldol reacts with OH to form an a-B-unsaturated product, initial enolate forming compound requires 2 alpha H
Requirements for starting materials for aldol addition and condensation

the alpha hydrogen is acidic because of the resulting stable resonance structure
Formation of enolate (why it occurs)

OH helps the initial aldehyde form the enolate so that it can attack another carbonyl C
Catalytic base

Advantages – safer, non-toxic
Disadvantages – reaction rate is much slower than CN
Benzoin condensation
Advantage and disadvantage of thiamine catalyst

Aldol product: B-hydroxycarbonyl
Aldol starting reactants need 2 alpha hydrogens
Benzoin product: alpha-hydroxycarbonyl
Benzoin starting reactants don’t need alpha hydrogens
differences between benzoin and aldol condensation – product and starting material

One to neutralize HCl, the other to remove H+ to form the free base
Why two moles of hydroxide ion necessary

To monitor the progress of the reaction
Why TLC used

To remove thiamine and unreacted benzaldehyde from product (so final product is more pure)
Why recrystallization might have been carried out

If benzaldehyde is not fresh, it oxidizes easily to benzoic acid
Or if acid is present, it can neutralize the OH and deactivate the thiamine by protonating it
Also the usual TLC, recrystallization, washing, collecting, melting point errors
Sources of error.

So you don’t fan the flames (prevents the flames from growing), prevent hot vapors/smoke from rising past the person’s mouth and being breathed
Why is it important to prevent a person with burning clothing from running

Get the person under the safety shower and have the TA help get his/her clothes off
What is the best course of action when chemicals are spilled over a large area of a person?

Keep the work area well ventilated, work under the fume hood
What is the best way to prevent flammable vapors from accumulating?

Push them down and roll them on the floor
If a safety shower is not nearby and a fire blanket is not advised, how does one help a person with burning clothing?

The oxygen supply is cut off and the flames are not fueled
A very small fire in a beaker or a flask could possibly be put out by simply covering the vessel with a flat object. Why is this possible?

It should be directed at the base of the fire
How should a fire extinguisher be directed when trying to put out a fire?

Other chemicals can be harmful to you as well?
Why should you not use a solvent (other than water) to remove chemicals from your skin?

You will not achieve good, smooth airflow
You won’t have adequate room for experiments
List two detriments of working in a hood that is cluttered with chemicals.

The vapor pressure of combined X=Y is lower than just pure X, thus the melting point is lowered
A melting point sample that has a mixture of X and Y (thus, impure) melts at a lower melting point than a melting point sample of pure X. What aspect of the solution of X and Y compared to pure X lowers the melting point

There was too much sample, the sample was too high in the capillary (more than 2-3mm)
What mistake made during the preparation of a melting point sample may cause a differential of temperature throughout the sample?

It will melt over a wider range of temperatures and at a lower melting point than the original
Describe how an impure solid would melt.

The sample could be contaminated
The thermometer in the Mel-Temp was not calibrated
If you can rule out moisture in the air as a factor, what two other reasons would there be for melting point samples of pure compounds not agreeing with literature values if sample fulfilled all the necessary criteria for filling the melting point capillary AND the determination of the melting point was carried out correctly?

Eutectic points have narrow melting points, in the data provided, the melting range of 13 degrees C is large
You have a mixture of 60% mandelic acid: 40% cinnamic acid that has a melting range of 95ºC-108ºC. Based on the data provided, explain why it is not possible that 60% mandelic acid: 40% cinnamic acid is the concentration at the eutectic point for a mixture of the two substances

The thermometers are calibrated differently in each Mel-Temp
You take the melting point of a sample of benzoic acid on one Mel-Temp apparatus and you obtain data that agrees with the literature value. The melting point of an identical sample of benzoic acid, which was similarly packed, is taken on a different Mel-Temp apparatus and you obtain data that does not agree with the literature value. What is the most likely source of error in this situation?

To establish the purity of compound X
You have carried out an experiment that reliably produces a known characterized compound X. Which of the 3 reasons for a melting point determination would best describe why you would take the melting point of compound X?

The melting point would not be depressed (p.48)
How would the presence of an insoluble impurity affect the melting point of a compound?

Once the solution boils, it occurs with explosive violence. To prevent this, use a boiling stick or chip to promote even boiling
What is the danger of superheating?

Many nuclei will form, and the crystals will be small and have a large surface area (p. 72)
Too many nuclei for crystallization form; leading to leading to small crystals that come out of solution along with many impurities
If you move or disturb a crystallizing solution, crystals that are too small come out of solution along with impurities. Why does this occur?

Removes high molecular weight byproducts that are highly colorized
What specific impurities are removed by decolorizing a crystal solution with palletized Norit

Ethanol has a lower melting point than water and evaporates readily from crystals, water has a high boiling point and a high heat of vaporization to make it difficult to remove from crystals
What is the advantage of using ethanol as a recrystallization solvent rather than water if the solid to be recrystallized can be recrystallized using both solvents?

Recrystallization involves dissolving the material to be purified (the solute) in a hot solvent to yield a solution (solubility) (p.61)
Describe specifically what is done when employing the concept of solubility in the process of recrystallization.

As the solvent cools, the solution becomes saturated with respect to the solute (saturation level)
Describe specifically what occurs when employing the concept of saturation level in the process of recrystallization.

As the crystals form, impurities are excluded (exclusion), and this the crystal is a single pure substance
Describe the concept of exclusion as it relates to the process of recrystallization.

Stratches on the surface of the test tube
A speck of dust
A seed crystal is an example of a point of nucleation. Provide 2 other examples of points of nucleation in the process of recrystallization.

Dissolve the crystals in the better solvent and then add the poorer solvent to the hot solution until it becomes cloudy, and the solution is saturated with the solute
How do you use a solvent pair to recrystallize a solid? (Explain procedure briefly) (p.64)

Even though the solvent that you use to wash the crystals is ice cold, some of the crystals may still dissolve and pass through the filter when washing
In the recrystallization experiment, you found that you did not get 100 % recovery. Explain how solute may have been lost (specifically) in the washing step.

Some crystals will adhere to the filter paper
In the recrystallization experiment, you found that you did not get 100 % recovery. Explain how solute may have been lost (specifically) in the collecting step.

While recrystallizing, some solute is bound to stay dissolved in the solvent and is not crystallized
In the recrystallization experiment, you found that you did not get 100 % recovery. Explain how solute may have been lost (specifically) in the crystallizing step.

Drying, the product is wet
You find that your percent recovery in the recrystallization experiment is larger than 100%.
What step was not done efficiently?

The melting point would be depressed because the solvent acts as an impurity
What other effect would you observe on a physical characteristic of the crystal if the step mentioned above is not carried out efficiently?

when some saturated solutions are cooled, especially those containing water, deposit not crystals but small droplets referred to as oils
Crystals come out of the solution as an oil
Define oiling out.

Lowering the temperature at which the solution becomes saturated with the solute
One of the remedies for oiling out in a recrystallization process is to add more solvent to the solution. What does this serve to do?

The solution would cool too fast and the solid will come crashing out of the solution as a powder, precipitating impurities along with it
What would occur if you forget to insulate the test tube containing the saturated solution with paper towels in the recrystallization experiment?

So crystals do not form on the boiling stick
What would occur if the boiling stick is left in the crystallization solution while it is cooling?

Remove the crystals and press them on filter paper, then let them sit on a watch glass to dry
How do you complete the drying of crystals after keeping the crystals for a few minutes on the Hirsch funnel?

when saturated solution cools and no crystals form
Define supersaturation with regards to recrystallization.

when the vapor pressure of the pure liquid equals the atmospheric pressure as it is heated
When does a pure liquid boil?

Keep the thermometer bulb below the sidearm so it accurately records the temperature of the vapor distilling through the sidearm
What should be done in the distillation apparatus to ensure that the temperature of the vapor distilling through the sidearm is accurately recorded?

Never distill in an airtight system
How do you avoid the rupturing of apparatus while carrying out a distillation?

Once the liquid cools, the capillaries become filled with liquid and cannot be reused
Why can a boiling chip only be used once?

The boiling point of the liquid = the temperature of the vapor
The temperature remains constant throughout distillation
What two observations can be made about the distillation of a pure liquid?

Keep the end of the sidearm inside the receiver vial
When you set up the apparatus for distillation, how do you minimize the release of vapors into the hood? Assume that all connectors are snug-fitting.

Left over residue can react with explosive violence
Why is it important not to distill liquids to dryness

Wrap the condenser and sidearm with a wet paper towel
How do you ensure that vapor condenses in the sidearm of the distillation apparatus?

A temperature higher than the boiling point of the liquid
What would a thermometer placed in a superheated liquid register?

It has a smaller surface area and has less points of attraction to adjacent molecules
What is the main reason 2,2-dimethylpropane would have a lower boiling point than pentane?

An alcohol with 2 hydroxy groups can form more hydrogen bonds
What is the main reason an alcohol with 2 hydroxyl groups will have a higher boiling point than an alcohol with one hydroxyl group?

So that the condensed liquid can percolate down through the rising hot vapors
Why is it important that the column be vertical in a fractional distillation?

loose connectors will cause vapor to escape, loss of liquid
A part of the microscale kit that was used in the simple and fractional distillation apparatus set up is shown below. (See figure (m) from pg 15 in textbook)
What source of error is introduced if the specific indicated part (the actual connector) of the piece above is loose?

The column is packed with material that provides a large surface area for heat exchange between the ascending vapor and the descending liquid
A piece of glassware from the microscale kit that was used in the fractional distillation apparatus set up is shown below. (See figure (o) from pg 15 in textbook)
What is done to this piece of glassware for it to perform its specific function in the fractional distillation set-up?

To allow the condensed liquid to flow into a separate chamber
A piece of glassware from the microscale kit that was used in the simple and fractional distillation apparatus set up is shown below. (See figure (h) from pg 15 in textbook)
What is the specific function of the specific indicated part (the sidearm) of the glassware?

To hold the thermometer in place and to prevent vapor from escaping through the top of the tube
A part of the microscale kit that was used in the simple and fractional distillation apparatus set up is shown below. (See figure (c) from pg 15 in textbook)
What is the function of this part in the apparatus?

So that ascending vapors and descending liquid attain equilibrium
Why is it important that the rate of distillation (especially fractional distillation) be slow?

(Height Equivalent to a Theoretical Plate) smaller the HETP, the more plate the column will have and the more efficient it will be
What is the significance of a small HETP in a fractional distillation?

To reduce the amount of unrecoverable distillate, so you get a higher yield of the product
Why is it important that the packing material for a fractional column have low holdup?

It is easy to insert into the column, it has a large surface area, good heat transfer characteristics, and low holdup. It does not come out of the column as beads do
What is the advantage of using steel or copper sponge in a fractional column over glass beads?

Azeotrope – a mixture of liquids of a certain definite composition that distills at a constant temperature without a change in composition
Minimum boiling azeotrope – below pure boiling points of both liquids
What is a minimum boiling azeotrope? (Make sure you provide the definition of an azeotrope as well as what it means to be minimum boiling)

As long as adequate amounts of both water and the organic component are present to saturate the vapor space
In steam distillation, what requirement is necessary in order for the boiling point of the mixture to remain constant?

Vacuum distillation allows the liquid to boil because of the reduced pressure
Steam distillation – if a mixture between 2 immiscible liquids is distilled, the boiling point of the mixture will be below the boiling point of each pure compound, each liquid exerts its vapor pressure independently and when the combined partial pressures equal the external pressure, boiling will occur
Both steam and vacuum distillations allow liquids to boil below the actual boiling points. What differentiates these two types of distillations?

Azeotrope – a mixture of liquids of a certain definite composition that distills at a constant temperature without a change in composition
Maximum boiling azeotrope – above pure boiling points of both liquids
What is a maximum boiling azeotrope? (Make sure you provide the definition of an azeotrope as well as what it means to be minimum boiling)

As column chromatography is carried out, the solvent is collected in a number of small flasks, unless the desired compounds are colored, the various fractions must be analyzed with TLC to determine which ones have the desired components of the mixture. TLC is fast and effective.
TLC can be used to monitor column chromatography. What specifically does the TLC do to achieve this purpose?

TLC is not good for purifying and isolating macroscopic quantities of materials; however, the adsorbents (silica gel and alumina) used for TLC are also used for column chromatography. The correct adsorbent and solvent to use can be rapidly determined by TLC
TLC is typically used to determine the correct adsorbent and solvent for column chromatography. Why is that possible?

Knowing the number and relative amounts of the components aids in planning further analytical and separation steps
What advantage is obtained when a TLC is used to determine the number of components in a mixture?

By sampling a reaction at regular intervals, it is possible to watch the reactants disappear and the products appear using TLC
Which use of TLC allows for one to determine the optimum time to halt a reaction?

a polar solvent
You spot a sample that contains a mixture of 3 compounds of varying polarities on a TLC plate and develop it. Assuming that all other factors (adsorbent, technique) were appropriate, what kind of solvent would cause all the spots to congregate in the manner shown below when visualized under UV light? (spots are all meshed together at solvent front)

a nonpolar solvent
You spot a sample that contains a mixture of 3 compounds of varying polarities on a TLC plate and develop it. Assuming that all other factors (adsorbent, technique) were appropriate, what kind of solvent would cause all the spots to congregate in the manner shown below when visualized under UV light? (spots are all meshed together at spotting line)

very nonpolar substances chromatographed on silica gel will travel with the solvent front and give high Rf values
You spot two very nonpolar compounds on a TLC plate and develop it in order to distinguish the two compounds. Which adsorbent would most likely produce the result shown below when visualized under UV light if all other factors (solvent, technique) are accounted for? (two spots are both at solvent front)

very polar substances chromatographed on alumina will not migrate very far from the starting point and give low Rf values
You spot two very polar compounds on a TLC plate and develop it in order to distinguish the two compounds. Which adsorbent would most likely produce the result shown below when visualized under UV light if all other factors (solvent, technique) are accounted for? (two spots are both at spotting line)

water has a high boiling point and is not very volatile
Water is rarely used as an eluent for TLC. What aspect of water, other than its polarity, makes it an undesirable TLC eluent?

to avoid contamination, so your samples are not mixed together
Why is it important to use a separate spotter for each of your samples when carrying out a TLC experiment?

by capillary action
How does the solvent rise up the TLC plate?

use a capillary tube and spot it down lightly and vertically onto the plate, lift the capillary tube up when the spot is about 1mm in diameter
How do you make spots small and compact when spotting a TLC plate?

the developing chamber will not be saturated with solvent vapors
A student develops a TLC plate without placing filter paper in the developing chamber. Why would this omission hinder the rising of the solvent up the TLC plate?

the compound does not have any conjugation in it, use iodine vapor
What conclusion can you make about a colorless spotted compound if you cannot see the traveled spot under UV light during the visualization stage? What alternative visualization technique can be suggested in this case?

using a solvent of high polarity
You take a TLC of a compound and you find that the Rf of the compound is much higher than expected. What action would have produced the above result?

calculations5 = ((12-x g)/30 ml)/(x/45) = (12-x/30)(45/x)
5= (x/30)/(12-x/45) = (x/30)(45/12-x) = 45x/(360-30x)
1800-150x=45x
1800=195x
x=9.23
12-9.23=2.77g
If 12 grams of an organic solid is dissolved in 45 mL of water, how many grams remain in the water after extraction with 30 mL of a solvent which has a partition (distribution) coefficient with water of 5? Show calculations

remedied by shaking the solution to be extracted very gently until you see that the 2 layers will separate readily or add NaCl to make the aqueous layer highly ionic
Provide one method of preventing or remedying the problem of emulsion.

they are soluble in water
Why are ethanol and methanol not good extraction solvents?

most organic compounds are immiscible in water and therefore can be separated from inorganic compounds
Besides the fact that water is not miscible with the organic solvents used in extraction, why is water used with organic solvents in extraction?

heat from one’s hands or heat from acid/base reactions will cause pressure buildup in an extraction mixture that contains a very volatile solvent
Besides the evolution of CO2 in acid/base extractions, what is another source of buildup of pressure?

sodium phenoxide (phenol is extracted)
You dissolve a mixture of phenol (weak acid), aniline (a base) and naphthalene (neutral compound) in ether. You extract the mixture with NaOH. What is contained in the aqueous layer? (Use actual name(s) of compound(s) in the layer).

anilinium chloride (aniline is extracted)
You dissolve a mixture of aniline (a base) and naphthalene (a neutral compound) in ether. You extract the mixture with HCl. What is contained in the aqueoues layer? (Use actual name(s) of compound(s) in the layer).

it is impossible to ascertain by appearance whether enough has been used
What is the disadvantage of using molecular sieves over calcium chloride pellets as a drying agent?

the pellets do not disintegrate into a fine powder
What is the advantage of using calcium chloride pellets over using the older granular form?

it is slow and not very efficient
List two reasons that make sodium sulfate a poor drying agent as compared to calcium chloride pellets.

it is so finely powdered that it requires careful filtration for removal
What are the disadvantages of using magnesium sulfate as the drying agent instead of calcium chloride pellets?

the high vapor pressure of the solvent will cause it to dribble out of the pipette
When transferring a layer from one test tube to another using a pipette with a rubber bulb attached, it is advised to grasp an empty tube in the same hand as the full tube to avoid losing any solution. What is the cause of the possible loss of solution from the pipette?

decrease rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN1 reaction. Answers should follow what you learned in the lab lecture.

Decrease temperature

decrease rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN1 reaction. Answers should follow what you learned in the lab lecture.

Change the leaving group from Br to F

no effect
Indicate whether the following situation would increase, decrease or not affect the rate of an SN1 reaction. Answers should follow what you learned in the lab lecture.

Decrease concentration of nucleophile

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN1 reaction. Answers should follow what you learned in the lab lecture.

Change solvent from ethanol(relatively nonpolar) to water(polar protic)

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN1 reaction. Answers should follow what you learned in the lab lecture.

Increase concentration of alkyl halide

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Increase temperature

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Change the leaving group from Br to I

decrease rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Decrease concentration of nucleophile

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Change solvent from ethanol to acetone

decrease rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Decrease concentration of alkyl halide

decrease rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Decrease temperature

decrease rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Change the leaving group from Br to Cl

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Increase concentration of nucleophile

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Change solvent from acetone to water

increase rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Increase concentration of alkyl halide

decrease rate
Indicate whether the following situation would increase, decrease or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.

Change solvent from water to acetone