Organic Chemistry II Lab Exam

Why must the oxidation of an alcohol take place in an ice bath?

The reaction is extremely exothermic, so the ice bath is used to cool the reaction down.

What was the oxidizing agent used in the oxidation of an unknown alcohol lab?

Bleach (sodium hypochlorite - Na⁺ OCl^-)

Draw the mechanism for the oxidation of isopropanol to acetone with bleach as the oxidizing agent.

During a KI Starch paper test, what are the two colors that can be produced and what do they mean?

• Yellowish white - negative result, no starch is present or no oxidizing agent is present

• Deep Blue - positive result, there is starch or an oxidizing agent present

What is the purpose of a base wash when oxidizing an alcohol?

To neutralize and remove acidic components. In this lab it would have been the residual glacial acetic acid.

What should be different in the IR spectra of an alcohol and ketone.

An alcohol should have an O-H stretch around 3500-3200 cm^-1, while a ketone should have a C=O stretch around 1750-1680 cm^-1.

What should be different in the H NMR spectra of an alcohol and ketone.

There should be a singlet peak for the hydrogen that is attached to the oxygen in an alcohol, while a ketone would not.

What is the structure for R-Carvone?

Draw the reaction mechanism for the conversion of R-carvone to carvacrol using sulfuric acid as the acid catalyst.

How can an acid catalyst help form an enol from a ketone?

The acid catalyst will protonate the oxygen of the carbonyl creating a “super-electrophile”. The now deprotonated acid catalyst will take an alpha hydrogen and fold down its electrons creating a double-bond between the alpha and carbonyl carbons. This will force the electrons in the pi bond of the “super-electrophile” up and solve the charge, leaving an alcohol in its place.

Why is R-carvone chiral and carvacrol not?

The chiral carbon that was once denoted as R loses its chirality as the cyclohexene becomes a benzene. This means it is now sp² hybridized, which cannot be chiral.

Why is there an inversion during an S_N2 mechanism?

Since an S_N2 happens in a concerted fashion, the nucleophile must attack as the leaving group leaves. The leaving group would still be occupying a certain orientation, leaving the nucleophile to choose the other.

Why does an alkoxide or alkyl halide need to be primary of secondary for a substitution reaction?

As tertiary compounds, more often than not, elimination is favored.

Draw out a reaction that includes a base catalyzed formation of phenoxide followed by a nucleophilic attack from chloroacetic acid.

How would you purify a final product based on relative acidities?

Do an acid-base extraction. The charged particles will end up in the aqueous layer, while the neutral molecules will end up in the organic layer.

Draw the reaction mechanism for the acid catalyzed opening of a 1,2-epoxycyclohexane. Use HBr.

Draw the reaction mechanism for the base catalyzed opening of a 1,2-epoxycyclohexane. Use NaOH.

How does the opening of an epoxide ring differ between acid and base catalysis in terms of nucleophilic attacks?

In base the nucleophile always attacks the less substituted carbon. In acid, the nucleophile will attack a tertiary carbon if it is present, but if not it will attack the less substituted carbon as well.

A Fischer esterification is the conversion of which type of molecule to what other type?

A carboxylic acid into an ester

How does Le Chatlier’s principle work in shifting equilibrium to favor the products in a Fischer esterification?

The over-addition of reagents will stress that side and work to drive equilibrium forward. Also, the addition of heat will cause the water by-product to evaporate, preventing it from influencing the reaction back to the reactant side.

Draw out the mechanism for a Fischer esterification using acetic acid, methanol, and sulfuric acid.

Why is aluminum oxide needed as a catalyst to convert an anhydride into an amide, despite the reactivity of an anhydride being enough?

It neutralizes the carboxylic acid byproduct which prevents the amine from being protonated and stopping this reaction.

Why is a reaction concerning the conversion of an anhydride to a amide sensitive to polar solvents?

Polar solvents like water and alcohol are slightly more reactive than amines and thus will participate in the reaction over the amine. The water will hydrolyze the amine back into a carboxylic acid, while the alcohol will react with it to form an ester.

Why was ethyl acetate used to extract the reaction mixture between the anhydride and amine instead of dichloromethane?

Ethyl acetate is safer to work with and less volatile than DCM.

How does ethyl acetate work as a solvent for the extraction of the reaction mixture formed between anhydride, amine, and aluminum oxide?

It dissolves the newly synthesized amide product and leaves behind the solid aluminum oxide and other components.

Draw the reaction mechanism to form an amide from acetic anhydride and methylamine.

Is heat necessary for the reduction of a ketone or aldehyde to alcohols?

No, because common reducing agents like NaBH₄ and LiAlH₄ use H^- to reduce, which is already reactive at room temperature.

Why is a strong reducing agent unnecessary for aldehydes and ketones?

Their carbonyls are already electrophilic, meaning the oxygen atom in the bond pulls electron density away from the carbon giving it a partial positive charge. This makes it more accepting of nucleophilic attacks.

Is the reduction of an aldehyde or ketone stereospecific? Why or why not?

No, because that negative hydrogen has an equal chance of attacking from either plane. This means that the product is found in a racemic mixture.

What should be some visible differences in the IR of the starting material and final product in the reduction of an aldehyde or ketone?

The starting product should feature a carbonyl peak between 1750-1680 cm^-1, while the final product should feature an alcohol stretch around 3500-3200 cm^-1.

Draw the reaction mechanism for the reduction of a ketone. Use acetone and NaBH₄ in this drawing.

What becomes what in a Wittig reaction?

An aldehyde or ketone becomes an alkene

What is a ylide typically comprised of?

A phosphorous attached to three phenyl groups and a carbon

Why can a Wittig Reaction not form tetrasubstituted alkenes?

It is prevented from doing such by steric hinderance.

Draw the Wittig reaction mechanism. Use acetone and create a ylide of your choice.

Why is a Wittig reaction considered an elimination reaction?

It includes the loss of a leaving group—triphenyl phosphorous double bonded to oxygen—due to the formation of a double bond.

What type of reaction does a Friedel-Crafts alkylation fall under?

Electrophilic aromatic substitution

What was the nucleophile and electrophile used in the Friedel-Crafts alkylation lab?

1,4-dimethoxybenzene as the nucleophile and an unknown alcohol as the electrophile

Why is sulfuric acid needed as a catalyst in a Friedel-Crafts alkylation?

It is used to create a carbocation from the alcohol, which is then attacked by the benzene ring. The deprotonated acid then returns to take back a hydrogen and fold down its electrons, restoring aromaticity.

Draw the reaction mechanism for a Friedel-Crafts alkylation using isopropanol, sulfuric acid, and toluene.

How do you know if the product formed is ortho, para, or meta after a Friedel-Crafts alkylation?

A Friedel-Crafts alkylation is only possible when there is an electron-donating group. This is also what incoming molecules listen to when being added to structures. For that reason the product formed would be ortho or para, with a preference for para.

Draw a Diels-Alder reaction mechanism using 1,3-butadiene and ethene.

Why is heat typically used in a Diels-Alder reaction?

Heat helps the reaction overcome the activation energy barrier, as well as placing the diene in the less stable s-cis confirmation which is required for this reaction.

Why does an electron-withdrawing group on a dienophile speed up a Diels-Alder reaction?

It lowers the energy in the LUMO of the dienophile, which decreases the gap between the dienophile’s LUMO and the diene’s HOMO. This will lower the activation energy of the reaction and increase the rate of the reaction.

Why can a Grignard not form in the presence of water?

In the presence of water, the Grignard will preferentially react with it instead of the target molecule since the Grignard is a very strong base and nucleophile.

Draw the reaction mechanism for a Grignard using acetone, bromoethane, and the reagent mix necessary for creating Grignards.

Why can a Grignard not be created by a alkyl halide that has acidic protons or electrophilic carbonyl functionality?

For the same reason water cannot be present. The Grignard will react preferentially with acidic protons because it is a very strong base and nucleophile.

What is the name of your lab TA?

Yizhi Chi