MCAT Biology : Reactions with Ketones and Aldehydes

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #2 : Reaction Types

Which of the following statements is false concerning the Wittig reaction?

Possible Answers:

A mixture of cis and trans isomers are created in the reaction

The product compound that includes the phosphorous from the ylide will be attached to the reactant's carbonyl oxygen

The ketone or aldehyde is bound by the negatively charged phosphorous atom

The ylide attaches to the substrate via nucleophilic addition

Correct answer:

The ketone or aldehyde is bound by the negatively charged phosphorous atom

Explanation:

The Wittig reaction is used to make an alkene from a ketone or aldehyde. The ylide (RPPh3) attaches to the substrate via nucleophilic addition, with the negative carbanion attaching to the ketone or aldehyde. The phosphorus atom carries a positive charge, balancing the carbanion.

All other listed answer choices are true. The products of the reaction are an alkene (cis and trans) with the addition of the R group of the ylide and Ph3PO.

Example Question #2 : Reactions With Ketones And Aldehydes

All of the following are characteristics of a Wittig reaction except __________.

Possible Answers:

it produces a trialkylphosphine oxide or triarylphosphine oxide as a by-product

it proceeds through a phosphaoxetane intermediate

it involves the reaction of a phosphonium ylide with a carbonyl

it results in the exclusive formation of trans double bonds

it results in the formation of a carbon-carbon double bond

Correct answer:

it results in the exclusive formation of trans double bonds

Explanation:

The Wittig reaction involves the reaction of a phosphonium ylide (generated by treating a phosphonium salt with a strong base) with a ketone or aldehyde.

The reaction proceeds through a phosphaoxetane (4-membered ring containing both phosphorus and oxygen) intermediate to generate a new compound containing a carbon-carbon double bond, plus a phosphine oxide byproduct. It does not form trans double bonds exclusively; sometimes, a mixture of cis and trans isomers are obtained, and sometimes the cis isomer is the predominant product. 

Example Question #3 : Reaction Types

What is created when a ketone is reacted with a phosphorus ylide?

Possible Answers:

Alkane

Alkene

Aldehyde

Ester

Correct answer:

Alkene

Explanation:

The Wittig reaction involves a ketone or aldehyde reacting with a phosphorus ylide, a molecule with a negatively charged carbanion. The ketone will undergo nucleophilic addition and form a betaine. This intermediate will then form an alkene with a triphenylphosphine oxide being released. The Wittig reaction will form a mixture of both cis and trans isomers if the carbanion has two different substituents.

Wittig general reaction:

Example Question #28 : Organic Chemistry, Biochemistry, And Metabolism

The reaction between one mole of acetone and one mole of 1-propanol in aqueous acid will result in the formation of what product?

Possible Answers:

A hemiacetal compound

An acetal compound

A ketal compound

A hemiketal compound

Correct answer:

A hemiketal compound

Explanation:

A hemiketal compound is the result of nucleophilic attack by an alcohol (1-propanol) on the carbonyl of a ketone (acetone). The previously double-bonded oxygen now bears a negative charge, and deprotonates the now positively charged attacking alcohol. The hemiketal of acetone will have a hydroxy group and an -OCH3 group bound to the central carbon.

Example Question #4 : Reaction Types

Imine

What is the product of the above reaction?

Possible Answers:

An amide compound

No reaction occurs

An amino compound

An enamine

An imine

Correct answer:

An imine

Explanation:

This reaction shows nucleophilic acyl attack on a carbonyl group by the ammonia molecule. This reaction leads to the loss of oxygen as water, and eventually the formation of an imine. Note the difference between each product type. An imine is a nitrogen with one nitrogen-carbon double bond and one substituent. An enamine is formed from a nitrogen with no double bonds. An amide is a nitrogen bound to a carbonyl carbon.

 

Imine_reaction

Example Question #5 : Reaction Types

An aldehyde undergoing one round of nucleophilic attack by an equivalent of alcohol results in a(n) __________ product, where two rounds of nucleophilic attack (two alcohol equivalents) results in a(n) __________ product.

Possible Answers:

acetal . . . ketal

acetal . . . hemiacetal

hemiacetal . . . acetal

hemiketal . . . ketal

Correct answer:

hemiacetal . . . acetal

Explanation:

The correct answer is hemiacetal and acetal. The partial positive character of a carbonyl carbon makes it susceptible to nucleophilic attack. An alcohol's oxygen has free electrons, and therefore can serve as a nucleophile. As the bond between the attacking alcohol's oxygen and the carbonyl carbon forms, the pi bond of the carbonyl group is lost. Once the attacking alcohol's oxygen is deprotonated, the resulting product is an ether (from the attacking alcohol) and a new hydroxyl group (from the now protonated carbonyl oxygen). The resulting molecule is referred to as the hemiacetal product.

Subsequent addition of alcohol leads to the loss of the hydoxyl group as water, resulting in the second ether chain to the original carbon, which is referred to as the acetal product.

The same reaction takes place with ketones as well. With ketones, the first addition of alcohol results in the hemiketal product, and the second addition of alcohol results in the ketal product.

It is important to remember the following material for analyzing acetal and ketal reactions.

1. These alcohol addition reactions only occur with aldehydes and ketones.

2. Hemiacetal and hemiketal products can be identified by having one ether chain and one hydroxyl group attached to the original carbonyl carbon. Acetal and ketal groups have two ether groups attached to the original carbonyl carbon.

3. Acetal and hemiacetal groups will have a hydrogen attached to the original carbonyl carbon, where ketal and hemiketal groups will have carbon chains attached to the original carbonyl carbon (with no bound hydrogens).

Example Question #6 : Reactions With Ketones And Aldehydes

Which of the compounds below could not be made from an aldehyde reduction?

Aldehyde_reduction

Possible Answers:

II only

I, III, and IV

 

I only

IV only

III only

Correct answer:

II only

Explanation:

Of the choices given, all can be made from some type of aldehyde reduction except choice II. Choices I, III, and IV each have the terminal (or primary) alcohol that is characteristic of a former aldehyde. In contrast, choice II has a secondary alcohol, characteristic of a former ketone. In other words, if choice II was oxidized then the product would be a ketone, not an aldehyde.

Example Question #6 : Reaction Types

Acetaldehyde  undergoes a Wolf-Kishner reaction, which is the addition of hydrazine  with subsequent addition of a base and heat. In this reaction, the aldehyde is __________, resulting in a(n) __________ product.

Possible Answers:

reduced . . . alcohol

reduced . . . alkane

oxidized . . . amide

oxidized . . . carboxylic acid

Correct answer:

reduced . . . alkane

Explanation:

The correct answer is that the aldehyde is reduced to an alkane. In viewing the final product, we see that acetaldehyde would be reduced to ethane. The reaction of any aldehyde or ketone with hydrazine and the subsequent addition of base and heat will result in that aldehyde or ketone being reduced to an alkane, and is referred to as the Wolf-Kishner reaction. The Wolf-Kishner reagent is a commonly tested reducing agent.

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