MCAT Organic Chemistry Review

Aldehydes and Ketones I: Electrophilicity and Oxidation—Reduction

Answers and Explanations

1.    DThe reactivity of the carbonyl can be attributed to the difference in electronegativity between the carbon and oxygen atoms. The more electronegative oxygen atom attracts the bonding electrons and is therefore electron-withdrawing. Thus, the carbonyl carbon is electrophilic, and the carbonyl oxygen is nucleophilic. One resonance structure of the carbonyl pushes the π electrons onto the oxygen, resulting in a positively charged carbonyl carbon.

2.    BAssuming the length of the carbon chain remains the same, the alkane consistently has the lowest boiling point. The boiling point of the ketone is elevated by the dipole in the carbonyl. The boiling point of the alcohol is elevated further by hydrogen bonding.

3.    CThe reaction between a ketone and one equivalent of alcohol produces a hemiketal. This has an –OR group, an –OH group, and two alkyl groups attached at the same carbon. Choice (A) is a ketal, with two –OR groups and two –R groups. Choice (B) is a hemiacetal, with an –OH group, an –OR group, one R group, and a hydrogen atom. Choice (D) is a ketone. Note that a hemiketal is a very unstable compound, and will react rapidly with a second equivalent of alcohol to form a ketal in acidic conditions.

4.    AAldehydes and ketones react with ammonia and nitrogen-based derivatives to form imines—compounds with a double bond between carbon and nitrogen.

5.    BAldehydes are easily oxidized to the corresponding carboxylic acids by KMnO4. In choice (A), the aldehyde has been reduced to an alcohol. In choice (C), the molecule has not reacted. In choice (D), the aldehyde has been oxidized, but a –CH2– group has been removed.

6.    BLiAlH4 reduces aldehydes to primary alcohols and ketones to secondary alcohols. In this reaction, a ketone is converted to a secondary alcohol.

7.    DBecause an excess of ethanol is present, the product of the reaction between this aldehyde and ethanol will be an acetal. The benzaldehyde will first be converted to a hemiacetal, shown in choice (C), but will then proceed to completion as an acetal. Choices (A) and (B) are incorrect because they show the presence of two benzene rings in the final product.

8.    BHemiacetals and hemiketals are usually short-lived because the –OH group will rapidly become protonated in acidic conditions and is lost as water, leaving behind a carbocation that is very susceptible to attack by an alcohol. Once the alcohol has been added, the acetal or ketal becomes more stable because the newly added group is less likely to become protonated and leave as compared to –OH.

9.    AA hemiacetal is a molecule in which one equivalent alcohol has been added to a carbonyl (–OR) and the carbonyl oxygen has been protonated (–OH). Otherwise, there is the same alkyl group (–R) and hydrogen atom (–H) as the parent aldehyde. Choice (B) describes an acetal,choice (C) a hemiketal, and choice (D) a ketal.

10.AAlthough both the aldehyde and ketone listed will be reactive with the strongly nucleophilic hydrogen cyanide, aldehydes are slightly more reactive toward nucleophiles than ketones for steric reasons, so the aldehyde and HCN will form the major product (which will be a cyanohydrin).

11.CPCC is a mild anhydrous oxidant that can oxidize primary alcohols to aldehydes, and secondary alcohols to ketones. It is not strong enough to oxidize alcohols or aldehydes to carboxylic acids.

12.BIn a hydration reaction, water adds to a carbonyl, forming a geminal diol—a compound with two hydroxyl groups on the same carbon. Hydrogen peroxide and potassium dichromate are oxidizing agents, which can convert an aldehyde to a carboxylic acid. Ethanol will react with a carbonyl compound to form an acetal or a ketal, if excess ethanol is available.

13.AAmmonia, or NH3, will react with an aldehyde like glutaraldehyde to form an imine. This is a condensation and a substitution reaction, as the C=O of the carbonyl will be replaced with a C=N bond.

14.CHydrides like LiAlH4 and NaBH4 are reducing agents; as such, they will reduce aldehydes and ketones to alcohols. The other reagents listed are oxidizing agents, which will not act on a ketone.

15.DDuring tautomerization, the double bond between the carbon and nitrogen in an imine is moved to lie between two carbons. This results in an enamine—a compound with a double bond and an amine.