(D), the aldehyde has been oxidized, but a –CH 2 – group has been removed.6 . B
LiAlH 4 reduces aldehydes to primary alcohols and ketones to secondary alcohols. In this reaction,
a ketone is converted to a secondary alcohol.7 . D
Because 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 . B
Hemiacetals 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 . A
A hemiacetal is a molecule in which one equivalent of 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 . A
Although 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 . C
PCC 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 . B