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16 Cards in this Set

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Aldehydes and ketone react reversibly with two equivalents of an alcohol in the presence of an acid catalyst to yield ?
acetals, R2C(OR')2
If acetals are derived from a ketone, they are sometimes called
Ketals
Acetal formation is similar to the ..?
hydration rxn
Like water, alcohols are weak nucleophiles that add to aldehydes and ketones only ...
slowly under natural conditions
An Acetal formation under acidic conditions, the reactivity of the carbonyl group is
increased by protonation so addition of an alcohol occurs rapidly
Nucleophilic addition of an alcohol to the carbonyl group initally yields
a hydroxy ether called a hemiacetal

- analogous to the gem diol formed by the addition of water.
Hemiacetals are formed reversibly, with the equilibrium
normally favoring the carbonyl compound
Hemiacetals are formed
reversibly
A Hemiacetal in the presence of acid, a futrher rxn can occur such as
Protonation of the -OH group followied by an E1 like lost of water leads to an oxonium ion, R2C=OR+, which undergoes a second nucleophilic addition of alcohol to yield the acetal
Since acetal formation are reversible, the rxn can be driven either
forward (from carbonyl compound to acetal) or backward (from acetal to carbonyl compound), depending on the conditions
The forward rxn of an acetal formation is favored by ?
conditions that remove water from the medium and thus drive the equilibrium to the right
The backward rxn of an acetal formation is favored by
treating the acetal with a large excess of aqueous acid to drive the equilibrium to the left
Acetals are useful because they can serve as
protecting groups for aldehydes and ketones in the same way that trimethylsilyl ethers serve as protecting groups for alcohols
Treatment of the starting keto ester with LiAlH4 of a acetal formation would
reduce both the keto and the ester groups to give a diol product
Treatment of the starting keto ester with LiAlH4 of a acetal formation would reduce both the keto and the ester groups to give a diol product

By protecting the keto group as an acetal,
the problem can be circumvented
Like other esters, acetals are unreactive to ...
bases, hydride reducting agents, Grignard regents, and catalytic reducing conditions