Organic Chemistry Aldehydes And Ketones - Nucleophilic Addition Of Alcohols: Acetal Formation

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Aldehydes and ketone react reversibly with two equivalents of an alcohol in the presence of an acid catalyst to yield ?

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acetals, R2C(OR')2

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If acetals are derived from a ketone, they are sometimes called

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Ketals

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Acetal formation is similar to the ..?

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hydration rxn

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Like water, alcohols are weak nucleophiles that add to aldehydes and ketones only ...

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slowly under natural conditions

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An Acetal formation under acidic conditions, the reactivity of the carbonyl group is

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increased by protonation so addition of an alcohol occurs rapidly

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Nucleophilic addition of an alcohol to the carbonyl group initally yields

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a hydroxy ether called a hemiacetal

- analogous to the gem diol formed by the addition of water.

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Hemiacetals are formed reversibly, with the equilibrium

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normally favoring the carbonyl compound

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Hemiacetals are formed

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reversibly

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A Hemiacetal in the presence of acid, a futrher rxn can occur such as

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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

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Since acetal formation are reversible, the rxn can be driven either

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forward (from carbonyl compound to acetal) or backward (from acetal to carbonyl compound), depending on the conditions

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The forward rxn of an acetal formation is favored by ?

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conditions that remove water from the medium and thus drive the equilibrium to the right

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The backward rxn of an acetal formation is favored by

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treating the acetal with a large excess of aqueous acid to drive the equilibrium to the left

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Acetals are useful because they can serve as

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protecting groups for aldehydes and ketones in the same way that trimethylsilyl ethers serve as protecting groups for alcohols

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Treatment of the starting keto ester with LiAlH4 of a acetal formation would

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reduce both the keto and the ester groups to give a diol product

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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,

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the problem can be circumvented

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Like other esters, acetals are unreactive to ...

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bases, hydride reducting agents, Grignard regents, and catalytic reducing conditions