06 Enols, Enolates, and Carbon–Carbon Bond Formation Free Online FlashCards

What are the α carbon and an α hydrogen?

The α carbon is the carbon adjacent to a carbonyl carbon; an α hydrogen is attached to that carbon.

What structural feature defines an enol?

An enol is a neutral compound with an –OH group attached to a carbon–carbon double bond.

How does the step order differ in acid- and base-catalyzed enolization?

Under acid catalysis, the carbonyl oxygen is protonated before an α proton is removed; under base catalysis, α deprotonation occurs first.

What makes an enolate useful as a carbon nucleophile?

An enolate is a resonance-stabilized anion with negative charge distributed between oxygen and the α carbon. Its carbon end commonly forms C–C bonds.

What conditions often favor a kinetic enolate?

Rapid deprotonation with a strong, bulky base such as LDA often favors removal of the more accessible α hydrogen, giving the kinetic enolate.

What is the key bond-forming event in an aldol reaction?

An enolate attacks another aldehyde or ketone’s carbonyl carbon. Protonation gives a β-hydroxy carbonyl, with a new bond between the donor α carbon and acceptor carbonyl carbon.

How can selectivity be improved in a crossed aldol?

A crossed aldol uses two different carbonyl compounds. Selectivity can improve by using an acceptor with no α hydrogens, such as benzaldehyde, or by preforming one enolate.

How does α-alkylation form a new C–C bond?

An enolate replaces an α hydrogen with an alkyl group by SN2 substitution on a suitable alkyl electrophile, commonly a primary alkyl halide.

What product characterizes a Claisen condensation?

An ester enolate attacks another ester’s carbonyl; the tetrahedral intermediate expels an alkoxide to form a β-keto ester.

Where does an enolate add in a Michael addition?

An enolate adds to the β carbon of an α,β-unsaturated carbonyl by conjugate, or 1,4-, addition, forming a C–C bond at the donor α carbon.

How do you identify donor and acceptor in an enolate reaction?

The donor forms the enolate; the acceptor is the electrophile. The new bond forms between the donor’s α carbon and the electrophilic carbon specified by the reaction.

Why are α hydrogens relatively acidic?

They are more acidic than ordinary alkane hydrogens because deprotonation forms a resonance-stabilized species.