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

What are the α-carbon and β-carbon?

The α-carbon is directly adjacent to a carbonyl carbon; the next carbon is the β-carbon.

Why are carbonyl α-hydrogens relatively acidic?

An α-hydrogen is more acidic because removing it forms a conjugate base stabilized by the neighboring carbonyl group.

What structural feature defines an enol?

An enol has a carbon–carbon double bond with an –OH group attached directly to one of its alkene carbons.

How does keto–enol tautomerism differ from resonance?

Keto–enol tautomerism interconverts constitutional isomers as a hydrogen and a double bond change position; the forms are not resonance contributors.

Which tautomer usually predominates for simple aldehydes and ketones?

Most simple aldehydes and ketones exist mainly in the keto form at equilibrium. Conjugation or intramolecular hydrogen bonding can increase the enol content.

How does a carbonyl compound form an enolate?

An enolate forms when a base removes an α-hydrogen. Its negative charge is delocalized between oxygen and the α-carbon.

How does LDA compare with a weaker base in enolate formation?

A strong base such as LDA can convert a carbonyl compound substantially into its enolate; weaker bases often produce only a small equilibrium fraction.

Where do enols and enolates typically react with electrophiles?

Enols and enolates act as nucleophiles at the α-carbon, allowing reaction with an electrophile to replace an α-hydrogen with a new group.

Which alkyl halides are best for enolate alkylation?

The enolate carbon attacks the alkyl halide in an SN2 substitution. Methyl and primary halides work best; tertiary halides generally favor competing elimination.

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

One carbonyl partner forms an enolate donor, whose α-carbon attacks the carbonyl carbon of the electrophilic partner; protonation then gives a β-hydroxy carbonyl compound.

What α-hydrogen requirement applies to an aldol reaction?

At least one partner must have an α-hydrogen so it can form the enol or enolate donor.

What product forms when two ethanal molecules undergo aldol addition?

The aldol addition of ethanal gives 3-hydroxybutanal, a β-hydroxy aldehyde.