Ch 7 - Aldehydes and Ketones: Enolates Flashcards
What is the alpha carbon?
the carbon adjacent to the carbonyl carbon
What are the alpha hydrogens?
the hydrogens attached to the alpha carbon
How can alpha hydrogens be removed?
they are relatively acidic and can be removed by a strong base
What weakens the CH bonds on an alpha carbon?
the electron withdrawing oxygen of the carbonyl
How can the enolate resulting from deprotonation be stabilized?
by resonance with the carbonyl
Why are ketones less reactive toward nucleophiles?
steric hindrance and alpha-carbanion destabilization
- the presence of an additional alkyl group crowds the transition step and increases its energy
- the alkyl group also donates electron density to the carbanion, making it less stable
What form do aldehydes and ketones exist in?
- in the traditional keto form (C=O) and in the less common enol form ( ene + ol = double bond + hydroxyle group)
What are tautomers?
- isomers that can be interconverted by moving a H and double bond
- the keto and enol forms are tautomers of each other, keto being thermodynamically favored
What is the enol form deprotonated to?
to form enolate, which are good nucleophiles
What happens in the Michael addition?
an enolate attacks an alpha, beta-unsaturated carbonyl, creating a bond
What is the difference between the kinetic and thermodynamic enolate?
- kinetic favored by by fast, irreversible reactions at low temperatures with strong, sterically hindered bases
- thermo: favored by slower, reversible reactions at higher temperatures with weaker, smaller bases
What are enamines?
tautomers of imines
- like enols, enamines ar the less common tautomer
What happens in aldol condensation?
the aldehyde or ketone acts as both nucleophile and electrophile, resulting in the formation of a carbon-carbon bond in a new molecule called an aldol
What does an aldol contain?
both aldehyde and alcohol functional groups
What is the nucleophile and electrophiles in aldol condensation?
- nucleophile is the enolate formed from the deprotonation of the alpha-carbon
- electrophiles is the aldehyde or ketone in the form of the keto tautomer
What is a condensation reaction?
- the two molecules come together (electro/nucleophile)
- first step of aldol condensation
What occurs after the aldol is formed in aldol condensation?
a dehydration reaction (loss of water molecule) occurs resulting in an alpha, beta-unsaturated carbonyl
What are retro-aldol reactions? How are they catalyzed?
- reverse of aldol condensation
- catalyzed by heat and base
- the bond between an alpha and beta-carbon are cleaved
How do electron withdrawing/donating groups affect organic anions?
- withdrawing: like oxygen, stabilize organic anions
- donating: like alkyl groups, destabilize organic anions
Why are the alpha hydrogens of aldehydes and ketones acidic?
- due to both inductive effects and resonance effects
- the electronegative oxygen atom pulls electron density from the CH bond, weakening it
- once deprotonated, the resonance stabilization of the negative charge between the alpha carbon, carbonyl carbon, and electron withdrawing carbonyl oxygen increases the stability of this form
Which has lower pKa: 1-pentanone or pentanal?
pentanal > 3-pentanone
- the alpha hydrogens of aldehydes are slightly more acidic than those of ketones due to the electron-donating characteristics of the second alkyl group in ketones
- this extra alkyl group destabilizes the carbanion, which slightly disfavors the loss of the alpha-hydrogen in ketones as compared to aldehydes
How does steric hindrance affect the relative reactivity of aldehydes and ketones?
- steric hindrance is one of the 2 reasons that aldehydes are slightly more reactive than ketones
- the additional alkyl group gets in the way and makes for a higher-energy, crowded intermediate
What happens when an aldehyde/ketone is treated with a strong base?
it forms the nucleophilic enolate carbanion
What is the final product of an aldol condensation?
alpha, beta-unsaturated carbonyl
