Ch 7 - Aldehydes and Ketones: Enolates

Question 1

What is the alpha carbon?

Answer 1

the carbon adjacent to the carbonyl carbon

Question 2

What are the alpha hydrogens?

Answer 2

the hydrogens attached to the alpha carbon

Question 3

How can alpha hydrogens be removed?

Answer 3

they are relatively acidic and can be removed by a strong base

Question 4

What weakens the CH bonds on an alpha carbon?

Answer 4

the electron withdrawing oxygen of the carbonyl

Question 5

How can the enolate resulting from deprotonation be stabilized?

Answer 5

by resonance with the carbonyl

Question 6

Why are ketones less reactive toward nucleophiles?

Answer 6

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

Question 7

What form do aldehydes and ketones exist in?

Answer 7

• in the traditional keto form (C=O) and in the less common enol form ( ene + ol = double bond + hydroxyle group)

Question 8

What are tautomers?

Answer 8

• 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

Question 9

What is the enol form deprotonated to?

Answer 9

to form enolate, which are good nucleophiles

Question 10

What happens in the Michael addition?

Answer 10

an enolate attacks an alpha, beta-unsaturated carbonyl, creating a bond

Question 11

What is the difference between the kinetic and thermodynamic enolate?

Answer 11

• 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

Question 12

What are enamines?

Answer 12

tautomers of imines

- like enols, enamines ar the less common tautomer

Question 13

What happens in aldol condensation?

Answer 13

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

Question 14

What does an aldol contain?

Answer 14

both aldehyde and alcohol functional groups

Question 15

What is the nucleophile and electrophiles in aldol condensation?

Answer 15

• 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

Question 16

What is a condensation reaction?

Answer 16

• the two molecules come together (electro/nucleophile)

- first step of aldol condensation

Question 17

What occurs after the aldol is formed in aldol condensation?

Answer 17

a dehydration reaction (loss of water molecule) occurs resulting in an alpha, beta-unsaturated carbonyl

Question 18

What are retro-aldol reactions? How are they catalyzed?

Answer 18

• reverse of aldol condensation
• catalyzed by heat and base
• the bond between an alpha and beta-carbon are cleaved

Question 19

How do electron withdrawing/donating groups affect organic anions?

Answer 19

• withdrawing: like oxygen, stabilize organic anions

- donating: like alkyl groups, destabilize organic anions

Question 20

Why are the alpha hydrogens of aldehydes and ketones acidic?

Answer 20

• 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

Question 21

Which has lower pKa: 1-pentanone or pentanal?

Answer 21

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

Question 22

How does steric hindrance affect the relative reactivity of aldehydes and ketones?

Answer 22

• 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

Question 23

What happens when an aldehyde/ketone is treated with a strong base?

Answer 23

it forms the nucleophilic enolate carbanion

Question 24

What is the final product of an aldol condensation?

Answer 24

alpha, beta-unsaturated carbonyl