CH8: Aldehydes and Ketones-Aldol Reactions

Question 1

What are properties of alpha hydrogens in carbonyl compounds and why

Answer 1

Alpha H are super acidic due to the resonance stabilized enolate anion that forms

Question 2

What forms during protonation of the enolate anion

Answer 2

It can be protonated at the O or the C forming keto or enol forms. This is tautomerization.

Keto enolate anion enol form

Question 3

Keto enol tautomers are:

Answer 3

Constitutional isomers, ketones and aldehydes almost always exist in the keto form

Question 4

What form do beta-dicarbonyl systems primarily exist in and why?

Answer 4

In the enol form due to conjugation

Question 5

Racemization via enols and enolate anions

Answer 5

An optically active aldehyde or ketone with a stereocenter at the alpha carbon can lead to racemization if in keto form or an achiral molecule in the enol when reacted with acid or base

Question 6

Base and acid catalyzed racemization

Answer 6

A base is more important…OR- removes H from Alpha carbon forming enolate anion which then gets its O protonated forming an enol

Question 7

Enols are

Answer 7

achiral

Question 8

Ketone halogenation

Answer 8

Ketones can be halogenated at the alpha carbon in the presence of acid or base.

Acid-Goes via enol
Base-Goes via enolate anion

Question 9

Haloform reaction

Answer 9

Reaction of ketone with 3 alpha hydrogens in the presence of excess base
1) Remove H with base
2) Grab X with a carbanion
NO SHIFTING OF O UNTIL HALOFORM IS FORMED
3) Base attacks carbonyl carbon

Question 10

Haloform

Answer 10

CHX3 + carboxylate anion. Haloform is a good L.G.

Question 11

What is the Aldol rxn

Answer 11

Ketone + aldehyde via enolate anion. Aldol=Aldehyde and alcohol. It is 2 of an aldehyde or ketone with dilue NaOH

Question 12

Dehydration of Aldol product

Answer 12

If it is heated with base present, dehydration to an unsaturated Alpha,beta-unsaturated carbonyl compound. It is stabilized because of conjugation.

Question 13

Alpha, beta-Unsaturated carbonyl compound

Answer 13

From dehydration of an aldol product with heat leading to db in conjugation

Question 14

How to undo the aldol rxn

Answer 14

Strong base

Question 15

What does acid catalyzed acid yield

Answer 15

The condensation product. Key step: The db of the enol formed attacks the carbonyl carbon that has been protonated. Goes via the enol.

Question 16

How mny products can be formed during crossed aldol reactions

Answer 16

4

Question 17

What are practical crossed aldol reactions

Answer 17

They give predictable reactions when one of the compounds has no alpha hydrogens. The one without any alpha H is put in basic solutions and the one with is added slowly. Dehydration normally occurs immediately.

Question 18

Claisen-Schmidt reactions

Answer 18

These are crossed aldol reactions in which one of the reactants is a ketone. Ketone self-condensation is not observed…only 1 product.

Question 19

Cyclization via aldol condensations

Answer 19

Dicarbonyl compounds can react to form 5 or 6 membered rings in the presence of base. The aldehyde is attacked over the carbon. The condensation product forms.

Question 20

Lithium enolates

Answer 20

In the presence of a strong base such as LDA, enolate formation is greatly favoured

Question 21

LDA

Answer 21

Strong, sterically hindered base

Question 22

Regioselective enolates

Answer 22

Unsymmetrical ketones have 2 places to remove alpha H from there can be the kinetic and thermodynamic products.

Question 23

Thermodynamic enolate

Answer 23

Favoured by using weak base because an eq forms

Question 24

Kinetic enolate

Answer 24

Favoured with LDA, THF -78degrees

Question 25

LDA and ketones in crossed aldol

Answer 25

Rxns work best when LDA and aldehyde are added slowly

Question 26

Direct alkylation of enolates with LDA and SN2

Answer 26

1) LDA forms kinetic enolate | 2) SN2 of DB on an alkyl halide