McMurry (kap.17,1) - Keto-Enol Tautomerism

Question 1

A carbonyl compound (keton, carboxyl, esters, amides, etc.) with a hydrogen atom on its (a-carbon) is in equilibrium with its corresponding enol isomer.

What is this spontaneous interconversion between the two isomers called?

Answer 1

tautomerism

The individual keto and enol isomer are called tautomers.

Question 2

are tautomers and ressonance forms the same?

Answer 2

No, ressonance only differs in the position of their pi electrons and lone pairs, whereas…

Tautomers have their atoms arranged differently.

Question 3

Does the enol tautomer or the keto tautomer dominate in the equilibrium?

Answer 3

Most monocarbonylic compounds exist dominatly in their keto tautomer form, and therefore the enol form is difficult to isolate.

Question 4

What helps to make the enol form dominate?

Answer 4

1) can be stabilized by conjugation

2) can be stabilized by inermolecular hydrogen bond formation

(see page 601 for the pentane-2,4-dione example) from ketone to enol form.

Question 5

Keto-Enol tautomerism of carbonyl compounds is catalyzed by both bases and acids.

Describe the acid catalyzed reaction, and the base catalyzed reaction and the yield of these two.

(see page 602)

Answer 5

For both of the reactions, the yield is the enol form.

Acid catalyzed enol formation:

1) the carbonyl oxygen is protonated by an acid, giving a cation with 2 ressonance structures.

2) Loss of H+ from the a position by reaction with a base gives the enol tautomer.

Basic catalyzed enol formation:

1) Base removes the acidic a hydrogen yielding an enolate ion with two ressonance structures.

2) Protonation of the enolate ion on oxygen gives the enol form.

(see page 602)

Question 6

Are only the hydrogen on the a position of carbonyl compounds acidic, or the others also? (B, y, sigma ets.)

Answer 6

ONLY the a-position.