A Guidebook to Mechanism in Organic Chemistry

Aldol Reactions

(XXX) which can attack the carbonyl carbon atom of a second mole­
cule of aldehyde to yield, ultimately, the aldol (XXXI):

CH 8 —C=0

H

(0 ©OH

<*-o

Me-*-C*TC

H

CH 2 —O O "

I

H

t

CH 2 =C—Oe

I

H

(ii)

e

o

Me—C—CH 2 — C=0

H H

MeCH(OH)CH 2 CHO

(XXX) (XXXI)

The forward reaction (ii) and the reversal of (i) are essentially in com­

petition with each other but, as carrying out the reaction in D 2 0 fails

to result in the incorporation of any deuterium in the methyl group,

(ii) must be so much more rapid than the reversal of (i) as to make

the latter virtually irreversible. The corresponding^reaction of acetone

to diacetone alcohol (XXXII) proceeds much more slowly and.^hen

carried out in D 2 0, deuterium is incorporated into the methyl group;

this is the result of a less rapid attack of the carbanion on a carbonyl

carbon atom which is markedly less positive than that in an aldehyde:

CHs—C=0

I

Me

(i) ©OH

Me

Me

/pcHj—C=0 I

I

Me

t

CH,=C—O©

I

Me

(ii> Me— C— CH 2 —C=0

— I I

Me Me

H.O

Me 2 C(OH)CH 8 COMe

(XXXII)