A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

Aldehydes having no a-hydrogen atoms cannot form carbanions

and they therefore, undergo the Cannizzaro reaction (p. 167) with

concentrated alkali; but as this reaction is slow, such aldehydes are

often able to function as carbanion acceptors. Thus formaldehyde,

in excess, reacts with acetaldehyde:

CH 2 =0 CH,Oe H.O CH,OH

^CH,CHO CHaCHO CH 8 CHO

CH 2 OH CHjOH

I <i;eOH | (i)CH.O |

HOCHj—CCHO " ~ CHCHO ' 7 CHCHO

J (ii)CH.O I (ii)H.O e

CHjOH (,">H'° CHjOH

©OH /c'H.O

CH 2 OH •

HC0 2 e + HO CH 2 —C—CH 2 • OH

I

CH 2 OH (XXXIII)

In tile last stage (flt)CH 2 ) 3 CCHO, which can no longer form a
carbanion, undergoes a crossed Cannizzaro reaction with formal­
dehyde to yield pentaerythritol (XXXIH) and formate anion. The
reaction proceeds this way rather than to yield (HO'CH^aC-COg®
and CH3OH as the carbonyl carbon atom of formaldehyde is the
more positive of the two aldehydes so that it is attacked preferentially
by eOH, with resultant transfer of H© to (HO-CHaJaC-CHO rather
than the other way round.
A further useful synthetic reaction is the base-catalysed addition
of aliphatic nitro-compounds to carbonyl groups (see p. 175). Here
the aldehyde itself can also form a carbanion and aldol formation
could be a competing reaction, but the carbanion from the nitro­
compound tends to be the more stable (due to the more effective
delocalisation of its charge) and isrfhus formed more readily, resulting
in the preponderance of the above reaction.
The elimination of water from a hydroxy compound usually
requires acid-catalysis (p. 192) but the possibility of carbanion forma­
tion in the first formed aldol, coupled with the presence of a group