A Guidebook to Mechanism in Organic Chemistry

Hydration

Thus it has been shown that the percentage hydration at 20° of
formaldehyde, acetaldehyde and acetone is 99-99, 58. and «0 per
cent, respectively. The latter is confirmed by the fact that if acetone
is dissolved in H 218 0, when the following equilibrium could, theoreti­
cally, be set up,
OH
Me,C==0+H,0 ^ Me 2 C ^ MesC=0+HjO
\
"OH

no^18 0 is incorporated into the acetone. In the presence of a trace of

acid or base, however, while no equilibrium concentration of the

hydrate can be detected, the incorporation of lsO occurs too rapidly

to measure, indicating that a hydrate must now, transiently, be

formed. The acid or base catalysis is presumably proceeding:

«+

Me,C==0-

HA

MesC=0

8-

HA

©OH

H.O

Me 2 C

O©

\

OH

/

OH

Me.C

OH

H

-H®

OH

H.O

Me 2 C

\

OH

The acid catalysis exhibited in this case is general acid catalysis, thatls

to say the hydration is catalysed by any acid species present in the

aqueous solution and not solely by H 3 Oe as is so often the case.

The fact that such catalysis is necessary with acetone, but not with

the aldehydes, reflects the less positive nature of the carbonyl carbon

atom of the ketone, which necessitates initial attack by^3 OH (or by

He on oxygen), whereas with the aldehydes HgO: will attack the

more positive carbon atom directly?

The presence«of electron-withdrawing substituents in the alkyl

groups makes hydration easier and stabilises the hydrate once formed;

thus glyoxal (I), chloral (II) and triketohydrindene (III) all form

isolable, crystalline hydrates: