A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

charge. That the migrating hydride ion is transferred directly from

one molecule of aldehyde to another and does not actually become

free in the solution is shown by carrying out the reaction in D 2 0,

when no deuterium becomes attached to carbon in the alcohol as it

would have done if the migrating hydride ion had become free and so

able to equilibrate with the solvent. In some cases, e.g: with formal­

dehyde in very high concentrations of alkali, a fourth-order reaction

takes place: rate « [CH 2 O]^2 [^0 OH]^2. This is believed to involve

formation of a doubly charged anion and transfer of hydride ion by

this to a second molecule of aldehyde to yield carWxylate and

alkoxide ions:

OH OS, H

©OH I ©OH K. | #*V

H-C-H ==^ H-C-H H-CQ^C=Lo

J52 O© O© H

O H

II I

H—C + H—C—O©

^* O©. H

Intramolecular Cannizzaro reactions are also known, e.g. gly-

oxal—hydroxyacetate (glycollate) anion:

H O©

I I

-» HO—C—C=0

H

As expected

rate * [OHCCHO] [eOH]

and no deuterium attached to carbon is incorporated in the glycollate
produced on carrying out the reaction in D 2 0.

(vii) Reactions with metals *

(a) Magnesium or sodium and ketones: Magnesiwm, usually in the
form of an amalgam to increase its reactivity, will donate one electron
each to two molecules of a ketone to yield a bimolecular product
(XIII). This contains two unpaired electrons which can then unite to