A Guidebook to Mechanism in Organic Chemistry

Carbonium Ions, Electron-deficient N and O Atoms

Me

I

Me—C—CMe 2

I I

OH OH

H-Hy

Me

I -HaO

Me—C—CMe 2 ~ " Me

I U

OH OH

H

'Me

1

CMe,

OH

.(XIII)

Me—C—CMe 3

II

O

Me—C—CMe 3

o*lh

It might be expected that an analogous reaction would take place

with any other compounds that could yield the crucial carbonium

ion (XIII) and this is, in fact, found to be the case; thus the corres­

ponding bromohydrin (XIV) and hycVoxyamine (XV) yield pinaco­

lone when reacted wijh Ag® and NaN0 2 /HCI, respectively:

Me

I

Me—C—CMe 2

I I

OH Br

(XIV)

Me

Ag© I

> Me—C—CMe 2

e.—AgBr I ©

OH

Me

(XIII)

-N,

Me

Me—C—CMe 2 NaN°'> Me—C—CMe 2

II

/HC

' Id

OH NH 2 OHNsN

(XV) ®

It seems likely that the migration of the alkyl group follows

extremely rapidly on the loss of HaO, Br® or N 2 , or probably takes

place simultaneously, for in a compound in which the carbonium

ion carbon is asymmetric, the ion does not get Time to become

planar and so yield a racemic product, for the product obtained is

found to have undergone inversion of configuration

(c) The pinacol/pinacolone rearrangement: Another case of migra­
tion of an alkyl group to a carbonium ion carbon atom occurs in the
acid-catalysed rearrangement of pinacol (cf. p. 168) to pinacolone,
Me 2 C(OH) C(OH) • Me 2 -Me • CO • CMe 3 :