A Guidebook to Mechanism in Organic Chemistry

Syi Mechanism

higher proportion of inversion. A good solvating solvent such as water
is particularly effective in this respect because of the rapidity with
which the incipient carbonium ion collects a solvating envelope around
itself.

(iv) SNi mechanism

Despite what has been said above of replacement reactions leading to

inversion of configuration or racemisation or, in some cases, a mixture

of both, a few cases are known of reactions that proceed with retention

of configuration, i.e. in whicbAhe starting material and product have

the same configuration. One case in which this has been shown to

occur is in the replacement of OH by CI by the use of thionyl chloride.

This reaction has been shown to follow second order kinetics, i.e.

rate «[R - OH] [SOCl 2 ], but it clearly cannot proceed according to an

unmodified SN 2 mechanism for this would lead to inversion of con­

figuration which is not observed. It has been interpreted mechanis­

tically as follows:

R R O R o

No change in configuration can take place in stage (i) as the C—O

bond is not broken and in the second stage, where this bond is broken,

attack by CI takes place from the same side of the carbon atom because

of the orientation of the intermediate (VIII). A close study of the

reaction has suggested that the second stage exhibits some SN 1

character in that the breakdown of (VIII) probably proceeds through

an ion pair (cf. p. 80):

The chlorosulphite anion then breaks down to S0 2 and CI^9 so rapidly

that Cl° is available for frontal attack on the carbonium ion before the

R'^C—OH+SOCl 8 S=0 RVC S=0

* / \

• R" CI..

s=o