A Guidebook to Mechanism in Organic Chemistry

Investigation of Reaction Mechanisms

readily with the trans member of a pair of cis/trans isomerides, as
seen in the conversion of syn and anti aldoxime acetates to nitriles:

Ph H Ph Ph H

\ / I \ /

C ©OH C eoH C

II —* III < II

N. ""'y N '«?, .N

y • readily • ^

MeCOO OOCMe

Anti Syn

This again s«ts limitations to y^uch any mechanism advanced for the

reaction will have to conform.

A study of the fate of asymmetric centres in optically active com­

pounds has also been of great value, particularly in the case of

nucleophilic substitution at a saturated carbon atom, for example, in

the hydrolysis of alkyl halides (p. 66). Thus reaction via the type of

transition state that wfe have already mentioned (XLVI, p. 31), will

lead to a product having the opposite configuration to the starting

material as the asymmetric centre will have been 'turned inside out':

HOs + R'»-C—I

/

R*

ir r

«- > «-

HO C 1

I

R*

HO—C-R +IQ

R*

This establishes a test iy which the occurrence of this type of

mechanism may be detected (p. 66).

The degree of success with which a suggested mechanism can be

said to describe the course of a particular reaction is not determined

solely by its ability to account for the known facts; the acid test is

how successful it is at forecasting a change of rate, or even in the

nature of the products formed, when the conditions under which the

reaction is carried out or the structure of the substrate are changed.

Some of the suggested mechanisms we shall encounter measure up to

these criteria better than do others, but the overall success of a

mechanistic approach to organic reactions is demonstrated by the way

in which the application of a few relatively simple guiding principles

can bring light and order to bear on a vast mass of disparate inform­

ation about equilibria, reaction rates and the relative reactivity of

organic compounds. We shall now go on to consider some simple

examples of this.

C 37