A Guidebook to Mechanism in Organic Chemistry

Investigation of Reaction Mechanisms

In some cases, for example the Hofmann reaction mentioned
above, intermediates can actually be isolated, but where this is not
possible, various potential intermediates can be introduced into the
reaction to see whether any of them will speed it up, as a true inter­
mediate in the process should. Alternatively, another species of mole­
cule may be introduced into the system in an endeavour to fix or trap
a transient intermediate. An example of this is provided in reactions
proceeding via radical intermediates where, by introducing an olefine
into the system, the radical intermediate induces polymerisation of
the olefine ajjd is itself therebyJixed at the end of the polymer chain
(c/.p.247): • W
nCH,= CH,
Ra- + CH,=CH 2 -»• Ra—CH,—CH 2 - Polymer

The use of isotopes has also shed»a good deal of light on a number
of difficult mechanistic problems. Thus the aqueous hydrolysis of
esters could proceed,*in theory, by cleavage at (a)—alkyl/oxygen
Ravage, or (b)—acyl/oxygen cleavage:

ii

"'R—C—OH + HO—R

ii!. 1 ' 18

R—C-^-O-J-R' H 20

bI ft \

ifcK o

\ II 18

R—C—OH + H—OR'

If the reaction is carried out in water enriched in the isotope lsO,

(a) will lead to an alcohol which is lsO enriched and an acid which is

not, while (b) will lead to an lsO enriched acid but a normal alcohol.

Most esters are found to yield an lsO enriched acid indicating that

hydrolysis, under these conditions, proceeds via acyl/oxygen cleavage.

It should, of course, be emphasised that these results only have

validity provided neither acid nor alcohol can, after formation, itself

exchange oxygen with water enriched in^18 0, as has in fact been

shown to be the case. Heavy water, D 2 0, has been much used in a

rather similar way, especially to investigate whether particular hydro-.

gen atoms participate in a given reaction. Thus in the Cannizzaro

reaction with benzaldehyde (p. 167) \

(a)

~ _ / • _ m v«

(^35) '