A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

the question arises of whether the second hydrogen atom that be­

comes attached to carbon in the benzyl alcohol formed comes from

the solvent (water) or from a second molecule of benzaldehyde.

Carrying out the reaction in D 2 0 leads to the formation of no

PhCHDOH, indicating that the second hydrogen could not have

come from the solvent and must, therefore, arise by direct transfer

from a second molecule of aldehyde.

Compounds suitably 'labelled' with deuterium, or the radioactive

hydrogen isotope tritium, have also been used to decide whether a

particular C—H bond is broken during the rate-determining stage of

a reaction. Thus in the nitration of nitrobenzene (p. 105)

a C—H bond is broken and a C—NOa bond formed, and the
question arises whether either or both processes are involved in the
rate-determining step of the reaction. Repeating the nitration on
deuterium- and tritium-labelled nitrobenzene shows that there is no
detestable difference in the rate at which the three compounds react,
thus indicating that C—H bond fission cannot be involved in the
rate-determining step as it can be shown tfeat if it were there would
be a considerable slowing in reaction rate, on going C—H -»• C—D -*•
C—^3 H (kinetic isotope effect).
Another technique that has been of the utmost value is observing
the stereochemical course followed by a number of chemical reac­
tions. Thus the addition of bromine, and a number of other reagents,
to suitable defines has been found to yield trans products

indicating that the bromine cannot add on directly as Br—Br for-this
would clearly lead to a cis product. The prevalence of trans addition
reactions also provides further information about the mechanism of
the reactions (p. 139). Many elimination reactions also take place more

Br