ABSTRACT
CHBrCD
.
This confirmed that the carbon to hydrogen bond was broken in the rate limiting step of the
mechanism. In the concerted elimination mechanism there should be both a hydrogen and leaving
group isotope effect. Since the measurement of heavy atom isotope effects requires special mass
spectroscopy equipment, Bunnett
suggested an “element effect” as a substitute for the heavy atom
isotope effect as a criterion of mechanism for aromatic nucleophilic substitution reactions. Bartsch
and Bunnett
applied this concept to the methoxide-promoted dehydrohalogenations of
CH
CHBrCH
CH
CH
CH
and CH
CHClCH
CH
CH
CH
. The element effect, k
/k
, for
the E2 eliminations were 38 for the formation of 1-hexene, 51 for the formation of trans-2 hexene
and 46 for the formation of cis-2-hexene. Since then it was accepted that a combination of a
hydrogen PKIE and a significant k
/k
was experimental evidence that an alkoxide-promoted
dehydrohalogenation occurred by the concerted E2 mechanism. The stereochemistry of the E2
concerted mechanism should also be anti to allow a proper alignment of the orbitals.
