A Guidebook to Mechanism in Organic Chemistry

ELIMINATION REACTIONS

ELIMINATION reactions are those in which two groups are removed
from a moWBle without being replaced by other groups. In the great
majority of such reactions the groups are lost from adjacent carbon
atoms, one of the groups eliminated commonly being a proton and
the other a nucleophile, Y: or Y@, resulting in the formation of a
multiple bond:
H
"| |« -HY \ / \ / -HY
H—C—C—Y > >C=C< C=C > -C=C-
II

X X
/ \
Y
Among the most familiar examples are the base-induced elimination
of hydrogen halide from alkyl halides ^
®OH •
RCH,CH,Hal —• RCH=CH,+H,0 + Hale
the acid-catalysed dehydration of alcohols

R-CHi-CHi-OH RCH=CH,+HsOe

and the Hofmann degradation of quaternary alkylammonium
hydroxides:
OH
R-CHj-CHj-NR, —> RCH=CH,+H.O+NR,

^-ELIMINATION
The carbon atom from which Y is removed is generally referred to as
the cc-carbon and that losing a proton as the jS-carbon, the overall
process being designated as a ^-elimination, though as this type of
elimination reaction is by far the most common the j3- is often omitted.
Some estimate of the driving force behind such elimination reactions
may be gained from calculations of the energy released in forming the
multiple bond. Thus ^C—C^ -*• ^>C=C<^ releases approximately