A Guidebook to Mechanism in Organic Chemistry

Elimination Reactions

Apart from any preparative or diagnostic value the reaction may

have, it is of course the above state of affairs that makes impossible

the preparation of Grignard and similar organo-metallic compounds

from 1,2-dihalides. Similar eliminations can be made to proceed with

1,2-halo-esters and 1,2-halo-ethers. The reactions normally proceed

stereospecifically trans.

A similar stereospecificity has been observed in the preparatively

more useful denomination of 1,2-dibromides by iodide ion. The

kinetic law followed is of the form:

but there is reason to believe that the mechanism, in some cases at

least, is a little more complicated than that with zinc, involving

an SJV2 displacement to form the bromo-iodide as the rate-deter­

mining step

followed by attack by iodide ion on the iodine atom that has been

introduced: I

Practical use is made of this reaction in the purification of olefines.

The usually crystalline dibromides are purified by recrystallisation

and the pure olefine then regenerated, as above, under extremely

mild conditions.

A small number of cases are known of the elimination of hydrogen
halide where both atoms are lost from the same carbon: these are
known as a-elimination reactions. The best known example occurs in
the hydrolysis of chloroform with strong base:

Rate oc [I©] [1,2-dibromide]

I© + BrCH, • CHaBr -> Br© + ICH,CH,Br

a-ELIMINATION

CI

HO ©^ H-^C^-Cl H 2 0*heCCI 3 > C1© + :CC1 2

slow

\

CI • fast H.O

HC0 2 ©

slow

©OH CO