A Guidebook to Mechanism in Organic Chemistry

Elimination Reactions

The kinetic law followed is of the form

Rate <x [ester]

and the reaction is believed to proceed through a cyclic transition

state (XXXVII):

(XXXVII)

The strongest evidence in favour of such a route is that these

eliminations are found to proceed stereospecifically cis in contrast

to the almost universal trans eliminations that we have encountered

to date. It is thus probable, in most cases, that the breakage of the

C—H and C—O bonds occur simultaneously, but the fact that small,

though detectable, amounts of trans elimination have been observed

to take place suggests that in some cases the latter breaks before the

former, leading to *he transient formation of an ion pair, which can

then»undergo mutual reorientation. It seems unlikely in such cases

that radicals, rather than ion pairs, are ever formed as the reactions

appear to be unaffected by either radical initiators or inhibitors (cf.

p. 231).

A very closely analogous elimination is the Chugaev reaction which

involves the indirect conversion of an alcohol to the corresponding

olefine via pyrolysis of the methyl xanthate (XXXVIII) obtained by

the action of CS^OH/Mel on the original alcohol:

\/\/ X/V

C-r-C C=C

O J H O H

^ \ /

C=t=S C—S -* MeSH + COS

MeS^ MeS^*

(XXXVIII).

This reaction has the greater preparative value as the pyrolysis
may be successfully carried out at lowef temperatures (ca. 150° as