A Guidebook to Mechanism in Organic Chemistry

The Beckmann Rearrangement

has been shown by rearrangement of benzophenone oxime to benzan-
ilideinH 218 0:
Ph Ph HO Ph O Ph
\ / \ / \ /
c -* c ^ c
II II I
.N .N HN

• \ * \ \
  OH Ph Ph
  Provided that neither the initial oxime nor the anilide produced
  will exchange their oxygen for^18 0 when dissolved in H 218 0 (as has
  been confirmed), a mere intramolecular exchange of Ph and OH
  cannot result in the incorporation of any lsO in the rearranged pro­
  duct. In fact, however, the benzanilide is found to contain the same
  proportion of mO as did the original water so that the rearrangement
  must involve loss of the OH group and the subsequent replacement
  of oxygen by reaction with water.
  The rearrangement is believed to take place as follows:
  R R
  \ R COCI _\
  C=N > C=N (XXX)
  / V

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£

a

/ \ «

R' OH e,c- R' OX • OX©

(T\ R

H,OL V ©J © /

>N-yC=*C > C=N

R

\

C=N

/ \m • / " /

R' OH R' R'

H (XXIX)

H

O HO R HO® R

|| \ / -H9 \ /

R'—C—NHR <— C=N <— C=N

/ /

R' R'

In fairly strong acid, the rearrangement proceeds by protonation

of the oxime, followed by loss of water to yield the species (XXIX)

having an electron-deficient nitrogen atom; while with acid chlor­

ides, etc., the ester (XXX) is obtained which loses an anion, eOX, to

yield the same intermediate. Support for the latter interpretation is

provided by the fact that such O-esters may be prepared separately

and shown to undergo the subsequent rearrangement in neutral