A Guidebook to Mechanism in Organic Chemistry

The Beckmann Rearrangement

O

R-C-NHOH <-

(XXV)

eOH

\

O

II •

R_C—OEt

O

O

• R-C-NHNH,

NaNO^

o

HQ

(XXIV)

0=C=N—R

derived from amyl nitrite and hydrogen chloride), the urethane is
obtained:

R—C—Ng

O

II- ..

R—C—N

ROH

R—N=C=0 > R—NHCOjR'

In all these cases, the R group that migrates conserves its configur­

ation as in the carbon -^carbon rearrangements already discussed and,

as with them, no mixed products are formed wfcen two different, but

very similar, compounds are rearranged in the*ame solution, sjn»wing

that the R groups never became free in the solution when migrating,

i.e. these too are vrt/YzmolecuIar rearrangements.

(ii) The Beckmann rearrangement

The most famous of the rearrangements in which R migrates from

carbon to nitrogen is undoubtedly the conversion of ketoximes to

N-substituted amides, the Beckmann transformation:

RR C=N OH — R'CONHR or RCONHR'

The reaction is catalysed by a wide variety of acidic reagents, e.g.

H 2 S0 4 , P 206 , SOa, SOCl 2 , BF 3 , PC1 6 , etc., and takes place not only

with the oximes themselves but also with their O-esters. Only a very

few aldoximes rearrange under these conditions but more can be

made to do so#by use of polyphosphoric acid as a catalyst. The most

interesting feature of the change is, that unlike the reactions we have

already considered,it is not the nature, e.g. relative electron-releasing

ability, but the stereochemical arrangement of the R, R' groups that