beckmann
TRANSCRIPT
Beckman rearrangementBeckman
rearrangement
Oximes
compounds, containing the grouping C = N-OH, derived from aldehydes and ketones by condensing them with hydroxylamine
R2CO NH2OH R2C=NOHH
+
+
Types of Oximes
Oximes are of two types:Aldoxime: combination of aldehydes
with hydroxylamine.Ketoxime: Combination of ketones with
hydroxylamine.
R2CO
NH2OH
R2C=NOH
H+RCHO RHC=NOH
+
Chemical Properties
The ketoximes by the action of acids undergo a peculiar intramolecular re-arrangement known as the BECKMAN REARRANGEMENT yielding an acid-amide or anilide as a final product.
Named: German Chemist Ernst Beckmann
Isomers
If we use aldehydes or asymmetrical ketones, there is the possibility of getting TWO STEREOISOMERS of the oximes.
R R'
NOH
R R'
NOH
Possible Isomers
2, p-bromophenyl ethyloxime is an asymmetric oxime, hence two possible products.
CH3
NOH
Br
CH3
N
Br
OH
Beckman Rearrangement
It is an acid-catalyzed rearrangement of an oxime to an N substituted amide
R1R2C == N - OH
R1- CO - NR2
R2- CO - NR1
ORH
H
Beckman Rearrangement
Open Chain oxime gives an open chain amide
Cyclic oximes give lactam
Beckman Rearrangement
The Beckmann solution, acetic acid, hydrochloric acid and acetic anhydride, is widely used to catalyze the rearrangement
Other acids, such as polyphosphoric acid, sulfuric acid or phosphorous pentachloride can also be used
Beckman Rearrangement
The reaction mechanism of the Beckmann rearrangement is generally believed to consist of:
1. An alkyl migration (trans to –OH) with expulsion of the hydroxyl group to form a nitrilium ion
2. Followed by hydrolysis
Beckman Rearrangement
R1 R2
C
N
OH
OH R2
C
N
R1
BeckmanRearrangementMechanismIn the presence of acid chlorides/acid halides
R1 R2
C
N
OH
R1 R2
C
N
OCl
RCOCl
R1 R2
C
N +
-OCl-
R2
C
N
R1
+
R2
C
N
R1
OH2
+ R2
C
N
R1
OH
-H+
H2O
R2CO N H R1
BeckmanRearrangementMechanismIn the presence of strong acids
R1 R2
C
N
OH
R1 R2
C
N
OH2+
H+
R1 R2
C
N +
-H2OR2
C
N
R1
+
R2
C
N
R1
OH2
+ R2
C
N
R1
OH
-H+
H2O
R2CO N H R1
Cyclic oxime
Application
Amides can also be prepared from the Beckmann Rearrangement
Because it is an acid catalyzed rearrangement of an oxime to an amide