Level 1 – Case 12
Elimination Reactions of
Benzaldehyde O-Benzoyloximes
Key point: Hammett constants. Isotope effects
X
O C
N
(D) H
O
C
Y Y Y
DBU
MeCN
C N
(D) H
O C
O
Y Y Y
X
X
CN
Y Y Y
COO
Z-isomer Z Z
E-isomer
Elimination Reactions of Benzaldehyde O-Benzoyloximes
E- and Z-Benzaldehyde Z Z
O-benzoyloximes 1 and 2, give benzonitriles in quantitative yield upon treatment with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) (Scheme 12.1).
Discuss the mechanism of the elimination reaction and justify the differences in
reaction rates, k H
k /k
H H D
k and Hammett U values found for Z- and E-isomers. Interpret
the experimental data and propose a reasonable structure for the transition state
in both cases.
Experimental Data
1. A complete kinetic study has been carried out to determine the mechanism of
r r
the elimination reaction for both isomers and the results obtained are compiled
in Table 12.1.
2. The elimination reactions exhibit general-base catalysis in both cases.