8

PCE          p-
Cl\  /C
cl/CKCcl

acetylene
CI-C-C-CI

1,1-DCE
CI\ /H
CI/C=\H
trans-DCE
H,\ C=/01
CI/  H
cis-DCE <
H \ ./H
CI/= Ci
Chloro-
acetylene
Cl-C-C-H

I \  -./H
,I/C:=C\H

Ethylene
H x. ..H
AZC=UH

Acetylene
H -C C-H

Reductive Elimination
CI\  .=/H  u-eliminatior.
CI/U= H
H x  .H   P-eliminaiorL
ci/0=0Ci,

H -C-C- H
H -C-C- H

Dehydrohalogenation

H x C=./H
CI/ =0H

pHp-C-C- H

Hydrogenolysis
Cl\"     0CIl\  /H  H \0  CI0   H\   /H
ci/CI "CI  Cl/C=C\CI *CI / H     cI/C\H
Fig. 2.1.  Pathways of PCE Reduction by Zinc Metal to Ethylene (Roberts et
al. 1996, Drawing from Elder 2000).