Chemistry Reference
In-Depth Information
bonds involved. Polar effects are not involved in these reactions. Electrophilic
chlorine atoms exhibit the reverse selectivity, however. This can be attributed to
the repulsion between the electron-seeking chlorine and carbon atom 2, which is
rendered relatively electron deficient by the carbonyl group in the aliphatic acid.
Similarly, electrophilic CH
3
radicals add faster to ethylene than to tetrafluor-
ethylene, in which the inductive effects of the fluorine substituents make the dou-
ble bond somewhat electron deficient. The reverse selectivity is shown by CH
3
radicals, which are nucleophilic in character
[24]
. This parallels the observations
in organic chemistry that carbonium ion formation is facilitated by replacement of
hydrogens by alkyl groups and hindered by CF
3
groups. There is a polarization of
σ
electrons toward the trifluoromethyl group, whereas these electrons can be
more effectively released by alkyl substituents.
Studies primarily of effects of substituents on reactions on phenyl rings permit
the assignment of a nucleophilic (electron-releasing) character to various groups.
These include alkyls, vinyl, hydroxyl, ether, phenyl, and
O
C
R
O
species. Electrophilic (electron-withdrawing) substituents include halogen, nitro,
cyanide, carboxyl, and carbonyl groups. Thus the CH
2
site in styrene is relatively
electron rich while that in acrylonitrile has the opposite character, and the radicals
derived from these monomers (9-2 and 9-3, respectively) also share the same ten-
dency. Note that the isomers vinyl acetate (9-4) and methyl acrylate (9-5) differ
in their selectivity. The double bond in vinyl acetate is weakly
H
H
CH
2
C
CH
2
C
CN
9-2
9-3
H
H
CH
2
CO
C
CH
3
CH
2
CC
OCH
3
O
O
9-4
9-5
electron rich whereas that in methyl acrylate is electron poor, because it is conju-
gated with a carbonyl group on which a slight excess charge can be stabilized.
Monomers with opposite polarities will tend to copolymerize in alternating
fashion. Thus, styrene and acrylonitrile copolymerize normally with an
r
1
r
2
