Chemistry Reference
In-Depth Information
chance to form. Greater than five-membered rings are not planar and six- and seven-membered rings
can also form freely, though not quite as easily as five-membered ones. Six-membered rings are more
favored than seven-membered ones. Rings with eight to twelve members are relatively strain-free
from bond angle distortion, but they are thermodynamically unstable. This is because substituents
(hydrogens or others) are forced into positions of repulsion due to crowding. Also, there is little ring
formation with eight to twelve members.
Whether cyclization will take place or not during polymerization also depends on the kinetic
feasibility to cyclize. This feasibility is a function of the probability that functional end groups on a
molecule will approach each other. As the size of the monomer increases, so does the size of the
potential ring. An increase in the size of the monomer, however, also means an increase in the number
of different configurations that the monomer molecule can assume. Very few of these configurations
are such that the two ends become adjacent [
4
]. With fewer chances of the end groups encountering
each other, there is decreased probability of ring formation. From practical considerations, ring
formations are mainly a problem when five-, six-, or seven-membered rings can form. Formation
of large rings with more than twelve members is seldom encountered [
5
].
It was reported that at least in one instance, the products of a reaction can be affected by the isomer
used [
244
]. Thus, in the following Knoevenagel condensation, a polymer will form when an
ortho
isomer is being reacted:
NN
H
5
C
2
C
2
H
5
H
5
C
2
N
N
C
2
H
5
CHO
CHO
NC
CH
HC
CN
+
CN
n
NC
O
O
NH
NH
NH
NH
O
O
By contrast, the
meta
isomer forms only ring structures instead [
242
]:
N
C
2
H
5
H
5
C
2
N
H
5
C
2
N
N
C
2
H
5
CHO
CHO
+
CN
NC
CN
NC
O
O
NH
NH
NH
NH
O
O
