Advices For Studying Organic Chemistry

occur perfectly end-on. This leads to weaker “bent” bonds and to

angle strain. (b) Bond distances and angles in cyclopropane. (c) A

Newman projection formula as viewed along one carbon-carbon bond

shows the eclipsed hydrogens (Viewing along either of the other two

bonds would show the same pictures.)

2. Angle strain: the potential energy rise resulted from compression of the internal
   angle of a cycloalkane from normal sp^3 -hybridized carbon angle.

1. The sp^3 orbitals of the carbon atoms cannot overlap as effectively as they do in
   alkane (where perfect end-on overlap is possible).

HH

H H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

88 o

(a) (b)
Figure 4.11 (a) The “folded” or “bent” conformation of cyclobutane. (b) The
“bent” or “envelop” form of cyclopentane. In this structure the front
carbon atom is bent upward. In actuality, the molecule is flexible and
shifts conformations constantly

2) The C—C bonds of cyclopropane are “bent” ⇒ orbital overlap is less

effectively (the orbitals used for these bonds are not purely sp^3 , they contain

more p character) ⇒ the C—C bonds of cyclopropane are weaker ⇒

cyclopropane has greater potential energy.

3) The hydrogen atoms of the cyclopropane ring are all eclipsed ⇒ cyclopropane

has torsional strain.

4) The internal angles of cyclobutane are 88° ⇒ considerably angle strain.

5) The cyclobutane ring is not plannar but is slightly “folded” ⇒ considerably

larger torsional strain can be relieved by sacrificing a little bit of angle strain.

4.11A CYCLOPENTANE