Page 106 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

chloride toward solvolysis in ethanol, 118 owing to the fact that the cationic intermediate 85
is stabilized even more, 57.6 kcal/mol, according to the computational results on p. 22.
TOPIC 1.3
Bonding in
CH Cl CH 2 Cyclopropane and Other
2
:O +O Small Ring Compounds
CH 3 CH 3
stabilized by stabilized by
about 15 kcal about 58 kcal

As indicated at the beginning of this section, heteroatom hyperconjugation was
first recognized in carbohydrates. The anomeric effect has been particularly well
studied in cyclic systems, such as found in carbohydrates. We return to the anomeric
effect in cyclic systems in Topic 2.3.

Topic 1.3. Bonding in Cyclopropane and Other Small Ring
Compounds

Molecules such as cyclopropane that are forced by geometry to have nonideal
bond angles are said to be strained. This means that the bonds are not as strong as those
in comparable molecules having ideal bond angles and results in both lower thermo-
dynamic stability and increased reactivity. The increased reactivity has at least two
components. (1) Typically, reactions lead to a less strained product and partial relief of
strain lowers the energy barrier. (2) Strained molecules require orbital rehybridization,
which results in electrons being in higher energy orbitals, so they are more reactive. 119
There have been many experimental and computational studies aimed at understanding
how strain affects structure and reactivity.
The simplest VB description of cyclopropane is in terms of bent bonds. If the
3
carbons are considered to be sp hybrid, in accordance with their tetravalent character,
the bonding orbitals are not directed along the internuclear axis. The overlap is poorer
and the bonds are “bent” and weaker.

C
C C

The overlap can be improved somewhat by adjustment of hybridization. Increased
p character in the C – C bonds reduces the interorbital angle and improves overlap,
which means that the C–H bonds must have increased s character. Compared to
ethane or propane, cyclopropane has slightly shorter C – C and C–H distances and an
open CH bond angle, 120 which is consistent with rehybridization. The C–H bonds in
2
cyclopropane are significantly stronger than those in unstrained hydrocarbons, owing
118
P. Ballinger, P. B. de la Mare, G. Kohnstam, and B. M. Prestt, J. Chem. Soc., 3641 (1955).
119 A. Sella, H. Basch, and S. Hoz, J. Am. Chem. Soc., 118, 416 (1996).
120
J. Gauss, D. Cremer, and J. F. Stanton, J. Phys. Chem. A, 104, 1319 (2000).