CYCLOHEXANE STEREOCHEMISTRY

BAEYER STRAIN THEORY AND STRAIN

Anand S. Burange, Wilson College, Mumbai

asgburange@gmail.com.

Small rings like cyclopropane and cyclobutane have larger values of heat of combustion per –CH₂- group compared to other larger rings. The answer to this observation lies in a theory proposed by Adolf Von Baeyer from University of Munich in 1885. His theory is also known as Baeyer’s Strain Theory but all the aspects discussed in theory are not completely accepted as some of them were proven wrong in the course of history. Now a days this theory, is modernized and then discussed for the better understanding of students/researchers.

Baeyer’s Strain Theory Assumptions:

Baeyer in his theory, he did few arguments where he failed to convince many aspects while some of them are well accepted. Few important arguments from his theory are as follows.

# Argument 1

In cyclopropane and cyclobutane, all carbon atoms are SP³ hybridized therefore expected bond angle (C-C-C bond angle) should be 109.5⁰ but observed bond angles are 60⁰ and 90⁰ respectively. Therefore in cyclopropane and cyclobutane, there is a deviation of 49.5⁰ (109.5⁰-60⁰) and 19.5⁰ (109.5⁰ -90⁰) respectively and thus the bonds are compressed i.e. strained. This deviation in bond angles causes strain in molecule. Such strain observed in small rings is known as small angle strain.

His argument regarding cyclopropane and cyclobutane, is well accepted and proved. Since cyclopropane has much deviation in bond angle, it is more strained than cyclobutane which can be clearly seen from heat of combustion data. High strain in cyclopropane is well evidenced by its reaction to give propene on pyrolysis at 450-500⁰C and also bromination reaction to yield 1,3 –dibromopropane.

# Argument 2

According to Baeyer, cyclohexane (hexagon) molecule should be more strained compared to other small rings.

But cyclohexane exists in a chair form with all staggered bonds and has bond angles 109.5⁰. Therefore cyclohexane has no strain and his argument regarding hexagon fails.

# Argument 3

He proposed that, larger rings like cycloheptane, cyclooctane. etc. will have more angle deviation and thus more strain.

But larger rings have very little angle deviation and thus lesser strain compared to small rings.

Heat of combustion data

Ring size (n)	Heat of combustion per          –CH2- (Kcal/mol)
3	166.6
4	164.0
5	158.7
6	157.4
7	158.3

More the heat of combustion more is the strain.

In cyclohexane and larger cycloalkanes  with  n≥14 are strain free having heat of combustion 157.4 Kcal/mol

Strain(s)

In cyclopropane, along with angle strain, all C-H bonds are eclipsed. The strain created because of eclipsing of the bonds is called as Pitzer strain. Cyclobutane exists in puckered wing conformation while cyclopentane (very less angle strain) exists as open enevelope conformation. Both of them along with angle strain have Pitzer strain in them.

When ring size is in between 7-13:

 If during rotation of bonds, when all bonds become gauche, then there exists an interaction between C-H bonds across the rings. The strain developed due to this interaction is known as transannular strain.

Along with it, there are few other strains like back strain, face strain, etc not discussed here because of limitations.

References: Morrison and Boyd, J. March, J. Smith and Solomon and Fryhle.