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
More the heat of combustion more is the strain.
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
|
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.
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