Skip to main content
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.

Comments

Post a Comment

Popular posts from this blog

CYCLOHEXANE STEREOCHEMISTRY

CYCLOHEXANE STEREOCHEMISTRY Anand S. Burange, Wilson College, Mumbai asgburange@gmail.com Cyclohexane is an industrially important molecule mainly used for the production of epsilon caprolactum and adipic acid which are precursors to Nylon. Cyclohexane has a molecular formula C 6 H 12 and has a cyclic structure. At room temperature it exists as a liquid with boiling point 80.74 o C. Generally, representation of cyclohexane in a line structure form is shown as a hexagon but practically it is not the case. Cyclohexane has six C-C bonds and due to cyclic structure there is a restricted bond rotation. Rotations of bonds lead to the various conformations. The most important conformations of cyclohexane are enlisted below. Conformations of Cyclohexane 1. Boat 2. Chair 3. Twist boat 4. Twist Chair Boat Conformation In a boat conformation, four carbon atoms lie in a plane while remaining two carbons atoms (C1 and C4) are out of plane (above or below) but ...
Post a Comment
Read more

LOSCHMIDT'S BENZENE

While taking group photographs in conferences, seminars, workshops, etc.   Indian chemists and researchers prefer to say ‘benzene’....instead old one ‘cheese’ to make happy faces. But what do we know about benzene? As a chemist do we really know the struggle faced by the researchers to derive its structure? Who discovered benzene? Who synthesized benzene for the first time?   I hope this blog will be quite useful for those who are willing to know the answers of the above mentioned questions. Benzene What we know today about benzene? Benzene is a colourless organic flammable liquid with characteristic odour which boils and freezes at 80.1 and 5.5 °C respectively. It is immiscible with water and has low density than it (water). The molecular formula of benzene is C 6 H 6 where all the C-C and C-H bonds are of equal length. Benzene is a planar aromatic molecule where all carbon atoms are SP 2 hybridized. The aromatic nature can be evidenced from its NMR spectrum. I...
6 comments
Read more