Question

According to Baeyer’s strain theory, which of the following are the most stable cyclic compounds?
A ) Cyclopropane
B ) Cyclobutane
C ) Cyclopentane
D ) Cyclohexane

Answer 1

Hint: According to Baeyer's strain theory, the stability of cycloalkanes is dependent on the variation between the bond angles present in the ring system and the normal tetrahedral bond angle. Greater is this difference, greater is the strain on the ring system and lesser is the stability of the ring. To find out the most stable cyclic compound, determine the angle strain for each cycloalkane.

Complete answer:
Baeyer’s strain theory was proposed by the German chemist Dr. Bayer for the determination of stability order in rings.
The angle strain is the magnitude of the difference between the desired carbon-carbon bond angle and the actual carbon-carbon bond angle. Closer is the actual carbon-carbon bond angle to the desired carbon-carbon bond angle, lower is the angle strain and greater is the stability of cycloalkane.
Cycloalkanes are hydrocarbons containing carbon and hydrogen atoms. They contain all single bonds. They contain one (or more) ring of carbon atoms. Cyclopropane, cyclobutane, cyclopentane and cyclohexane contain 3,4,5, and 6 carbon atoms respectively. The structures of cycloalkanes are shown below:

                                  

Cycloalkane	Desired carbon-carbon bond angle	actual carbon-carbon bond angle	Angle strain
Cyclopropane	\[109^\circ \]	\[ 60 ^\circ \]	\[ 49 ^\circ \]
Cyclobutane	\[109^\circ \]	\[ 90^\circ \]	\[ 19 ^\circ \]
Cyclopentane	\[109^\circ \]	\[ 108^\circ \]	\[ 1 ^\circ \]
Cyclohexane	\[109^\circ \]	\[ 120^\circ \]	\[ 11 ^\circ \]

The decreasing order of the stabilities of cycloalkanes is cyclopentane > cyclohexane > cyclobutane > cyclopropane.
For cyclopentane, the angle strain is lowest. Hence, among the given cycloalkanes, cyclopentane is the most stable cycloalkane.
Hence, the option C ) cyclopentane is the correct option.

Note: Greater is the difference between the desired carbon-carbon bond angle and the actual carbon-carbon bond angle, greater is the angle strain and lower is the stability of the cycloalkane.