While looking for data about ring strain in cycloalkanes, I came across two types of data:
- Ring strain per methylene group (let’s denote this quantity by X) : The difference between heat of combustion of the cycloalkane per methylene group and the reference value of 658.6 kJ per mol.
- Total ring strain (let’s denote this quantity by Y) : The difference between molar heat of combustion of the cycloalkane and the reference value of 658.6 kJ per mol multiplied by number of carbon atoms in the cycloalkane.
From what I have read till now, it appears that the first quantity is the better representation of ring strain. (I say this on the basis that Y has the same value for cyclopentane as well as cycloheptane but still cycloheptane is considered to be less strained as it has lower value of X).
I want to know why X is a better measure for ring strain than Y. When we talk about a ring, we consider it as a whole; then why do we calculate ring strain per methylene group?