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Quantitative measurement of ring strain

While looking for data about ring strain in cycloalkanes, I came across two types of data:

  1. 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.
  2. 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?

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