Formation of a ring in a cycloalkane imposes (with the exception of cyclohexane!!) geometric restrictions that require carbon to deform from the ideal sp3 geometry: bond lengths of 1.54 Å, bond angles of 109.5°. Examine the five simplest cycloalkanes:
Cyclopropane Total strain: 27.6 kcal/mol 9.2 kcal/mol per CH2		Click on the link to display the measurement, or click here to remove them. C-C: 1.508 Å C-C-C: 60° C-C-H: 118.2°
Cyclobutane Total strain: 26.3 kcal/mol 6.6 kcal/mol per CH2		C-C: 1.553 Å C-C-C: 88.5° C-C-H: 111.5° H-C-C: 108.4° Measurements off
Cyclopentane Total strain: 6.5 kcal/mol 1.3 kcal/mol per CH2		C-C: 1.538-1.560 Å C-C-C: 104.2, 105.8° C-C-H: 110.1° H-C-H: 106.5° Measurements off
Cyclohexane Total strain: 0 kcal/mol 0 kcal/mol per CH2		C-C: 1.537 Å C-C-C: 111.5° C-C-H: 110.3, 109.1° H-C-H: 106.4° Measurements off
Cycloheptane Total strain: 6.4 kcal/mol 0.9 kcal/mol per CH2		C-C: 1.535-1.540 Å C-C-C: 115.0-118.2° C-C-H: 108.4-109.5° H-C-H: 105.3° Measurements off