Question

The heats of formation, ΔHf (rounded off to simplify the arithmetic) of cyclohexane and cyclooctane are both –30 kcal/mol. What is the strain energy (in kcal/mol) of cyclooctane?

A 0

B 5

C10

D 15

E20

Answer 1

we know that

cyclohexane is strain free

so

from cyclohexane

we can calculate the heat of formation of a strain free -CH2- group

there are 6 such groups in cyclohexane

so

heat of formation of a strain free -CH2- group = -30 / 8 = -5

now

consider cyclooctane

in this case

there are 8 -CH2- groups

so

theoretical heat of formation fo cyclooctane = 8 x -5

theoretical heat of formation of cyclooctane = -40

but

given

heat of formation of cyclooctane = -30

so

strain energy = -30 - (-40)

stain energy = 10

so

the answer is C) 10