Chapter 3

CONFORMATIONS OF ALKANES AND CYCLOALKANES

Chapter 3: Conformations of Alkanes and Cycloalkanes

HEATS OF COMBUSTION AND RING STRAIN

RING SIZE

CYCLO

ALKANE

MOLAR HEAT

OF COMBUSTION

PER

CH2

GROUP

RING

STRAIN PER CH2

TOTAL RING STRAIN

3 Cyclopropane 499.8 kcal/mol 166.69.2 kcal/mol

27.6

4 Cyclobutane 655.9 164.0 6.6 26.4

5 Cyclopentane 793.5 158.7 1.3 6.5

6 Cyclohexane 944.5 157.4 0.0 0.0

7 Cycloheptane 1108.3 158.3 0.9 6.3

Ref.Long-chain alkane 157.4 0.0 0.0

Chapter 3: Conformations of Alkanes and Cycloalkanes

STRAIN ENERGY VS. RING SIZE

0

5

10

15

20

25

30

3 4 5 6 7 8 9 10 11 12 13 14

Ring s ize

Str

ain

en

erg

y [

kc

al/

mo

l]

Chapter 3: Conformations of Alkanes and Cycloalkanes

TYPES OF STRAIN IN ORGANIC MOLECULES (SUMMARY)

1. Torsional Strain: The strain due to eclipsing of bonds at neighboring carbon atoms. Energy cost – about 1 kcal/mol for C-H --- C-H eclipse, or 1.3 kcal/mol for C-H --- C-CH3 eclipse.

2. Steric Hindrance: The strain due to repulsive interactions, when atoms or groups approach each other too closely. Energy cost – about 0.8 kcal/mol for C-CH3 --- C-CH3 gauche interaction.

NOTE: The C-CH3 --- C-CH3 eclipsed interaction is a cumulative torsional strain + steric hindrance!!!

3. Angle Strain: The strain due to expansion or compression of bond angles. Energy cost – varies, depending on the actual bond angle.

Chapter 3: Conformations of Alkanes and Cycloalkanes