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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