Download Chapter 3 STRUCTURE AND STEREOCHEMISTRY OF ALKANES

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
166.6
9.2
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
30
Strain energy [kcal/mol]
25
20
15
10
5
0
3
4
5
6
7
8
9
Ring size
Chapter 3: Conformations of Alkanes and Cycloalkanes
10
11
12
13
14
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