Cycloalkanes

1
Cycloalkanes

• Hydrocarbon compound that contains rings
  Formula CnH2n
• Nonpolar, insoluble in water, Compact shape
• Melting and boiling points similar to branched
  alkanes with same number of carbons

Naming Cycloalkanes

• Cycloalkane usually base compound
• Number carbons in ring if gt1 substituent.
• First in alphabet gets lowest number.
• May be cycloalkyl attachment to chain.

2-cyclopentylheptane
1,1-diethyl-3-methylcyclopentane
2
Cis-Trans Isomerism

• Cis like groups on same side of ring
• Trans like groups on opposite sides of ring

3
Heats of Combustion/CH2 Alkane O2 ? CO2 H2O
Stability of cycloalkanes and Ring strains
Long chain alkanes
(CH2)
4
Cycloalkane Stability

• 5- and 6-membered rings most stable
• Bond angle closest to 109.5?
• Angle (Baeyer) strain
• Measured by heats of combustion per -CH2 -
• Cyclopropane has ring strain energy of 27.6
  kcal/mol (115 kJ/mol).
• Cyclobutane has ring strain energy of 26.3
  kcal/mol (110 kJ/mol).
• 5-membered ring 6.5 kcal/mol (27 kJ/mol).
• 6-membered ring 0.0 kcal/mol
• 7-membered ring 6.4 kcal/mol (27 kJ/mol).
• 8-membered ring 9.7 kcal/mol (41 kJ/mol).

5
Cyclopropane

• Large ring strain due to angle compression
• Very reactive, weak bonds

Eclipsed Hydrogens
6
Cyclobutane

• Angle strain due to compression
• Torsional strain partially relieved by
  ring-puckering

Cyclopentane

• If planar, angles would
• be 108?, but all hydrogens
• would be eclipsed.
• Puckered conformer
• reduces torsional strain.

7
Cyclohexane

• Combustion data shows its unstrained.
• Angles would be 120?, if planar.
• The chair conformer has 109.5? bond angles and
  all hydrogens are staggered.
• No angle strain and no torsional strain.

Chair Conformer
8
Boat Conformer
9
Conformational Energy
gt
10
Axial and Equatorial Positions
Monosubstituted Cyclohexanes
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1,3-Diaxial Interactions
Conformational equilibrium of disubstituted
cyclohexane
12
Cis-Trans Isomers

• Bonds that are cis, alternate axial-equatorial
  around the ring.

Bulky Groups

• Groups like t-butyl cause a large energy
  difference between conformer.
• Most stable conformer
• puts t-butyl equatorial
• regardless of
• other substituents.

13
Bicyclic Alkanes

• Fused rings share two adjacent carbons.
• Bridged rings share two nonadjacent Cs.

14
Cis- and Trans-Decalin

• Fused cyclohexane chair conformers
• Bridgehead Hs cis, structure more flexible
• Bridgehead Hs trans, no ring flip possible.

Bicyclo4.4.0decane