Cycloalkanes

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Cycloalkanes

• Alkanes forming a ring of carbon atoms
• Similar properties to the alkanes

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Bridged/Fused Cycloalkanes

• Multiple rings joined by one or more carbon
  atoms
• Functionally the same as cycloalkane

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Flat Cyclohexane?
Also The C-C-C bond angles in a flat structure
would be 120. Since each carbon is sp3
hybridized, it must be 109 not 120.
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Flat Cyclohexane?
Cyclohexane is not planar!
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Cyclohexane Geometry

• Chair Conformation

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Chair ConformationA Closer look
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Another View
Newman projection!
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Geometric Isomerism
Opposite side trans

• What is different about these molecules?
• The orientation of the methyl groups
• These are not two conformations, since there is
  no bond rotation that will convert one to the
  other. Thus they are isomers.

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Geometric Isomers - Skeletal Structures
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Nomenclature of Cycloalkanes

• Named similar to alkanes.
• Parent cycloalkane is the base name.
• Lowest numbering system is desired.
• If the alkyl substituent has more carbons than
  the cycloalkane, then the alkane is the parent
  name and the cycloalkane is a substituent.

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Nomenclature - examples
1,6-dimethylcyclohexane
1,2-dimethylcyclohexane
incorrect name
correct name
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Nomenclature - more examples
1-ethyl-1-methylcyclopentane
trans-1,3-dimethylcyclopentane
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Nomenclature - one more

• The 6-carbon chain takes precedence in the name
  and becomes the parent name. The 5-carbon
  cycloalkane becomes the substituent on the
  6-carbon chain.

Name is thus 1-cyclopentylhexane not
1-hexylcyclopentane
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Physical Properties of Alkanes

• The H-C bond is not very polar and typically
  these small dipoles cancel, thus the alkanes are
  essentially non-polar.
• Since alkanes only have London Dispersion forces,
  they generally have low boiling points.
• The non-polarity also leads them to not dissolve
  with polar/ionic compounds such as water and
  salts.

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Alkanes - boiling points

• Boiling points increase as alkanes increase in
  size.

Alkane Boiling Point methane -162 ethane -89 p
ropane -42 butane -1 pentane 36 hexane 69 He
ptane 98
Increasing molecular weight, increasing boiling
point
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Alkanes - boiling point

• For alkanes of the same molecular weight, boiling
  points are lower as the alkane becomes more
  branched.

e.g. isomers of hexane, C6H14
boiling point(C) 69 63 58
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