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Cycloalkane Conformations

Article ID: 44
Last updated: 01 Aug, 2011
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The term cycloalkane literally means "cyclic alkane" - ring systems formed of only C-C and C-H bonds.  Such structures are commonly encountered in natural compounds such as steroids, with cyclopentanes and cyclohexanes being the most common.
 
Other than cyclopropane (which must be planar), cycloalkanes are "puckered" to relieve some of the ring strain (see below) by lowering angle and torsional strains.  This puckering occurs by rotating around multiple C-C bonds within the ring, using the same thought process as rotating around the C2-C3 bond in butane to achieve the most stable anti conformation.  This can be easily mimicked by manipulating a five or six member ring with a ball and stick model.  Five and six member rings have specific names for these lowest energy conformers, such as half-chair and chair.
 

Ring Strain: cyclopropane > cyclobutane > cyclopentane > cyclohexane (most stable)

 
The structures of some of the smaller cycloalkanes are shown below with thermodynamic information provided for further consideration of their relative stabilities.  Six member rings deserve special discussion, because of the unique conformations that are possible.  These are discussed separately.  In order to compare the strain in each member of the cycloalkane series below cyclohexane, the heat of combustion per methylene (i.e. -CH2-) is also given in green. The smaller this number is the less ring strain:
 
C3H6

CYCLOPROPANE
DHc / CH2 = -697 kJ/mol
(-166.6 kcal/mol)
 
C4H8

CYCLOBUTANE
DHc / CH2 = -681 kJ/mol
(-162.7 kcal/mol)
 
C5H10

CYCLOPENTANE
DHc / CH2 = -658 kJ/mol
(-157.3 kcal/mol)
 
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Conformational Analysis of Simple Alkanes     Drawing Cyclohexane Chair Structures

Department of Chemistry & Chemical Biology