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Cyclic Hydrocarbons
Cycloalkanes: general formula CnH2n

How many hydrogens are in the
formula for methylcyclopentane?
_____

Cycloalkanes

Nomenclature Rules
1. Find the parent (is the ring a substituent of some larger carbon chain?)

2. Assign substituents so as to obtain lowest possible numbering

Nomenclature of Cycloalkanes
Choose a point of attachment as carbon 1 and number the substituents so that the second
substituent has as low a number as possible.
Complex Substituents: the nonhyphenated prefix iso- is considered part of the alkane name for
alphabetizing purposes, but the hyphenated and italicized prefixes sec- and tert- are not. Thus,
isopropyl is listed alphabetically under i, but sec-butyl and tert-butyl are listed under b.
When two or more different alkyl groups that could potentially receive the same number are
present on a ring, number them alphabetically, ignoring numerical prefixes such as di- and tri-

1-bromo-3-ethyl-5-methylcyclohexane

2-ethyl-1,4-dimethylcycloheptane

1-sec-butyl-3-isopropylcyclohexane

2-tert-butyl-1,1-dimethylcyclopentane (not 1,2,2-)

Cis-Trans Isomerism in Cycloalkanes
• Rotation about C-C bonds in cycloalkanes is precluded by the ring structure
• Rings have two “faces” and substituents are labeled as to their relative facial
positions
• There are two different 1,2-dimethylcyclopropane isomers, one with the two
methyls on the same side (cis) of the ring and one with the methyls on opposite
sides (trans)

Stereoisomers
Constitutional Isomers
(different connections between atoms)

Stereoisomers
(same connections but different
three dimensional geometry)

Cis-Trans Isomerism in Cycloalkanes
Can indicate relative stereochemistry by Wedge and Hash perspective lines

Stereoisomers
Taking into consideration stereoisomers, how many isomeric dimethylcyclopropanes
are possible?

Taking into consideration stereoisomers, how many isomeric dimethylcyclopropanes
are possible?

Stability of Cycloalkanes:
The Baeyer Strain Theory
• Baeyer (1885): since carbon prefers to have bond angles
of approximately 109°, ring sizes other than five and six
may be too strained to exist
• Rings from 3 to 30 C’s do exist but are strained due to
bond bending distortions and steric interactions

The Nature of Ring Strain
• Rings larger than 3 atoms are not flat
• Cyclic molecules can assume nonplanar conformations to
minimize angle strain and torsional strain by ring-puckering

Types of Strain
• Angle strain - expansion or compression of bond angles away
from most stable
• Torsional strain - eclipsing of bonds on neighboring atoms
• Steric strain - repulsive interactions between nonbonded atoms in
close proximity


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