Nomenclature of cycloalcanes
Saturated hydrocarbons occur in three forms: straight-chain forms (alkanes), branched
chain forms (alkanes), and cyclic forms (cycloalkanes). The cycloalkanes contain only single
bonds, and have the general formula CnH2n. Cyclomethane and cycloethane obviously cannot
exist, but cyclopropane can; it is a triangular stable structure, though somewhat reactive because
the bond angles are somewhat strained to form the triangular structure. The bond angles in
cyclopropane are those of an equilateral triangle, 60 degrees, as compared to the tetrahedral bond
angle of 109.5 degrees.
Cyclobutane (C4H8 )
Cyclopropane(C3H6 )
Cyclopentane (C5H10)
Cyclobutane is a square structure; it is less reactive than cyclopropane because the bond
angle strain is less, 90 degrees compared to the tetrahedral bond angle of 109.5 degrees.
Cyclopentane and larger cycloalkanes are, like the normal alkanes, quite unreactive; there is no
significant bond angle strain in these molecules.
Because these diagrams are somewhat unwieldy, organic chemists use stylized drawings
to represent cyclic compounds. In the cyclic compound symbols, a carbon atom is understood to
be at the intersection of each pair of straight lines. Each carbon atom is understood to be bonded
to a sufficient number of hydrogen atoms to produce a total number of four bonds (the number
required for carbon). The standard symbols used to represent the first four cycloalkanes are:
Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane
Cycloalkanes with a single ring are named analogously to their normal alkane counterpart
of the same carbon count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. The
larger cycloalkanes, with greater than 20 carbon atoms are typically called cycloparaffins.
CH3CH2CH3
Propane
CH3CH2CH2CH3
Cyclopropane
Butane
CH3CH2CH2CH2CH3
Cyclobutane
Pentane
CH3CH2CH2CH2CH2CH3
Cyclopentane
Hexane
Cyclohexane
Polycyclic Alkanes
The naming of polycyclic alkanes such as bicyclic alkanes and spiro alkanes is more
complex, with the base name indicating the number of carbons in the ring system, a prefix
indicating the number of rings (eg, "bicyclo"), and a numeric prefix before that indicating the
number of carbons in each part of each ring, exclusive of vertices. For instance, a bicyclooctane
which consists of a six-member ring and a four member ring, which share two adjacent carbon
atoms which form a shared edge, is [4.2.0]-bicyclooctane. That part of the six-member ring,
exclusive of the shared edge has 4 carbons. That part of the four-member ring, exclusive of the
shared edge, has 2 carbons. The edge itself, exclusive of the two vertices that define it, has 0
carbons.
Bicyclic Alkanes
Fused ring systems that share more than two atoms are called bicyclic molecules.
To name bicyclic alkanes, you follow these three steps:
1.
Count the total number of carbons in the entire molecule. This is the parent name
(eg. ten carbons in the system would be decane)
2.
Count the number of carbons between the bridgeheads, then place in brackets in
descending order. (eg. [2,2,1])
3.
Place the word bicyclo at the beginning of the name.
Example: Name the following molecule by the IUPAC system of nomenclature.
1. Count the total number of carbons. In this case there are 7 carbons making the parent name
heptane.
2. Count the total number of carbons between bridgeheads. In this
case there are 2, 2 and 1 carbons between the bridgehead. Placed
in descending order between brackets it is [2, 2,1].
3. Place the word bicyclo in front of the name.
The name therefore is Bicyclo[2,2,1]heptane.
Bicyclo[4 3,1]decane
Bicyclo[2,2,2]Octaine
Bicyclo[3,3,1]nonane
We have seen that if two (or more) carbon rings in a molecule are completely separate
from each other (i.e. they have no carbons common to the two rings), then the naming is straight
forward. However it is possible for the two rings to be sharing one or two carbons, and special
ways have been formulated to name these situations.
Bicyclic alkanes are named differently depending whether the two rings are connected at
one carbon or at two. When naming rings sharing only one carbon the following rules will
apply. Examples
Example I
Example II
Example III
Note: The two rings are not in the same plane, in fact they are at right angles to each other at the
common carbon. This is often not shown when the structures are put into two dimensions.
Spiro Alkanes
Instead of naming these compounds as bicycloalkanes as might be supposed, the whole
two-ring structure is given the prefix spiro after the way the two rings connect at the single
carbon (in a sort of spiral).
To name these compounds, the root name is taken from the total number of carbons
in both rings.
Thus for example I above, there are 9 (nine) carbons in both rings together leading to the
name spirononane. However, the size of the two rings may vary, for if you look closely at
Examples II and III above, you will count 9 (nine) carbon atoms in both rings for each of these
compounds as well. So all are spirononanes, but quite obviously they are constitutional isomers
(differing in atom connectivity) of each other.
To complete the naming of these compounds, the size of the two rings is used to
differentiate them: a count is made of the number of non- common carbons in each ring, and the
two numbers obtained in this count are used in the middle of the name, in square brackets [..].
Thus, Example I above has counts 4 and 4 in the two rings and so is spiro[4.4]nonane.
Example II is spiro[3.5]nonane.
Example III is spiro[2.6]nonane.
Hint: the sum of the two numbers plus 1 equals the number of carbons in the two rings
giving the root name: 4 + 4 + 1 = 9; 3 + 5 + 1 = 9; 2 + 6 + 1 = 9.
Note the Rules of Syntax

where the brackets appear within the name

the separator (a dot, not a comma this time)

the order of the numbers...low to high.
Additional Examples
Spiro[3.4]octane
Spiro[5.5]undecane
Spiro[4.5]decane
Numbering for positioning ring substituents.
If substituents are present on the ring, numbering of the rings starts in the small ring next
to the ring junction, passes around this ring and through the ring junction into the larger ring.
The numbering goes around the two rings in the direction which produces the lowest numbers in
each ring. Examples:
2,6-dimethylspiro[4.5]decane
2,6-dimethylspiro[3.3]heptane
1,2,4,8tetramethylspiro[2.5]decane