Cycloalkanes are types of hydrocarbon compounds that have one or more rings of
carbon atoms in the chemical structure of their molecules. Alkanes are types of
organic hydrocarbon compounds that have only single chemical bonds in their
chemical structure. Cycloalkanes consist of only carbon and hydrogen atoms and
are saturated because there are no multiple C-C bonds to hydrogenate. A
general chemical formula forpila cycloalkanes would be CnH2(n+1-g) where
n = number of C atoms and g = number of rings in the molecule. For those
cycloalkanes that have one ring in their molecules, cycloalkanes can be treated
as isomers of their alkene counterparts, for example, cyclopropane and propene
both have the chemical formula C3H6. 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. Cycloalkanes are classified into small,
common, medium, and large cycloalkanes, where cyclopropane and cyclobutane are
the small ones, cyclopentane, cyclohexane, cycloheptane are the common
ones, cyclooctane through cyclotridecane are the medium ones, and the rest are
the larger ones. Nomenclature
See also: IUPAC nomenclature 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, and a numeric prefix before that indicating the number of
carbons in each part of each ring, exclusive of junctions. For instance, a
bicyclooctane that consists of a six-member ring and a four-member ring,
which share two adjacent carbon atoms that 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. There is more than one convention for
the naming of compounds, which can be confusing for those who are just
learning, and inconvenient for those who are well rehearsed in the older ways.
For beginners it is best to learn IUPAC nomenclature from a source that is up to
date, because this system is constantly being revised. In the above example
[4.2.0]-bicyclooctane would be written bicyclo[4.2.0]octane to fit the
conventions for IUPAC naming. It has then got room for an additional
numerical prefix if there is the need to include details of other attachments to
the molecule such as chlorine or a methyl group. Another convention for the
naming of compounds is the common name, which is a shorter name and it gives
less information about the compound. An example of a common name is terpineol,
the name of which can tell us only that it is an alcohol and it should then have
a hydroxide group attached to it. An example of the IUPAC method is given
in the image to the right. In this example the base name is listed first,
which indicates the total number of carbons in both rings including the
carbons making up the shared edge. Then in front of the base name is the
numerical prefix, which lists the number of carbons in each ring, excluding the
carbons that are shared by each ring, plus the number of carbons on the bridge
between the rings. In this case there are two rings with two carbons each and
a single bridge with one carbon, excluding the carbons shared by it and
the other two rings. There is a total of three numbers and they are listed in
descending order separated by dots, thus: [2.2.1].
Before the numerical prefix is another prefix indicating the number of rings.
Thus, the name is bicyclo[2.2.1]heptane. The group of cycloalkanes are also known
as naphthenes. Properties
Cycloalkanes are similar to alkanes in their general physical properties, but
they have higher boiling points, melting points, and densities than alkanes. This
is due to stronger London forces because the ring shape allows for a larger area
of contact. Containing only C-C and C-H bonds, unreactivity of cycloalkanes with
little or no ring strain are comparable to non-cyclic alkanes.
Ring strain The carbon atoms in cycloalkanes are sp3
hybridized and are therefore a deviation from the ideal tetrahedral bond angles
of 109°28'. This causes an increase in potential energy and an overall
destabilizing effect. Eclipsing of hydrogen atoms is an important
destabilizing effect, as well. The strain energy of a cycloalkane is the
theoretical increase in energy caused by the compound's geometry, and is
calculated by comparing the experimental standard enthalpy change of combustion
of the cycloalkane with the value calculated using average bond energies.
Ring strain is highest for cyclopropane, in which the carbon atoms form a
triangle and therefore have 60 degree C-C-C bond angles. There are also three
pairs of eclipsed hydrogens. The ring strain is calculated to be around 120
kJ/mol. Cyclobutane has the carbon atoms in a
puckered square with approximately 90-degree bond angles; "puckering"
reduces the eclipsing interactions between hydrogen atoms. Its ring strain
is therefore slightly less, at around 110 kJ/mol.
For a theoretical planar cyclopentane the C-C-C bond angles would be 108
degrees, very close to the measure of the tetrahedral angle. Actual
cyclopentane molecules are puckered, but this changes only the bond angles
slightly so that angle strain is relatively small. The eclipsing
interactions are also reduced, leaving a ring strain of about 25 kJ/mol.
In cyclohexane the ring strain and eclipsing interactions are negligible
because the puckering of the ring allows ideal tetrahedral bond angles to be
achieved. As well, in the most stable chair form of cyclohexane, axial
hydrogens on adjacent carbon atoms are pointed in opposite directions,
virtually eliminating eclipsing strain. After cyclohexane, the molecules are
unable to take a structure with no ring strain, resulting in an increase in
strain energy, which peaks at 9 carbons. After that, strain energy slowly
decreases until 12 carbon atoms, where it drops significantly; at 14, another
significant drop occurs and the strain is on a level comparable with 10 kJ/mol.
After 14 carbon atoms, sources disagree on what happens to ring strain, some
indicating that it increases steadily, others saying that it disappears
entirely. However, bond angle strain and eclipsing strain are an issue only for
smaller rings. Reactions
The simple and the bigger cycloalkanes are very stable, like alkanes, and their
reactions, for example, radical chain reactions, are like alkanes.
The small cycloalkanes - in particular, cyclopropane - have a lower stability
due to Baeyer strain and ring strain. They react similarly to alkenes, though
they do not react in electrophilic addition, but in nucleophilic aliphatic
substitution. These reactions are ring-opening reactions or ring-cleavage
reactions of alkyl cycloalkanes. Cycloalkanes can be formed in a
Diels-Alder reaction followed by a catalytic hydrogenation.
See also Cycloalkene
Cyclopropane Cyclobutane
Cyclopentane Cyclohexane
Cycloheptane Cyclooctane
References IUPAC, Compendium of Chemical
Terminology, 2nd ed.. Online corrected version: "Cycloalkanes".
Organic Chemistry IUPAC Nomenclature. Rule A-23. Hydrogenated Compounds from
Fused Polycyclic Hydrocarbons tp:www.acdlabs.comnomenclaturer79_73.htm
Organic Chemistry IUPAC Nomenclature.Rule A-31. Bridged
Hydrocarbons: Bicyclic Systems. p:www.acdlabs.comnomenclaturer79_163.htm
Organic Chemistry IUPAC Nomenclature.Rules A-41, A-42: Spiro
Hydrocarbons p:www.acdlabs.comnomenclaturer79_196.htm
Organic Chemistry IUPAC Nomenclature.Rules A-51, A-52, A-53,
A-54:Hydrocarbon Ring Assemblies p:www.acdlabs.comnomenclaturer79_158.htm
External links "Cycloalkanes" at the online
Encyclopædia Britannica "Cycloalkanes" by the Chemistry staff at
Westminster College