Cycloalkanes (also called naphthenes, but
distinct from naphthalene) 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 (C)
and hydrogen
(H) atoms and are saturated because there are no multiple C-C bonds to hydrogenate
(add more hydrogen to). A general chemical
formula for cycloalkanes would be CnH where n = number of C atoms and g =
number of rings in the molecule. 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
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 (e.g., "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 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 (method or nomenclature)
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 (because the suffix
'ol' is in the name) and it should then have a hydroxide (OH) 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
(e.g., heptane, which means hept or 7 carbons, and ane, which
indicates only single bonding between carbons). 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 (e.g., "bicyclo"). 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 (see below) 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 (around 50 kJ/mol). 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.
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