INTRODUCING ALKANES AND
CYCLOALKANES
This is an introductory page about alkanes such as methane,
ethane, propane, butane and the rest. It deals with their
formulae and isomerism, their physical properties, and an
introduction to their chemical reactivity.
What are alkanes and cycloalkanes?
Alkanes
Formulae
Alkanes are the simplest family of hydrocarbons - compounds
containing carbon and hydrogen only. They only contain carbonhydrogen bonds and carbon-carbon single bonds. The first six
are:
methane
CH4
ethane
C2H6
propane
C3H8
butane
C4H10
pentane
C5H12
hexane
C6H14
You can work out the formula of any of them using: CnH2n+2
Isomerism
All the alkanes with 4 or more carbon atoms in them show
structural isomerism. This means that there are two or more
different structural formulae that you can draw for each
molecular formula.
For example, C4H10 could be either of these two different
molecules:
These are called respectively butane and 2-methylpropane.
Note: If you aren't confident about naming organic
compounds, the various ways of drawing organic
compounds, or structural isomerism, then you really ought to
follow these links before you go on.
You should read the whole of the page about drawing
organic molecules, but there is no need to read the other
two beyond where they talk about alkanes.
Use the BACK button on your browser to return to this page.
Cycloalkanes
Cycloalkanes again only contain carbon-hydrogen bonds and
carbon-carbon single bonds, but this time the carbon atoms are
joined up in a ring. The smallest cycloalkane is cyclopropane.
If you count the carbons and hydrogens, you will see that they
no longer fit the general formula CnH2n+2. By joining the carbon
atoms in a ring, you have had to lose two hydrogen atoms.
You are unlikely to ever need it, but the general formula for a
cycloalkane is CnH2n.
Don't imagine that these are all flat molecules. All the
cycloalkanes from cyclopentane upwards exist as "puckered
rings".
Cyclohexane, for example, has a ring structure which looks like
this:
This is known as the "chair" form of cyclohexane - from its shape
which vaguely resembles a chair.
Note: This molecule is constantly changing, with the atom
on the left which is currently pointing down flipping up, and
the one on the right flipping down. During the process,
another (slightly less stable) form of cyclohexane is formed
known as the "boat" form. In this arrangement, both of these
atoms are either pointing up or down at the same time.
Physical Properties
Boiling Points
The facts
The boiling points shown are all for the "straight chain" isomers
where there are more than one.
Notice that the first four alkanes are gases at room temperature.
Solids don't start to appear until about C17H36.
You can't be more precise than that because each isomer has a
different melting and boiling point. By the time you get 17
carbons into an alkane, there are unbelievable numbers of
isomers!
Cycloalkanes have boiling points which are about 10 - 20 K
higher than the corresponding straight chain alkane.
Explanations
There isn't much electronegativity difference between carbon
and hydrogen, so there is hardly any bond polarity. The
molecules themselves also have very little polarity. A totally
symmetrical molecule like methane is completely non-polar.
Note: If you aren't sure about electronegativity and polarity,
then you really ought to follow this link before you go on.
Use the BACK button on your browser to return to this page.
This means that the only attractions between one molecule and
its neighbours will be Van der Waals dispersion forces. These
will be very small for a molecule like methane, but will increase
as the molecules get bigger. That's why the boiling points of the
alkanes increase with molecular size.
Note: If you aren't sure about Van der Waals forces, then
you should follow this link before you go on.
Use the BACK button on your browser to return to this page.
Where you have isomers, the more branched the chain, the
lower the boiling point tends to be. Van der Waals dispersion
forces are smaller for shorter molecules, and only operate over
very short distances between one molecule and its neighbours.
It is more difficult for short fat molecules (with lots of branching)
to lie as close together as long thin ones.
For example, the boiling points of the three isomers of C5H12
are:
boiling point
(K)
pentane
309.2
2-methylbutane
301.0
2,2dimethylpropane
282.6
The slightly higher boiling points for the cycloalkanes are
presumably because the molecules can get closer together
because the ring structure makes them tidier and less "wriggly"!
Solubility
The facts
What follows applies equally to alkanes and cycloalkanes.
Alkanes are virtually insoluble in water, but dissolve in organic
solvents. The liquid alkanes are good solvents for many other
covalent compounds.
Explanations
Solubility in water
When a molecular substance dissolves in water, you have to


break the intermolecular forces within the substance. In
the case of the alkanes, these are Van der Waals
dispersion forces.
break the intermolecular forces in the water so that the
substance can fit between the water molecules. In water
the main intermolecular attractions are hydrogen bonds.
Note: If you aren't sure about hydrogen bonds, then you
should follow this link before you go on.
Use the BACK button on your browser to return to this page.
Breaking either of these attractions costs energy, although the
amount of energy to break the Van der Waals dispersion forces
in something like methane is pretty negligible. That isn't true of
the hydrogen bonds in water, though.
As something of a simplification, a substance will dissolve if
there is enough energy released when new bonds are made
between the substance and the water to make up for what is
used in breaking the original attractions.
The only new attractions between the alkane and water
molecules are Van der Waals. These don't release anything like
enough energy to compensate for what you need to break the
hydrogen bonds in water. The alkane doesn't dissolve.
Note: The reason that this is a simplification is that you also
have to consider entropy changes when things dissolve. If
you don't yet know about entropy, don't worry about it!
Solubility in organic solvents
In most organic solvents, the main forces of attraction between
the solvent molecules are Van der Waals - either dispersion
forces or dipole-dipole attractions.
That means that when an alkane dissolves in an organic
solvent, you are breaking Van der Waals forces and replacing
them by new Van der Waals forces. The two processes more or
less cancel each other out energetically - so there isn't any
barrier to solubility.
Chemical Reactivity
Alkanes
Alkanes contain strong carbon-carbon single bonds and strong
carbon-hydrogen bonds. The carbon-hydrogen bonds are only
very slightly polar and so there aren't any bits of the molecules
which carry any significant amount of positive or negative
charge which other things might be attracted to.
For example, you will find (or perhaps already know) that many
organic reactions start because an ion or a polar molecule is
attracted to a part of an organic molecule which carries some
positive or negative charge. This doesn't happen with alkanes,
because alkane molecules don't have this separation of charge.
The net effect is that alkanes have a fairly restricted set of
reactions.
You can


burn them - destroying the whole molecule;
react them with some of the halogens, breaking carbonhydrogen bonds;

crack them, breaking carbon-carbon bonds.
These reactions are all covered on separate pages if you go to
the alkanes menu (see below).
Cycloalkanes
Cycloalkanes are very similar to the alkanes in reactivity, except
for the very small ones - especially cyclopropane. Cyclopropane
is much more reactive than you would expect.
The reason has to do with the bond angles in the ring. Normally,
when carbon forms four single bonds, the bond angles are about
109.5°. In cyclopropane, they are 60°.
With the electron pairs this close together, there is a lot of
repulsion between the bonding pairs joining the carbon atoms.
That makes the bonds easier to break.
The effect of this is explored on the page about reactions of
these compounds with halogens which you can access from the
alkanes menu below.