In this video you will learn about the homologous
series of the alkanes and alkenes and their
structure. Before we look at the structure
of these molecules, you may wish to recap
your
understanding of bonding by watching our covalent
bonding video.
Alkanes are saturated hydrocarbons meaning
they contain only single covalent bonds between
carbon atoms. A single covalent bond is shared
pair of electrons. Let us take our first alkane
in the
series, methane - CH4 . Carbon has 6 electrons
in total, 4 of these being in the outer shell.
Carbon
therefore always needs 4 more covalent bonds
to be stable. In methane, the 4 hydrogen atoms
have
only 1 electron and so covalently bond to
carbon atom, each sharing a pair of electrons.
Methane
is know as natural gas and is often used as
a fuel in homes for heating a cooking. It
is also that
gas released by cows by beltching and flatulence
as it is produced by the fermentation of food
by
bacteria in their many stomaches.
The next molecule in the homologous series
of alkanes in ethane, with two carbon atoms.
As you
can see, a single covalent bond is formed
between the two carbon atoms, witch 6 more
hydrogen
atoms covalently bonded to complete the molecule.
Now we understand the way these molecules
are bonded, we can show the next molecules
in
structural formula, representing the covalent
bonds simply with lines. Here is propane followed
by
butane. This alkane series continues and follows
the general forumula CnH2n+2
A general formula allows us to describe a
series of molecules that differ from each
other by a
constant unit. Let us look again ethane. Using
the formula CnH2n+2, where n represent 2 for
the
two carbon atoms in ethane, we can see that
the number of hydrogens will be 2x2=4 + 2
= 6 – so
6 hydrogen. C2H6. Using this we could find
the molecular formula of decane, an alkane
with 10
carbons. Pause the video now and use the general
forumula to work out the structure of decane.
You should have got C10, H22, where n = 10,
therefore 10 multiplied by 2 = 20 + 2 = 22.
So the alkane series is not limited to 4 carbons
as we have only previously discussed. There
is
methane, ethane propane….. decane – to
name just the first 10 in the alkane series.
Let us look now at the homologous series of
the alkenes. These are unsaturated and so
contain a
double covalent bond between two carbon atoms.
Here is ethEne – since there is a double
bond,
there are now two shared pairs of electrons
between the two carbon atoms. Each carbon
atom
now has only two electrons which covalently
bond with a total of 4 hydrogen atoms, two
on each
carbon, to complete the ethene molecule. You
are only likely to come across alkene molecules
with
just one double bond although there can be
more within a molecule. The next molecule
in the series
is propene. As before, there is a double covalent
bond between two of the carbon atoms but the
other is just a single bond. We now need to
fill the molecule with hydrogen atoms to ensure
that
each carbon has a total of four bonds. Two
hydrogen atoms are required at this end (ILLUSTRATION
SHOWS!), this middle carbon already has 3
bonds and so requires just one hydrogen and
this end
carbon has only one bond and needs three more
hydrogens. Since we now understand the covalent
bonding in this molecule, let us get replace
the dot cross diagram with a structural formula
of
propane.
Pause the video now and try and complete this
structural forumla of butane (SHOW C=C-C-C).
Here
is the completed structure – did you get
it right?
Here are the structural formulas of the first
four alkenes. Pause the video again and try
and
complete the general forumula – what is
the relationship between the number of carbons
and
hydrogens: CnH …..
Hopefully you got the structure CnH2n – quite
simply there is double the number of hydrogens
than
there is of carbons. So as an example for
decane, the molecular forumula would be C10H20.
Now at the end of this tutorial, you should
have a better understanding of the bonding
in the
alkanes and alkenes and how each homologous
series follows the pattern given by its general
formula.
