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.