Unsaturated molecules such as alkenes really only undergo one type of reaction -
addition reactions. You may recall from
your gcse science that small molecules can "add" across the carbon carbon
double bond (C=C) to form new saturated molecules. For example
bromine and chlorine
will add across the C=C bond in unsaturated
molecules such as ethene to form a saturated halogenalkane, this is outlined in the image below which shows the addition of a bromine molecule across a carbon carbon double bond (C=C) bond:
A bromine molecule (Br2) can add across the double bond of an ethene molecule to form the colourless halogenalkane 1,2-dibromoethane.
Chlorine will also add across the C=C double bond in an unsaturated molecule in exactly the same way as bromine to form the halogenalkane molecule 1,2-dobromoethane, this is outlined in the image below:
Fluorine (F2) reacts explosively with alkenes such as ethene (C2H4) and other unsaturated hydrocarbons, the can be best described as very violent rapid exothermic reaction, releasing a large amount of energy. This reaction can happen explosively under normal lab conditions due to fluorine's extreme oxidising power. Unlike other the halogens chlorine and bromine, iodine addition across a carbon carbon double bond (C=C) can be a rather slow reaction and might sometimes be reversible due to the weaker C-I bond compared to C–Cl or C–Br bonds.
The addition of small molecules to a carbon carbon double bonds (C=C) proceeds
by a type of mechanism called electrophilic addition. An electrophile
is an electron deficient
species, that is to say it is looking to gain electrons and so will be
attracted to areas in another molecule where there is
high electron density, such as the pi(π) covalent bond in an unsaturated molecule such as an alkene.
You may recall that a carbon carbon double bond contains one sigma
and one pi bond. The pi bond is formed
by the sideways or partial overlap of p-orbitals on the two atoms involved
in the covalent bond. A pi bond consists of two molecular orbitals or
lobes of electron density above and below the two
nuclei of the carbon atoms forming the covalent bond, this is shown in the image below.
These two molecular orbitals contain two electrons
and will be a large target for any electrophile.
It is this area of electron density, the pi bond which is responsible for many of the typical reactions of unsaturated alkene molecules. The image below shows how the sideways overlap of two p=orbitals results in the formation of molecular orbitals in the pi bond present in unsaturated molecules. It is these two lobes of electron density that are attacked by electrophiles, that is electron seeking species.
In the molecule of ethene shown opposite we usually just show the pi and sigma bonds as two lines between the carbon atoms. The problem with this is that it implies that the two covalent bonds are the same, which they are
obviously not. For example the bond enthalpy of a single C-C bond is of the order of 348 kJ mol-1, if the two bonds were identical then we would obviously expect the C=C bond to have a bond enthalpy of double the C-C, this is not the case. The bond enthalpy of the C=C bond is of the order 614kJ mol-1. So this means that the pi bond is
considerably weaker than the sigma bond.
Perhaps a better picture of the bonding in an alkene molecule might be the lower one of the two images shown opposite. Here the two lobes of the pi bond are shown. These lobes are areas of electron density where the two electrons needed to form the covalent pi bond are to be found.
The lobes of the pi bond, sticking out above and below the molecule leaves them open to attack by electron deficient species - electrophiles. Indeed this is the main type of reaction found in alkenes. Small electron deficient molecules will add across the C=C bond. These reactions are called electrophilic addition reactions simply because the small molecules “add” across the C=C bond.
The diagram below gives a step by step account of how a bromine
molecule adds to the carbon carbon double bond (C=C).
The reaction is very easy to carry out. The alkene can simply be
bubbled through liquid bromine
or a solution of bromine in an organic solvent.
One thing
to be aware of is the position of the curly arrows. The
curly arrows are used to show the movement of a pair of electrons and
they give an exact indication of where any new bonds will be formed. Since a
covalent bond requires two electrons and
curly arrow show the
movement of a pair of electrons the connection between the two should be obvious. I would
also caution you to take care with where you start and
end your curly arrows,
as any examiner will swiftly deduct marks if you get it wrong. I would ensure that:
When bromine reacts with ethene a colourless halogenalkane liquid called 1,2-dibromoethane is formed, the mechanism for this electrophilic addition reaction is shown below:
In fact the addition of bromine across a carbon carbon double bond (C=C) is used as a test for unsaturation. Here bromine is dissolved in water to form a red-brown solution called bromine water. When bromine water is added to a suspected unsaturated substance in a boiling tube and shaken; then if the substance is unsaturated the bromine water will decolourise almost immediately. If the substance in the boiling tube is saturated then the bromine water will decolourise very slowly, this is outlined in the image below, here the right-hand test tube contains cyclohexane (C6H12); a saturated hydrocarbon while the left-hand test tube contains cyclohexene (C6H10); an unsaturated hydrocarbon.
