Addition of bromine water to a substance is often used to test for unsaturation in a molecule, for example the addition of bromine water to an unsaturated alkene in a boiling tube will instantly result in the decolourisation of the reddish-brown bromine water. In the image below the left-hand test tube contains cyclohexane (C6H12); a saturated hydrocarbon while the right-hand test tube contains cyclohexene (C6H10; an unsaturated hydrocarbon.
If a small amount of bromine water is added to each test tube and then both test tubes are given a quick shake to mix the contents together, then the bromine water is instantly decolourised in the test tube containing the unsaturated cyclohexene while the bromine water stays orange in the test tube containing the saturated cyclohexane. It is worth mentioning that the orange colour of the bromine will switch layers in the test tube containing the saturated cyclohexane. Halogens such as bromine are more soluble in organic solvents than in water, so when the contents of the test tube are shaken up, although no chemical reaction will occur the majority of the bromine will move into the top less dense organic layer of cyclohexane while the aqueous layer which initially contained the bromine dissolved in it will turn clearer as the bromine dissolves in the cyclohexane layer, this is shown in in the left hand test tube in the image above.
As you may recall alkenes under electrophilic addition reactions. The mechanism below shows
how bromine adds to the carbon carbon double bond in an unsaturated alkene. You may recall that:
However if the ethene is swapped for benzene or another aromatic compound one difference in the reaction is immediately obvious; aromatic rings will not react with bromine or other halogens, they are much less reactive than alkenes towards electrophiles. I am sure you are aware that while unsaturated molecules such as alkenes readily undergo electrophilic addition reactions aromatic molecules such as benzene will not undergo electrophilic addition reactions but instead undergo electrophilic substitution reactions instead.
In the image opposite bromine which has been dissolved in a solvent such as chloroform or carbontetrachloride is added to a test tube containing benzene and another test tube containing the unsaturated hydrocarbon cyclohexene (C6H10). Both test tubes are stoppered and shaken. The results are shown in the image. Cyclohexene rapidly decolourises the orange bromine but in the test tube containing benzene the orange colour due to bromine persists and does not fade.
As mentioned above if bromine is added to a test tube containing benzene then no reaction occurs, the orange colour due to the presence of bromine persists. However if some iron filings or iron (III) bromide are added then the orange colour due to the presence of bromine decolourises and white misty fumes of hydrogen bromide are observed above the test tube. If a piece of moist pH paper or blue litmus paper is held above the test tube it turns red due to the acidic hydrogen bromide gas which is released. Equations for these reactions are shown below:
From the equations above I hope you can see that the bromine swaps or substitutes for one of the hydrogen atoms on the benzene ring. The reaction as you might expect from an aromatic ring is electrophilic substitution.
Why then do unsaturated alkenes undergo electrophilic addition reactions but aromatic rings such as benzene undergo electrophilic substitution reactions instead?
Well the pi(π) electrons in an alkene molecule are localised between two carbon atoms, whereas in an aromatic molecule such as benzene
the pi(π) electrons are delocalised through the whole ring of carbon atoms, this delocalisation makes benzene a very stable molecule and means basically that in order for an electrophile to react with benzene it needs to be an extremely "good electrophile". Non-polar molecules such as bromine will NOT react with aromatic rings, simply because they are "not good" enough electrophiles. For this reason a catalyst or a halogen carrier as they are often referred to are needed to increase the ability of a species to act as an electrophile. The delocalisation of the π-electrons as mentioned results in an
increase in the stability of the benzene ring and this will raise the activation energy for any reaction which will destroy this delocalisation energy.
We have seen from the equations above that benzene will react with bromine or indeed chlorine and iodine in the presence of catalysts such as iron or even aluminium. These metals react with halogens to form metal (III) halides such as aluminium chloride (AlCl3) or iron (III) bromide. This is shown in the equations below:
The catalysts or halogen carriers are they are often called work by producing the much more powerful electrophile Br+. The larger positive charge on the bromine ion (Br+) makes it a much better electrophile than a bromine molecule (Br2) and this makes it much easier for the delocalised π-electrons in the aromatic ring to attack it.
So it is possible to produce a more powerful electrophile by simply reacting the bromine with iron filings.
Lewis acids are substances which are able to accept a pair of electrons from another species, a Lewis base. In order to accept a pair of electrons the Lewis acid must have a space to put them- obviously! that is it must have empty orbitals available. Molecules which have less than an octet of electrons (8 electrons) fit the bill here. The diagram below shows two common Lewis acid molecules where the central atoms in each molecule only has 6 electrons in their valency or outer electron shell.
Iron (III) bromide(s) (FeBr3) is the Lewis acid catalyst usually used to brominate aromatic rings while aluminium chloride (AlCl3) or iron (III) chloride (FeCl3) is usually used to chlorinate aromatic rings. The Lewis acid catalysts basically polarises the bromine molecule (Br2) which will eventually form a Br+ ion. It is the Br+ ion which acts as an electrophile and substitutes onto the aromatic ring replacing one of the hydrogen atoms as it does. The formation of the bromide ion is outlined in the image below:
The mechanism for the bromination of benzene is shown below. It is no different to the other electrophilic substitution reactions that we have seen for other aromatic rings. Essentially the benzene π electrons act as a nucleophile and attack the electrophilic bromide ion (Br+). This results in the formation of a cation where the delocalised stability of the ring has been destroyed. To help offset this loss of stability the cation will be resonance stabilised (as shown below). Finally lost of a hydrogen ion (H+) will result in the restoration of the delocailsation π electrons in the aromatic ring system.
The final step involves the attack of the FeBr4- ion, which helps to remove the hydrogen ion from the intermediate cation. The FeBr4- will react with the hydrogen ion to regenerate the FeBr3 catalyst and also form hydrogen bromide gas (HBr). An equation for this reaction is shown below: