This page discusses the problems which arise if you try to write the mechanism for an electrophilic substitution reaction into a benzene ring which already has something else attached to it. There are two problems you might come across:
- Whereabouts in the ring does the substitution happen? Does this make a difference to how you draw the mechanisms?
- Can the group already attached to the ring get involved in any way?
Electrophilic substitution in methylbenzene
The nitration of methylbenzene
If you substitute a nitro group, -NO2, into the benzene ring in methylbenzene, you could possibly get any of the following products:
The carbon with the methyl group attached is thought of as the number 1 carbon, and the ring is then numbered around from 1 to 6. You number in a direction (in this case, clockwise) which produces the smaller number in the name - hence 2-nitromethylbenzene rather than 6-nitromethylbenzene.
In the case of methylbenzene, whatever you attach to the ring, you always get a mixture consisting mainly of the 2- and 4- isomers. The methyl group is said to be 2,4-directing, in the sense that it seems to "push" incoming groups into those positions. Some other groups which might already be on the ring (for example, the -NO2 group in nitrobenzene) "push" incoming groups into the 3- position.
How to write the mechanism for the nitration of methylbenzene
Reacting methylbenzene with a mixture of concentrated nitric and sulphuric acids gives both 2-nitromethylbenzene and 4-nitromethylbenzene. The mechanism is exactly the same as the nitration of benzene. You just have to be careful about the way that you draw the structure of the intermediate ion.
Making 2-nitromethylbenzene (the first step)
This just shows the first step of the electrophilic substitution reaction. Notice that the partial delocalisation in the intermediate ion covers all the carbon atoms in the ring except for the one that the -NO2 group gets attached to. That is the only point of interest in this example - everything else is just the same as with the nitration of benzene. The hydrogen atom is then removed by an HSO4- ion - exactly as in the benzene case.
Making 4-nitromethylbenzene (the first step)
Once again, the only point of interest is in the way the partial delocalisation in the intermediate ion is drawn - again, it covers all the carbon atoms in the ring apart from the one with the -NO2 group attached.
The electrophilic substitution reaction between methylbenzene and chlorine
This is a good example of a case where what is already attached to the ring can also get involved in the reaction. It is possible to get two quite different substitution reactions between methylbenzene and chlorine depending on the conditions used. The chlorine can substitute into the ring or into the methyl group. Here we are only interested in substitution into the ring. This happens in the presence of aluminium chloride or iron, and in the absence of UV light. Substituting into the ring gives a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene.
The mechanisms are exactly the same as the substitution of chlorine into benzene - although you would have to be careful about the way you draw the intermediate ion.
For example, the complete mechanism for substitution into the 4- position is:
Electrophilic substitution in nitrobenzene
Substitution into the 3- position (the first step)
Methyl groups direct new groups into the 2- and 4- positions, but a nitro group, -NO2, already on the ring directs incoming groups into the 3- position. For example, if the temperature is raised above 50°C, the nitation of benzene doesn't just produce nitrobenzene - it also produces some 1,3-dinitrobenzene. A second nitro group is substituted into the ring in the 3- position.
The mechanism is exactly the same as the nitration of benzene or of methylbenzene - you just have to be careful in drawing the intermediate ion. Draw the partial delocalisation to include all the carbons except for the one the new -NO2 group gets attached to.
In the second stage, the hydrogen atom is then removed by an HSO4- ion - exactly as in the benzene case. This isn't shown because there's nothing new.
Jim Clark (Chemguide.co.uk)