Benzene therefore reacts with electrophiles, electron pair acceptors. One typical reaction is sulfonation because sulfuric acid can (in a way) decompose yielding sulfur trioxide. Now although the molecule is neutral, the electron withdrawing effect of the oxygens yields a slight positive charge at the sulfur, enough to make it an electrophile and so react with benzene, pushing off hydrogen (see fig 1). Moreover this pushing off of the hydrogen is actually quite cool. We get what is known in the business as a sigma complex. An electron is quite literally plucked out of the pi system and the carbon that has snatched this electron is now sp3 hybridized, taking a typical tetrahedral shape. So a bond is formed between the electrophile that is reacting and the carbon is still bonded to a hydrogen. But the electrophile was positively charge right? So where has that positive charge gone? Well the pi system delocalizes it; take a look at the diagram below to see how (curly arrows indicate the movement of a pair of electrons). The positive charge is at the adjacent carbon where the electron was plucked out initially and then it moves around as it attracts electrons to it.
Now if we already have an extra group on benzene then we can get different products. As you can see with fig 2 there are three principle places for the electrophile to latch onto, ortho (carbons 2 and 6), meta (carbons 3 and 5) and para (carbon 4). Now given that there are 2 lots of ortho and meta and one lot of para the product ratio should be 2:2:1 (o:m:p) right? Wrong! The type of group plays a huge, gigantic, enormous, stupendous role in determining where the electrophile goes. If there is a slight negative charge next to the ring then it can get involved with the delocalisation of the positive charge when the sigma complex is generated. So with methylbenzene, by looking at We fig 3 we can see that the ortho and para complexes are the predominant products. Note there is less of para because: a) there is only one para position vs two ortho positions and b) because of steric effects (which is why the percentage of para is not exactly half the percentage of ortho).
Eventually the hydrogen ion will be plucked off the ring by the HSO4 anion made earlier and the electron will return to the ring.
fig 3
fig 2
This differing product composition happens because with the ortho and para complexes the positive charge lands on the carbon of the methyl group, which has a slight negative charge, so the methyl group can get involved in the delocalisation of the positive charge and hence the complex forms faster. Overall aromatic chemistry is a pretty cool topic. We have barely scratched the surface but already we have discovered that we can get different bonding places on benzene and also come across sigma complexes. Looking back it seems that Kekule almost got benzene right and hats off to him for doing so, but now it is time to go and make more discoveries. For example what happens when there are two pi systems joined together? Or maybe we should give our brains a rest for the time being‌
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