How many disubstituted isomers of benzene are there?
1 Answer
The distribution of the different products will be different, but there will be 3 isomers. These may be named as the 1-2, 1-3 and 1-4 isomers, or as the ortho, meta and para isomers.
Explanation:
The benzene ring has 6 carbon atoms in a roughly circular (or regular hexagonal) shape.
If the carbon atom to which the original substitution is attached is considered to be 'carbon number 1', then the ones on either side of that carbon can be considered to be number 2. Since the ring can flip over, those two positions are equivalent to each other, and have the same number. A substitution onto carbon 2 is also sometimes described as the 'ortho' meta - this word means 'close to'.
If we count one more carbon away from carbon 1 we come to carbon 3. So we might call this isomer the 1-3 isomer, or alternatively the 'meta' isomer.
Finally, if we count one more carbon again we come to the 4th carbon, which is directly across from carbon 1. The word 'para' means 'across', and is the name sometimes given to the 1-4 isomer.
If you imagine that the ring can flip over, you will realise that these are all of the possible isomers for a di-substituted methane ring.
Particular substitutions are sometimes called 'ortho-directing' or 'para-directing'. They lead to a higher proportion of the ortho or para isomer being formed. The proportions might be, say, 3% ortho, 8% meta and 89% para isomer for a particular reaction, but usually all three isomers will be present to some extent.
