[Emissionspectra]

When an Alkyl or halogen group is attached to the benzene ring, why is it that sometimes it's 2,4 and/or 6 that gets substituted. What dictates if the hydrogen on bond 6 also gets substituted and why does it only sometimes occur?

[Zishi]

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.

The 2 position is essentially the same as 6 position, so it doesn't matter whether you attach the incoming group to 2 or 6 position. The substitution on both positions occurs when we use harsh conditions. For example, reacting dilute nitric acid with a phenol will only give a mixture of 2-nitrophenol and 4-nitrophenol, whereas reacting phenol with concentrated nitric acid will give 2,4,6-trinitrophenol.

[Emissionspectra]

(Original post by Zishi)
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.

The 2 position is essentially the same as 6 position, so it doesn't matter whether you attach the incoming group to 2 or 6 position. The substitution on both positions occurs when we use harsh conditions. For example, reacting dilute nitric acid with a phenol will only give a mixture of 2-nitrophenol and 4-nitrophenol, whereas reacting phenol with concentrated nitric acid will give 2,4,6-trinitrophenol.

Ah right, thanks very much.

[TiTo20]

(Original post by Emissionspectra)
When an Alkyl or halogen group is attached to the benzene ring, why is it that sometimes it's 2,4 and/or 6 that gets substituted. What dictates if the hydrogen on bond 6 also gets substituted and why does it only sometimes occur?

It depends on the groups being added onto the benzene ring, and what subsequent groups does to the ring.

For example, if you nitrate methylbenzene with the temperature less than 50 degrees Celcius, you get a mixture of 2-methylbenzene, 4-methylbenzene, or 2,4-dinitromethylbenzene, but very rarely 2,4,6-trinitromethylbenzene because the nitro groups tend to attract the pi electrons in the ring closer to the nitrogen atom due to electronegativity, which results in a more... "positive" ring if you will. This means Carbon 6 is more positive than usual and will not react with the NO2 group unless a higher temp is used.

I know you talked about Alkyl and Halogen groups, but I found it easier to explain it with the nitro group.

Hope this helps =]


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[JMaydom]

The regioselectivity is determined by how the group stabilises the (wheland) intermediate. The respective resonance structures can place the formed carbocation on the same carbon as the attached group if it's ortho or para attack, or it cannot place it on the adjacent carbon in the case of meta attack.

If the group is electron donating, it is favorable to have the carbocation adjacent to the group, so ortho or para attack is favored. If the group is electron donating, it is disfavourable to place the carbocation next to the group, so meta attack is favoured