Reactions of benzene

Original post by sam72016

When benzene reacts with something, how do you know what position it will take on the benzene ring? e.g. if we add excess NO2+ to benzene does it go in the 1,2,3,4,5 or 6 positions on the ring? And does the positioning change if there's a group already on the ring? e.g. methylbenzene or phenol.

Plain benzene will nitrate at any position, since all positions are identical.

If you have groups already attached, then they will direct substitution reactions, depending on the e- donating or withdrawing effects.

What spec are you?

Reply 2

Plantagenet Crown

Electron donating groups are ortho-/para-directing and electron withdrawing groups are meta-directing.

Reply 3

OP

Original post by Pigster

If you have groups already attached, then they will direct substitution reactions, depending on the e- donating or withdrawing effects.

What spec are you?

Sorry I forgot to mention, A level OCR A. What does that explanation mean?

Reply 4

WillJabine

4

Nitrobenzene substitutes to 3 and 5 positions (electron withdrawing group)
Phenol and phenylamine substitute to 2 and 4 positions (electron donating group)
If it’s just a benzene on its own it will substitute anywhere
I’m on the same spec , OCR A

Reply 5

OP

Original post by WillJabine

Nitrobenzene substitutes to 3 and 5 positions (electron withdrawing group)
Phenol and phenylamine substitute to 2 and 4 positions (electron donating group)
If it’s just a benzene on its own it will substitute anywhere
I’m on the same spec , OCR A

Oh ok cheers, what if it was something else like methylbenzene? Would it not make a difference

Reply 6

Pigster

Original post by sam72016

Oh ok cheers, what if it was something else like methylbenzene? Would it not make a difference

The only three directing groups that are on the spec are -OH, -NH2 and -NO2. There are lots of other groups, but you don't need to recall any others and their effects.

It is possibly they could show you a reaction, e.g. methylbenzene to 2-nitromethylbenzene and ask you to explain whether the methyl group is e- donating or withdrawing.

And your answer would be?

Reply 7

OP

Would it be electron drawing because the C is partially positive ? I thought the N in NO2 and O in OH are both electronegative so wouldn't they both be electron donating ?

Reply 8

Plantagenet Crown

Original post by sam72016

Would it be electron drawing because the C is partially positive ? I thought the N in NO2 and O in OH are both electronegative so wouldn't they both be electron donating ?

Methyl groups are electron donating (hyperconjugation) as is OH due to oxygen’s lone pair, but NO2 is electron withdrawing.

Reply 9

OP

But N in NO2 has an electron pair as well doesn't it? Why can't it donate that

Reply 10

Plantagenet Crown