Benzene - complete ring substitution.

[RyanJ]

Hey there everyone!

 

I've been wondering about this for a while. We've been studying the electrophilic substitution of various things into the benzene ring.

 

This normally occurs best when something like a hydroxyl group is attached to the ring as the electronegative oxygen draws the electrons towards it, allowing electrophiles to more easily attack the bond.

 

I am curious however as to why there is almost never complete substitution into the ring, there are normally 2 or three substitutions but I haven't seen any (I can remember) examples where there has been full substitution.

 

I'm guessing it is possible, but maybe somewhat unstable if it affects the delocalized electron system. Can anyone tell me if it is possible to form a complete substation into the benzene ring and the chemistry behind such a reaction?

 

My guess would be high heat (high temperatures making the compound unstable and making it decompose?) or maybe something like is done for the substitution of a halogen into the ring using a halogen carrier but with some form of catalyst?

 

Thanks for your insights!

[YT2095]

how about hexanitrobenzene?

or pentanitromethylbenzene?

 

and I`m not going to give the instructions for making these, suffice to say it can and has been done

[RyanJ]

how about hexanitrobenzene?

or pentanitromethylbenzene?

 

and I`m not going to give the instructions for making these, suffice to say it can and has been done

 

Actually I was more interested in the chemical reason for their formation (seeing as they do from your examples). I'm guessing as I said earlier they are probably not very stable due to the effect having multiple groups attached to the ring in place of hydrogen would have.

 

Can you at least tell if they require high temperatures or a catalyst as I would probably expect? (High temperatures probably isn't the correct solution if the compounds are in any way unstable though).

 

I'm also guessing that this probably shouldn't (or can't) be done by direct substation into the ring (maybe forming a temporary compound and then working from that to get the complete ring substation).

 

From the names I can see where your going by not specifying a way to make them by the way

 

-- Ryan Jones

[YT2095]

you`de start by nitrating 2,4,6-triaminobenzene, to get 1,3,5-trinitro-2,4,6-triaminobenzene., then oxidise the amines to convert them to nitroniums.

leaving your hexanitrobenzene.

 

so the theory goes

[RyanJ]

Interesting, like a form of indirect nitrating in this case. Looks like I'll have to dig out my organic chemistry books top find out what's going on here.

 

Thanks for the interesting information YT!

 

-- Ryan Jones

[jdurg]

Another reason for the difficulty in full ring substitution is steric hinderance. The benzene ring is basically a flat structure and the attachments to the ring will wind up on the same plane as well. As you increase the size of the attachments, you decrease the ability of other substituents to attack the central ring and latch on.

[YT2095]

JDurg, couldn`t it adopt a "Chinese Lantern" type structure?

 

whereby the benzene ring is flat on the page, 1,3 and 5 stick out above the page, and 2,4 and 6 go into the page?

 

probably not the Best description, but just a thought

[RyanJ]

I thought benzene always "liked" to form a planar structure as its more stable.

 

In nitrobenzene I think that the nitrogen is in plane with the carbon to which its attached with the two oxygen's raised above the plane of the ring? Maybe it would vary from the lone pair repulsion from the nitrogen's? (I don't know much about VSEPR unfortunatley.)

 

My guess would be that any other nitrite groups bonded directly too the ring would also be in plane with the ring but I have no basis for this belief

 

PS: Sorry mods, this should have been under organic chemistry.

[jdurg]

JDurg, couldn`t it adopt a "Chinese Lantern" type structure?

 

whereby the benzene ring is flat on the page, 1,3 and 5 stick out above the page, and 2,4 and 6 go into the page?

 

probably not the Best description, but just a thought

 

Yes, but wouldn't that put undue strain on the carbons in the benzene ring? You'd have the electrons kind of "squished" on one side of the carbon atom and "free" on the opposite side. As a result, it would wind up wanting to flatten out again and you'd run into that hinderance once more.

[John Cuthber]

Part of the problm for making a hexanitrobenzene is that each nitro group you add removes electrons from the ring and makes it more difficult for the next substitution to happen.

It's possible to add methyl groups to a benzene ring using chloromethane (or, more practically iodomethane) and a lewis acid catalyst like AlCl3.

If you do this you tend to get more than one methyl on each ring because the methyl benzene is more reactive than benzene itself and, similarly the dimethyl benzenes are more reactive still. Getting all 6 to react isn't as easy as it looks because of steric effects.

[RyanJ]

That's interesting John Cuthber.

 

That explains why the direct nitration of the benzene ring won't work fo 6 substitutions. It also explains the extreme instability of the compound, benzene normally being stable but with its pi delocalised electron system removed (or altered) will be very unstable.