Electrophilic Aromatic Substitution

[andafoo]

I'm not sure if this is still part of the MCAT req'd material, but nonetheless:

For aromatic substitution rxns, how do we know if they are first of second order? I just encountered a question:

Benzene reacts with bromine to form bromobenzene, but the reaction requires the use of an appropriate Lewis acid catalyst such as ferric bromide.

It is reasonable to assume that Reaction I will proceed via which of the following reaction mechanisms?

A. Nucleophilic substitution first order
B. Nucleophilic substitution second order
C. Electrophilic substitution first order
D. Electrophilic substitution second order


The correct answer is C, first order, but how is that? I thought second order because it depended on the concentration of two different species, which I wasn't sure if it is true or not.... but I have no better explanation. Anyone?

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[Ninjaface]

edit: oops, nope, think I may be wrong...

back to the books!

 

[andafoo]

bump this, anyone ... anyone?

 

[Phlame217]

Well you know its electrophilic becuase the ferric bromide creates a positively charged bromine ion complex and the pi bonds attack it the end bromine sticking out.

The only way to find its order is to experimentally determine it, and it should be learned knowlege that an halogenation of a benzene is a first order.

 

[andafoo]

Well you know its electrophilic becuase the ferric bromide creates a positively charged bromine ion complex and the pi bonds attack it the end bromine sticking out.

The only way to find its order is to experimentally determine it, and it should be learned knowlege that an halogenation of a benzene is a first order.

When you say it should be learned knowledge that halogenation of a benzene is first order you mean memorize it because you see it mentioned somewhere or experimentally determine it?

I've memorized it now since it showed up, but seems a bit unfair 😛.