How Does Electrophilic Addition Differ from Substitution in Aromatic Compounds?

[Canadian]

Homework Statement

Show the complete mechanism for the electrophilic addition reaction of toluene and bromine, isopropylbenzene and bromine. Draw the structure of the intermediate hydrocarbon.

Homework Equations

None

The Attempt at a Solution

I am having a really hard time with this question, I understand how electrophilic addition works in alkenes, and have reviewed electrophilic substitution reaction mechanisms in molecules with benzene rings.

For the toluene

would the bromine firstly separate to form two separate bromine ions, and then would one bromine attack the top most hydrogen, forming CH2BR instead of CH3, the remaining Br combine with the free H ion to form HBR??

thanks

 

[GCT]

Homework Statement

Show the complete mechanism for the electrophilic addition reaction of toluene and bromine, isopropylbenzene and bromine. Draw the structure of the intermediate hydrocarbon.

Homework Equations

None

The Attempt at a Solution

I am having a really hard time with this question, I understand how electrophilic addition works in alkenes, and have reviewed electrophilic substitution reaction mechanisms in molecules with benzene rings.

For the toluene

would the bromine firstly separate to form two separate bromine ions, and then would one bromine attack the top most hydrogen, forming CH2BR instead of CH3, the remaining Br combine with the free H ion to form HBR??

thanks

From what I remember it is more of a concerted radical mechanism.

 

[chemisttree]

If you are talking about electrophilic bromination, you are going to brominate the aromatic ring.

 

[GCT]

If you are talking about electrophilic bromination, you are going to brominate the aromatic ring.

That does not seem to happen without a catalyst. This is probably electrophilic addition and not electrophilic aromatic substitution.

 

[chemisttree]

It would be substitution for the benzylic bromination as well... free radical substitution. Benzylic bromination with Br₂ requires a catalyst as well although you could do it at high enough temperature to avoid the catalyst. The question didn't preclude the use of catalysts.

Addition reactions using bromine are typically ones that add Br₂ across a double bond, although not in aromatic systems like toluene. Thus, Br₂ is added to the structure of the reactant whereas the substitution of hydrogen (whether aromatic or benzylic or aliphatic) by bromne is properly identified as an electrophilic substitution reaction.