[unistudent]

So  in our chem lab we reacted cyclohexane, cyclohexene, toluene, ethylbenzene, isopropylbenzene and t-butylbenzene with bromine and let them sit and then used UV lamps to see if reactions continued further.  We determined as a class that cyclohexene was most reactive, then isopropylbenzene, ethylbenzene, toluene and then tied for last was t-butylbenzene and cyclohexane.

I am now supposed to draw out the reactions of each compound with bromine and show both products and reactants.  I think I am on the right track but I am having problems determining where the bromine is "attacking" the compounds that have a benzene ring as the functional group.

I think for the cyclohexane reaction, it is a radical substitution reaction producing the original compound but just with a bromine branch and HBr.  For the cyclohexene reaction I think it is an electrophilic addition where the double bond will move so that it looks like cyclohezane with a bromine branch and then there will be a bromine radical also produced.  There are just guesses though.  The the part that is really stumping me.  For the other 4 compounds, I think it is an electrophilic aromatic substitution reaction for all of them but I dont know if the bromine will "attack" a carbon on the branch, or it it will "attack" the carbon on the benzene ring that is also attached to the branch, or if it will "attack" a carbon on the benzene ring that isn't attached to the branch.  I am leaning towards the carbon on the ring attached to the branch but I don't know how I would "push" electrons to form a possible compound, if there would be a positive charge formed on the ring as a result of the electrons moving to accomodate the bromine and what other compounds would be formed.

Any help would be greatly appreciated!

[ooosh]

I think your analysis is great!

If only use UV light, without any other catalyst, alkanes will undergo radical substitution reaction with bromine.And for the substituted benzene ,the reaction will occur only on the alkylic branch.


The reaction occurs on benzene ring is electrophilic substitution reaction, the reaction of this type needs calalyst,e.g. FeX₃,X=Cl or Br.(always in the formation of FeX₄^-X⁺ )
X⁺ acts as the electrophile.

[unistudent]

Okay perfect, so I am glad I am on the right track.  Here is another thing, we were talking about radicals on the branch so obviously one needs to be present for this reaction to occur on the alkylic branch but how does it form?  Its a reaction intermediate I am guessing, so how do I know which carbon it forms on in the compounds where there is more that one carbon in the alkylic branch?
Thanks again!