[briteyellowness]

first of, is bromotoluene and benzyl bromide the same thing?

kay, the experiment: a student investigated the relative rate of bromination of benzene and toluene.  in the reaction, a solution of Br2 in HNO3 (5% in H20) was treated with a 2:1 mole ratio of benzene: toluene and the mixture was vigorously stirred for 15 min.  care was taken to exclude light by wrapping flask in aluminum foil.  the flask was uncovered and sodium sulfite was added to destroy excess Br2.  the mixutre was then extracted with diethyl ether, treated with Na2CO3, and dried over Mg2SO4.  The products were analyzed by gas chromotography and showed approximately equimolar amts of bromobenzene to bromotoluene.

if the student forgot to cover the rxn flask with foil, how might the outcome be affected?

answer: bromobenzene would have been the major product b/c UV light catalyzes the side-chain bromination of toluene thru a radical mechanism.

so... if UV light catalyzes the radical addition of bromine to toluene... then wouldn't that be the major product?

thanks again guys.

[Donaldson Tan]

first of, is bromotoluene and benzyl bromide the same thing?

no. toulene is methylbenzene. benzyl is just a synonym for phenyl.

[laotree]

Again, it is a mechanism question. The bromination on phenyl ring is a electrophilic substitution, and on benzylic position must be a radical reaction. Your reaction condition (polar solvent) fovors the phenyl ring bromination.

The UV light could initiate the radical reaction. In my Lab, The radical reaction condition is CCl4 (nonpolar SMs), or CF3Ph (for polar starting materials and high temperature.) as solvent, NBS/AIBN 60oC or high temperature.

[movies]

benzyl is just a synonym for phenyl.

No!  Benzyl means CH₂C₆H₅.  Phenyl is just C₆H₅.

Bromotoluene is not a complete name; you need something to define where the Br is.  e.g. 2-bromotoluene is different from benzyl bromide, but alpha-bromotoluene is the same as benzyl bromide.

In this question the "bromotoluene" they refer to in the correctly executed procedure would have the Br on the ring while in the incorrectly executed procedure the Br would be on the sidechain, so identical to benzyl bromide.

[Donaldson Tan]

 :french_blank:  ooo.. i learn something new today.

[briteyellowness]

i thought the radical reaction would be better because it's kind of allylic in a way, so more stability?  and radical reactions can happen in the presence of UV light, which is possible because the flask was left uncovered?

sorry, i'm still not understanding how the electrophilic substitution in the presence of light will become the major product....

and i don't understand how in the absence of light the radical reaction could have happened in those conditions (for the equilimolar amts).

[Dude]

Let's break this down so that it is understood what you are trying to do (and I'm guessing):

1.  The experiment was designed to prove that a methyl group activates aromatic electrophilic substitution (benzene and toluene).

2.  You were supposed to get higher relative amounts of 2-bromo and 4-bromotoluene (methyl group is an ortho para director), however, you only saw equimolar amounts of bromobenzene and the 2 and 4-bromotoluene isomers.  Presumably, the conditions are set to minimize polybromination.

3.  Does light impact your experiment?

My take - Light will increase the probability of forming benzyl bromide from toluene through a radical process - there is no such radical process for benzene (no benzylic site).  This may impact the reaction by reducing the relative amount of available toluene (and forming benzyl bromide) with the benzene concentration constant.  However, the amount would seem to be quite low in 15 minutes.  I would guess that having the foil removed would not significantly change your expected results in 15 min time.