Adding bromine

[SaintJude]

A compound to be analyzed is known to be a six carbon cyclic hydrocarbon. The compound is found to be inert to bromine in water and to bromine in dichloromethane, yet it decolorizes bromine in carbon tetrachloride when a small quantity if FeBr3 is added. Which of the following could be the identity of the compound?

a. cyclohexane OUT
b. benzene Edit!! It's benzene!!
c. 1,3 cyclohexadiene
d. 1,4 cyclohexadiene

Can someone explain?

All I know is "decolorizes bromine in carbon tetrachloride" = means alkene and that's how I eliminated choice a & b.

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[MedPR]

Are you sure Benzene is wrong? :/ The FeBr3 is probably a catalyst, which lead me to believe that Benzene was the answer since the unknown is unreactive without a catalyst, which sounds a lot like the bromination of benzene.

If it's not Benzene, then I'd pick the 1,4 diene because the 1,3 diene might lose one of the double bonds by donating an electron via resonance.

 

[chiddler]

Benzene is possible.

I do'nt know answer, but it looks like benzene.

 

[MrNeuro]

Benzene is possible.

I do'nt know answer, but it looks like benzene.

FUUUUUU i dont remember any of this

 

[syoung]

FUUUUUU i dont remember any of this

Lawl. Me neither... just shows that I gotta study mucho. when my books come.

 

[chaser0]

It is a problem because BR took all of this bromination and benzene reactions out of its book.

I dont know if they are implying that they wont be on the exam anymore. Or if they just decided to save paper this year.

 

[chiddler]

benzene chemistry is not required knowledge for the purposes of the mcat. i just remembered it from ochem class.

 

[MedPR]

Well, benzene is not directly tested on the MCAT. The decolorization of bromine indicates an alkene though.

 

[SaintJude]

It is a problem because BR took all of this bromination and benzene reactions out of its book.

I dont know if they are implying that they wont be on the exam anymore. Or if they just decided to save paper this year.

posting answer now....

 

[MrNeuro]

Well, benzene is not directly tested on the MCAT. The decolorization of bromine indicates an alkene though.

yeah but i just assumed that because benzene is aromatic it wouldn't give up electrons...then i saw the mechanism didn't even think about it gaining its aromaticity back...i imagine that it has a really high activation energy though

 

[MedPR]

yeah but i just assumed that because benzene is aromatic it wouldn't give up electrons...then i saw the mechanism didn't even think about it gaining its aromaticity back...i imagine that it has a really high activation energy though

Yea, but I figured C and D would both react without the catalyst.

 

[SaintJude]

NOooo, it' it's benzene!

Edit: And MedPr, is so right. But maybe this ain't on the MCAT anymore...i mean this was such a recent change (like this year, right?)

Anyway, if anyone sees that it reacts in the presence of FeBr3--it's likely a benzene.

Kaplan:

The question stem informs us that the unknown cyclic hydrocarbon does not react with bromine in dichloromethane, carbon tetrachloride, or water. This indicates that the unknown does not contain any non-conjugated double or triple bonds (otherwise the bromine would have added across the π bonds). However, since the unknown compound did react when FeBr3 was added, its identity must be benzene. The FeBr3 acts as a Lewis acid, catalyzing aromatic electrophilic addition reactions. By itself, bromine is not a strong enough electrophile to disrupt the conjugated system in the benzene ring. Yet, the addition of the iron (III) bromide catalyst converts the bromine into a strong enough electrophile, enabling its addition to the benzene ring-choice (B) is the correct answer.

 

[MedPR]

NOooo, it' it's benzene!

Aside from knowing the mechanism (like chiddler), I think the giveaway is the requirement of a catalyst.

Also, maybe this reaction doesn't discriminate between dienes and conjugated dienes? If so, then that pretty much rules out C and D.

 

[syoung]

I think I would have thought benzene like chiddler, but that's a good catch on the catalyst!