Question on Aromatic Substitution

[dynococus]

Does anyone know the major products of the following 2 reactions? I keep picturing propylbenzene but it's not the right answer. Is there a hydride or methyl shift going on?

A. Benzene ---> 1.Br2, FeBr3 2.CH3CH2CH2MgBr

B. Benzene ---> 1. CL2, FeCL3 2.CH3CH=CH2, AlCl3

---

Members don't see this ad.

 

[UCB05]

Does anyone know the major products of the following 2 reactions? I keep picturing propylbenzene but it's not the right answer. Is there a hydride or methyl shift going on?

A. Benzene ---> 1.Br2, FeBr3 2.CH3CH2CH2MgBr

B. Benzene ---> 1. CL2, FeCL3 2.CH3CH=CH2, AlCl3

CH3CH2CH2- should rearrange into isopropyl anion. The second step should proceed by nucleophilic aromatic substitution to form m-bromoisopropylbenzene. Am I close or completely off? Ochem was one of my weaker subjects

 

[sixkiller]

CH3CH2CH2- should rearrange into isopropyl anion. The second step should proceed by nucleophilic aromatic substitution to form m-bromoisopropylbenzene. Am I close or completely off? Ochem was one of my weaker subjects

I don't think carbanions readily rearrange; at least not that I know of.

Sure you're not confusing it with a carbocation?

Also, nucleophillic aromatic substitution is rather unlikely (due to severe kinetic constraints) without the presence of a meta director (one with both electron withdrawing resonance and inductive effects) prefereably a nitro group ortho or para to the halogen.

The intermediate of nucleophillic aromatic substitution's slow step involves a carbanion intermediate and having the nitro group orth/para to the halogen....a more stable transition state preceeds the carbanion intermediate by way of hammond's postulate.

I think there may be a typo in the op....

 

[dynococus]

I don't think carbanions readily rearrange; at least not that I know of.

Sure you're not confusing it with a carbocation?

Also, nucleophillic aromatic substitution is rather unlikely (due to severe kinetic constraints) without the presence of a meta director (one with both electron withdrawing resonance and inductive effects) prefereably a nitro group ortho or para to the halogen.

The intermediate of nucleophillic aromatic substitution's slow step involves a carbanion intermediate and having the nitro group orth/para to the halogen....a more stable transition state preceeds the carbanion intermediate by way of hammond's postulate.

I think there may be a typo in the op....

It's a question from ACS Ochem (page 115 #5 under Electrophilic and Nucleophilic Aromatic Substitution)

The full question asks: Which sequence of reagents would produce propylbenzene from benzene? The answer is Benzene ----> 1.Propanoyl chloride, AlCl3 2.Zn(Hg), conc HCl. I was just wondering what products you would get from the other two benzene reactions I posted above.

 

[sixkiller]

It's a question from ACS Ochem (page 115 #5 under Electrophilic and Nucleophilic Aromatic Substitution)

The full question asks: Which sequence of reagents would produce propylbenzene from benzene? The answer is Benzene ----> 1.Propanoyl chloride, AlCl3 2.Zn(Hg), conc HCl. I was just wondering what products you would get from the other two benzene reactions I posted above.

Choices C and D are more or less "dummy" reactions.

Choice A is the best answer. Acylation is followed by the clemenson reduction of the aryl ketone yielding propyl benzene.

Friedel Crafts alkylation is a poor choice because the primary carbocation will rearrange to a secondary carbocation and isoproplyl benzene results.

In addition, recall that alkylated bezene product is MORE reactive than the starting bezene. Thus, Friedel Crafts alkylation also leads to mixtures because of further alkylations; something to think about if you wanted to make toluene or ethyl benzene.

Friedel Crafts Acylation doesn't have that problem because the aryl ketone product is LESS REACTIVE than the starting aromatic compound and thus, further acylations do not occur. If the aryl ketone is followed by wolf kishner or clemenson reduction, a mono alkylated aromatic compound results.

 

[2PacClone23]

Didn't Chad say in his video that Benzene does NOT react because it's so stable? I'm pretty sure it was in one of this videos. The one about EAS (elec aro sub)

 

[SupahShyGuy]

I'm quite sure that the bromination of benzene doesn't create a reactable subtstrate for the nucleophilic attack from the grignard as stated by the author of this thread..

R-MgBr attacks ketones and aldehydes because of the carbonyl carbon's partial positive character. R-MgBr could not attack bromobenzene successfully because it will disrupt aromaticity. very bad...

i do believee that UCB05 is right about the rearrangement part though. Lewis acids like AlCl3 + R-Cl will give you an electrophilic R-group that will easily attach to the benzene (Friedel-Crafts alkylation).

Keep in mind that alkyl groups of 3 carbons or greater will rearrange so that the carbocation is secondary (to be more stable!) which in this case, isopropyl on a benzene ring.