Benzene question

[uthopeful]

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There was a question on the april test that said what added to a benzene would speed up the reaction. Reading the forums people said it was small and electronwithdrawing.

I thought withdrawl groups deactivate the ring and electron donating groups activate it, so wouldn't the donating group speed up the reaction because it makes it more active?

thanks

 

[TTSD]

No idea what the question was, but electron withdrawing would make the thing-a-ma-bob more acidic since it delocalizes the electrons.

Hope that helps.

 

[Rapid Decomposition]

O Chem is kind of a long time ago for me, but I think the question may have been referring to nucleophilic aromatic substitution, in other words a SN2 reaction where the electrophile is benzene. An electron withdrawing group would draw electrons away from the ring, making it more electrophilic and susceptible to nucleophilic attack. A small group would be less likely to get in the way of the reaction.

 

[zinjanthropus]

however, it is more common for benzene's to undergo an electrophilic aromatic substitution correct?

usually, the pi bonds of benze are acting as the nucleophile, i think. in that case, an electron withdrawing group would make the ring less nucleophilic and less able to carry out and substitution reaction where it is the nucleophile.

 

[uthopeful]

So if you have a withdrawing group on a ring like Cl- then the ring is more stable and therefore it is more likely to react then if you and a donating group like methy.

I know that withdrawing groups stabilize rings but does that mean that they are more likely to react than if they did not have a withdrawing group or if there was a donating group. Thanks

 

[zinjanthropus]

see that's the problem....it depends on what kind of reaction you are talking about..

if there is a Cl- on the ring, it is less likely to undrego an electrophilic aromatic substitution because the Cl is taking on the electron density, making the ring a poor nucleophile

 

[deleted9493]

The % yield of substituted benzene is exceedingly low. In fact, the amount of Sn2 that benzene undergoes is generally considered negligible. Even with an electron withdrawing group attached, I think the steric hindrance of a ring (remember, 3 pairs of delocalized pi electrons)...in addition to the stability of the ring makes it quite unreactive. I wouldn't worry about it for the mcat.

 

[Mudd]

Not to rain on any parade here, but an SN2 reaction cannot transpire on an sp2-hybridized carbon. Much like a carbonyl, the nucleophile attacks the carbon and forms a tetrahedral intermediate. The noteworthy example where benzene is electrophilic is the reaction of 2,4-dinitrofluorobenzene (Sangers reagent) with the N-terminal of a protein. The nitrogen attacks the carbon with the fluorine and forms a td intermediate before the electrons return to displace the fluoride leaving group.

EWGs enhance benzene's electrophilic nature and the acidity of phenols and benzoic acid derivatives.

EDGs enhance benzene's nucleophilic nature and the basicity of phenoxides and benzoate derivatives.

 

[mikemohan0482]

Originally posted by Mudd
The nitrogen attacks the carbon with the fluorine and forms a td intermediate before the electrons return to displace the fluoride leaving group.

So how is that not an SN2? The nitrogen is acting nucleophilically on the carbon forming a bimolecular intermediate and in the end the fluorine is displaced resulting in an overall substitution. Sounds SN2 to me. What don't I get?

 

[deleted9493]

Although the end product is the same as would be found in an Sn2, the Sangers reaction goes through a different step. In Sn2 you have direct displacement of the leaving group as a result of collision with a nucleophile. With the Sangers reagent, however, you do get a nuclephilic attack but this attack simply causes an initial displacement of electrons...not immediate displacement of the Fluorine atom which is found in Sn2. So the nucleophile forces a triple bond to be formed (intermediate)....then, a pair of electrons from this multiple bond goes on to displace the fluorine. So, you see, even though the products are the same, the reaction pathways are different. Reactions are generally classified by their pathways...not their products.