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Nucleophilic Aliphatic substitution Watch

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    Q: Ph2CHBr is hydrolysed in aqueous acetone 30 times faster than is Ph2CHCl, but if sodium azide is present during the reactions the ratio of Ph2CHOH to Ph2CHN3 formed is the same in each case; explain.

    Is Ph2CHBr hydrolysed in aqueous acetone faster than Ph2CHCl because of the fact that it will progress via an SN1 pathway and hence as Bromine is a better leaving group than Chlorine, the rate determining step will be faster for Ph2ChCl, hence leading to a faster rate of hydrolysis?

    However, if the sodium azide is present, then the reaction will proceed via an SN2 reaction mechanism and hence the leaving group ability of the Bromine/Chlorine is unimportant - hence leading to the same ratio of Ph2CHOH/PH2CHCHN3.

    This is just a bit of a guess so I may be completely wrong.... Also I'm thinking there may be some effect of the acetone solvent as well/maybe hydrogen bonding the H2O present but I'm not sure how this would tie in??? Edit: Would it be that the OH- ions in acetone solvent hydrogen bond and therefore are less effective nucleophiles and unable to participate in SN2 mechanism. Whereas, the N3- ions are unable to form hydrogen bonds with the acetone solvent and therefore are much stronger nucleophiles and able to push through with an SN2 mechanism?

    Or.....as I continually confuse myself:

    - would it progress via SN1 with the N3- ions as well, but as the rate equation for SN1 doesn't include the Nucleophile it will have no effect on the rate of the reaction only the composition. Therefore, you end up with the same ratios of both products.......? :P


    Q What would you expect to happen to the rate and to the product distribution is Ph2CHI were used?

    Faster rate.

    Same ratio of product distribution.

    ?



    Thanks for any help.
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    I think the acetone is there just to help solubilise the benzyl halides. Azide acts as a competing nucleophile.

    Does the question say if the kinetics stays the same after you add the azide? Or are you meant to assume that?
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    (Original post by =gabriel=)
    I think the acetone is there just to help solubilise the benzyl halides. Azide acts as a competing nucleophile.

    Does the question say if the kinetics stays the same after you add the azide? Or are you meant to assume that?

    The question doesn't mention it, but if it's SN1 then surely you must assume that the kinetics stays the same....
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    OK I'll be honest - I don't know a full answer to this question. But I don't think you can rule out SN2 just based on the first bit. Firstly, benzylic cations are stable, yes, but the benzylic trigonal bipyramidal transition state in SN2 is also pretty low energy, with good charge dispersion. Secondly, just because there is a difference in rate doesn't mean it's SN1. SN2 is a one-step reaction and there would be a difference in rate for different substrates too. If you did an experiment with primary alkyl halides, I'm sure they wouldn't all hydrolyse at the same rate.

    I think it's reasonable to say that at a near-neutral pH, water is not the best nucleophile. Azide will most probably be better. So it's not impossible that azide goes SN2 and water goes SN1, however when you come to the second statement it starts breaking down because pretty much any nucleophile can quench a carbocation and azide is negatively charged so it should get to the cation pretty quickly and will certainly compete with water, in addition to its SN2 running in the background. So I would expect to see a difference.

    Maybe you have to consider reversibility as well. If it's a thermodynamic as well as a kinetic problem here. The SN2 with azide will probably be reversible. On the contrary, the hydrolysis is probably not, at a normal pH you wouldn't see water leaving to form halides. But this still doesn't explain why the ratios are the same.

    The only model consistent with the ratios is the SN1 model where as soon as you form a carbocation, it's quenched by one of the two nucleophiles and since the benzylic carbocation formed is THE SAME for both halides, the ratio should be the same. I can't explain though, from an electronic perspective, why SN2 wouldn't go on this substrate, in fact I think it's quite likely to go with the azide...
 
 
 
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