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    Hi, I'm doing my AS ATM and cant completely wrap my head around the Hydrolysis rate in terms of the different mechanisms and the structure of the Haloalkane, along with Steric Hindrance and how does that all affect the reactivity?? :confused:

    I know the basics, like what steric hindrance is and what the different mechanisms are but cant see just how they all work together really. Id really appreciate any help be it website or self, Thanks.

    I have no idea what you are expected to know for your course so I'll just tell you some general info about these reactions and steric hinderance. Sorry if I waffle a bit. I'm not sure what you specfically want to know so I'll go over the lot.

    You probably know that SN2 reactions occur most readily with primary haloalkanes. This is because the carbon being attacked by OH- is most open to attack (least sterically hindered) so the new C-O bond forms at the same time as the C-X bond breaking. This is favourable as it avoids forming a carbocation altogether and a primary carbocation is much too unstable.

    The OH- nucleophile approaches at 180 degrees to the C-X bond, usually referred to as "backside attack". If you consider the shape of a tetrahedral carbon centre, the other 3 groups on the carbon being attack by the nucleophile initially point towards the direction the nucleophile approaches. The bigger these groups are, the more they hinder the approach of the nucleophile and the more they slow the reaction.

    Once these other groups get large enough, the approach of the nucleophile becomes high enough energy that the SN1 mechanism (via a carbocation) becomes more of a possibility. SN1 almost certainly happens for tertiary carbons as the carbon is far too hindered for SN2 nucleophilic attack and tertiary carbocations are most stable. For secondary carbons it depends on how large the groups are, how good the nucleophile is and what type of solvent you use.

    If you're only doing A level you might not need to know this bit, but you can get some unusual examples where if the carbon being attacked is a primary carbon, but the one alkyl group is a tertiary butyl group or larger, the reaction will not proceed by SN2 at all because the tertiary butyl group is so big it completely blocks the approach of the OH-. But it's still a primary carbon so the corresponding primary carbocation would be too unstable for SN1. Instead the molecule will rearrange itself by shifting a methyl so that it forms a tertiary carbocation and proceeds by SN1.

    This page explains nucleophilic substitution fairly well if my explanations were confusing: http://www.chemguide.co.uk/mechanism...ub/whatis.html
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Updated: January 27, 2010

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