[buttermellow]

Say I have cis-1-bromo-2-methylcyclopentane reacting with NaOCH3 in an Sn2 reaction. Would my product be trans-1-methoxy-2-methylcyclopentane?

I'm just a little confused as to how the inversion occurs in a ring.

Thanks for you *delete me*

[srihari]

We can take into account only the carbon containing the Br atom for this reaction forgetting temporarily the other substitutents

Using the molecular orbital theory we know that the the ABMO has a larger lobe away from the internuclear axis
in this case the C-Br axis . Only a filled and unfilled orbital can interact and form bonds , two filled orbitals cannot approach each other .
The unfilled ABMO and the filled BMO of OCH3- will interact the filled BMO of OCH3- will move towards the unfilled ABMO of C .
there will be a planar transition state and here there are two possibilities one the partial C-OCH3 bond can break or partial C-Br bond can break the more stable one sticks and the other is expelled .here Br- exits.

Hopefully this's clear .
Sorry that I can't upload an image as I work in a linux environment and I don't know how to make images there.

if u want a simpler or rather non-MO explanation do  tell me I'd be glad to help

Regards
Srinath

[Scatter]

Actually, I would like to hear a simpler non-MO answer.  This is interesting.  We're studying nucleophilic substitution reactions right now.

[srihari]

well take into account the simple carbon .

have  look at this image http://upload.wikimedia.org/wikipedia/en/f/f9/BromoethaneSN2reaction-small.png

I'll explain wrt the reaction in the picture first.
The OH- is attacking the C from the back . if it attacks form the front there is no reaction as it can't come near enough to the carbon
now there's transition state and then C-O bond being more strong than the C-Br bond stays and the C-Br breaks.

now instead of the two hydrogens in the given molecule consider them to be carbons and then connected by two other carbons in series .Remove the top methyl grp . draw this by replacing the hydrogens and see just don't draw in the mind , do it on paper.
now on the Carbon nearest to you let there be a methyl substitution cis to Br , Assuming u now the wedge representations . now OCH3- will attack from behind and thus there is inversion of configuration as above thus the incoming OCH3 grp will be trans to the methyl .

see in the transition state , u''ll come to understand better . the Br , methyl are cis and the OCH3 is opposite to Br and thus is trans to methyl .

hope u got it now.

regards
Srihari.

[jinclean]

Yes ,i agree with the MO answer,the transition in the reaction is so vague to understand it , why the carbon can form five bonds?this question confused me a long time untill the MO theory make a solution to this ,that is the formal answer i think