[thestein]

I believe I have this right, my explanation goes thus:

"For the ubove conditions (please see image), a secondary haloalkane, and a good nucleophile like I-, the mechanism is likely Sn2. The resulting abundance of the trans isomer could be explained by a front-side attack by the nucleophile. Regardless of which chair conformation the substrate assumes, there will be significant sterric hindrance from the methyl group on the 4-carbon, making a back-side attack less favored than a front side."

Please see the below image.



Link if image is too small:
http://i1226.photobucket.com/albums/ee405/TheStein67/My%20chem%20stuff/IMG_20110508_230711.jpg

[Dan]

Your answer is not correct. S_N2 reactions proceed with inversion by definition - when you say "front side attack by the nucleophile" I read this to mean S_N2 with retention of configuration. I think you're on the right track with S_N2 though - think a bit more about I^-, it's a good nucleophile but is is good at anything else...?

[thestein]

But if the configuration is reversed, wouldn't that produce more of the cis isomer than the trans?

[Dan]

But if the configuration is reversed, wouldn't that produce more of the cis isomer than the trans?

Yes, that's what you'd expect at first glance, that is the point of the question - there's something else going on. Again - think a bit more about I^-, it's a good nucleophile but is is good at anything else...?

[thestein]

But if the configuration is reversed, wouldn't that produce more of the cis isomer than the trans?

Yes, that's what you'd expect at first glance, that is the point of the question - there's something else going on. Again - think a bit more about I^-, it's a good nucleophile but is is good at anything else...?

Its also a good leaving group. Is there a... double Sn2 going on? The Iodine back-sides the Cl, forming the cis, then back-sides that iodine to give the lower energy trans isomer?

[Dan]

Yup, I think that's the answer.

[thestein]

It just seemed unlikely to me. I mean both the methyl and iodine group are pretty big, I was sure that would have something to do with it.

[Dan]

Stereochemical scrambling of alkyl iodides in the presence of iodide salts by an S_N2 mechanism is well known. You can find it in most textbooks and it pops up in organic chemistry exam papers quite regularly. It can happen with bromide too.

[thestein]

Stereochemical scrambling of alkyl iodides in the presence of iodide salts by an S_N2 mechanism is well known. You can find it in most textbooks and it pops up in organic chemistry exam papers quite regularly. It can happen with bromide too.

I'll watch out for that, then. Thank you.

[azmanam]

see also

http://www.chemicalforums.com/index.php?topic=34618.0

[thestein]

Ah, very good. Thanks to you both.