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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: thestein on May 09, 2011, 12:25:49 AM

Title: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: thestein on May 09, 2011, 12:25:49 AM
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.

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi1226.photobucket.com%2Falbums%2Fee405%2FTheStein67%2FMy%2520chem%2520stuff%2FIMG_20110508_230711.jpg&hash=5dfa2b10bc89c197f305c01f2606fcdcf27c85bd)

Link if image is too small:
http://i1226.photobucket.com/albums/ee405/TheStein67/My%20chem%20stuff/IMG_20110508_230711.jpg
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: Dan on May 09, 2011, 03:17:25 AM
Your answer is not correct. SN2 reactions proceed with inversion by definition - when you say "front side attack by the nucleophile" I read this to mean SN2 with retention of configuration. I think you're on the right track with SN2 though - think a bit more about I-, it's a good nucleophile but is is good at anything else...?
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: thestein on May 09, 2011, 11:48:41 AM
But if the configuration is reversed, wouldn't that produce more of the cis isomer than the trans?
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: Dan on May 09, 2011, 02:20:12 PM
Quote from: thestein on May 09, 2011, 11:48:41 AM
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...?
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: thestein on May 09, 2011, 03:16:30 PM
Quote from: Dan on May 09, 2011, 02:20:12 PM
Quote from: thestein on May 09, 2011, 11:48:41 AM
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?
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: Dan on May 09, 2011, 03:28:38 PM
Yup, I think that's the answer.
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: thestein on May 09, 2011, 04:00:58 PM
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.
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: Dan on May 09, 2011, 05:51:26 PM
Stereochemical scrambling of alkyl iodides in the presence of iodide salts by an SN2 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.
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: thestein on May 09, 2011, 05:56:42 PM
Quote from: Dan on May 09, 2011, 05:51:26 PM
Stereochemical scrambling of alkyl iodides in the presence of iodide salts by an SN2 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.
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: azmanam on May 09, 2011, 06:10:23 PM
see also

http://www.chemicalforums.com/index.php?topic=34618.0
Title: Re: Why a back-side Sn2 won't work on a 1-4 substituted chair.
Post by: thestein on May 09, 2011, 06:23:25 PM
Ah, very good. Thanks to you both.