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Topic: Sn2/Sn1 predictions involving solvent polarity  (Read 1881 times)

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Offline spirochete

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Sn2/Sn1 predictions involving solvent polarity
« on: November 20, 2016, 07:52:24 PM »
The following results from an answer key to a Sophmore level text (Vollhardt) are bothering me:

A) 2-bromobutane + Cl- ---> Sn2 reaction, when solvent is DMF

B) 2-bromobutane+ Cl- ---> Sn1 reaction, when solvent is nitromethane

C) t-butyl bromide + I- ---> no reaction when solvent is DMF. It says that DMF solvent is not sufficiently polar to support carbocation formation for the Sn1 reaction.

T-butyl bromide is tertiary, whereas the 2-bromobutane is secondary. I am especially confused about t-butyl bromide not reacting at all.

More polar solvent favors Sn1 as far as I know. The dielectric constant of nitromethane is 35.9 vs 38.5 for DMF. That means Nitromethane is less polar, right? So it seems that if anything, DMF should favor Sn1 more than nitromethane? And how is it even possible to remember all these random cases?
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Offline orgopete

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Re: Sn2/Sn1 predictions involving solvent polarity
« Reply #1 on: November 20, 2016, 07:59:29 PM »
It looks like an error to me. If A) is an SN2 reaction, then I'm expecting SN2 for B) as well.  The SN2 rate should still be greater than the SN1 rate.
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Offline Vidya

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Re: Sn2/Sn1 predictions involving solvent polarity
« Reply #2 on: November 21, 2016, 03:51:39 AM »

More polar solvent favors Sn1 as far as I know. The dielectric constant of nitromethane is 35.9 vs 38.5 for DMF. That means Nitromethane is less polar, right? So it seems that if anything, DMF should favor Sn1 more than nitromethane? And how is it even possible to remember all these random cases?

I can understand your frustration on exceptions in organic chemistry. In lewis dot structure of nitromethane ,one of the oxygen atom has formal negative charge and may be this oxygen is giving extra stability to carbocation .

Offline spirochete

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Re: Sn2/Sn1 predictions involving solvent polarity
« Reply #3 on: November 21, 2016, 07:58:23 PM »
The formal charge on oxygen in the nitro group seems like a reasonable after the fact rationalization. But I would think that would be accounted for in the dielectric constants. Perhaps not.

It seems very rare that teachers expect students to learn these subtleties at the sophomore level. I am more used to seeing a tertiary halide + a weak base, and simply assuming that it will be an Sn1 reaction.
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Offline spirochete

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Re: Sn2/Sn1 predictions involving solvent polarity
« Reply #4 on: November 21, 2016, 08:13:29 PM »
It looks like an error to me. If A) is an SN2 reaction, then I'm expecting SN2 for B) as well.  The SN2 rate should still be greater than the SN1 rate.

I did find some other much more blatant errors in the answer key. But they in the last problem they seem to focus on DMF as being not very polar, compared to nitromethane. I hope that it's a mistake!
I'm an online organic chemistry tutor and teacher. I have a Master's in organic chemistry, and have years of experience as a TA and teacher. Visit my website http://www.organicchemistrysolutions.com for more information.

Offline Babcock_Hall

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Re: Sn2/Sn1 predictions involving solvent polarity
« Reply #5 on: November 22, 2016, 05:51:46 PM »
I suggest sending an email with your suggestions to the author.

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