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parfee
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 Posted: Wed Jul 18, 2007 1:38 am Post subject: Question 31 |
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I understand from the explanation that SN1 can't be the mechanism since the reaction depends on both molecules. And also, there is no proton to extract to create a carbocation.
However, when I looked at this question during test, I thought the carbon is bounded to 2 carbons and doubled bonded to oxygen (what order is that? 3 or 4?), then SN2 can't happen due to steric hindrance.
So is this an exception or SN2 could work on highly substituted carbons, just slowly? |
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admin
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Joined: 08 Dec 2003
Posts: 2176
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| Posted: Thu Jul 19, 2007 1:13 pm Post subject: |
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| Quote: | | However, when I looked at this question during test, I thought the carbon is bounded to 2 carbons and doubled bonded to oxygen (what order is that? 3 or 4?), then SN2 can't happen due to steric hindrance. |
Actually, the carbonyl carbon is neither tertiary nor quartenary. The definition for the order is based on how many R groups are attached to the central carbon. Thus it is a secondary carbon.
Clearly, steric hindrance is not an important factor here because the central carbon is in the trigonal planar position as opposed to being in the center of a tetrahedron (which is a 3 dimensional structure).
And finally, the reaction in question is not SN2 but rather, as the question suggests, it is the mechanism "most consistent with" an SN2 reaction. Why? (1) it is 2nd order as you mentioned; (2) there is an O at the carbon nucleus which is substituted for an N. |
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jsfkt78927
Joined: 06 Jul 2010
Posts: 44
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| Posted: Wed Jul 21, 2010 10:27 pm Post subject: |
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| I have an additional problem with this question. I can't get around the fact that it is sterically hindered. BECAUSE for Sn2 there has to be a back side attack. With a the double ring strucutre, i would imagine that this would not be favorable. I'm going out on a limb here, but it seems like the way you're describing the geometry of the carbonyl carbon - you're suggesting the nucleophile attacks from either above or below the plane.... which doesn't happen. |
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dltkdgn896512
Joined: 26 Jun 2010
Posts: 36
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| Posted: Tue Aug 23, 2011 4:32 pm Post subject: |
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| jsfkt78927 wrote: | | I have an additional problem with this question. I can't get around the fact that it is sterically hindered. BECAUSE for Sn2 there has to be a back side attack. With a the double ring strucutre, i would imagine that this would not be favorable. I'm going out on a limb here, but it seems like the way you're describing the geometry of the carbonyl carbon - you're suggesting the nucleophile attacks from either above or below the plane.... which doesn't happen. |
choose the best answer, as the instruction says
other options are out of the loop
SN2 may not be good answer but it's the best one... |
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