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What is the order of speed of SN1 and SN2 for the following molecules?

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My idea:

For SN1 I think it is C>A>B>D

C - the carbocation has resonance

A - I believe the double bond acts as an electron donating group and helps stabilize the positive charge on the carbocation.

B - just a secondary carbocation

D - vinylic carbocation are highly unstable

For SN2 I think it is the same order C>A>B>D

C - the allyl group is completely planer and as a result it allows the nucleophile an easier approach to the alpha carbon

Honesty I'm not confident between B and A

A - while the CH2=CHCH2- group isnt planer I think the double bond allows for a more linear geometry of the group and a more condensed electron cloud which results in a smaller steric interference and an easier approach for the nucleophile.

B - all carbons in the propyl group are sp³ hybridization therefore the electron cloud is less linear and condensed resulting in a larger steric interference than compound A

D - vinylic carbons dont react in substitution since the nucleophile has to go through the plane of the electron dense double bond to attack the alpha carbon

What do you guys think is this correct?

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  • $\begingroup$ Compare the acidity of 3-Butenoic acid, (E)-2-Butenoic acid, and Butanoic acid. What do you think? $\endgroup$
    – aventurin
    Aug 21, 2023 at 22:38

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I think your rankings are all correct other than for SN2 I would say B>A and most of the rationale is as well.

In both cases C is definitely fastest, both because of the reasons that you gave but also due to the SN1’ and SN2’ mechanisms where the nucleophile attacks the double directly and the double bond shifts to either kick off the bromine or donate to the carbocation.

D definitely slowest in both cases for the reasons you gave.

For the SN2 I would have B>A just because the weaker rotation barriers will minimise steric interactions. When one considers the transition state in the SN2 this is reasonably considerable as the A transition state will be appreciably higher in energy

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  • $\begingroup$ Thanks a lot for answering!. $\endgroup$
    – sd s
    Aug 22, 2023 at 10:04
  • $\begingroup$ Actually there is something I don't understand, I thought the transitional state in SN2 mechanism has a trigonal bypyramid geometry which is was what I based my explanation upon. Can you try giving me a better insight on the mechanism and nucleophile attack since I'm not quite sure I understand how the double bond on C helps pushing the leaving group. Also another thing, you say A is only faster than B only in SN1 mechanism because in SN2 the double bond limits the movement of the substitutes which results in more steric interference? This is all very interesting I really appreciate the help! $\endgroup$
    – sd s
    Aug 22, 2023 at 10:21

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