1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

SN2 reactivity

  1. Aug 5, 2012 #1
    1. The problem statement, all variables and given/known data
    The decreasing order of rate of SN2 reaction is:
    a)CH3-Cl
    b)CH3-CO-CH2-Cl


    2. Relevant equations



    3. The attempt at a solution
    I have been trying hard to find the reason why i am wrong. It's obvious that less hindrance, more reactivity towards SN2. Using the same logic, the answer should be a>b but it is b>a. Now i don't understand where i went wrong. There is more hindrance in b than a.
     
  2. jcsd
  3. Aug 5, 2012 #2
    Hello?? Would someone like to help?
     
  4. Aug 5, 2012 #3

    AGNuke

    User Avatar
    Gold Member

    Which hindrance are you talking about? Steric?

    Steric hindrance will matter if the attacked carbon is secondary or tertiary. Since the attacked carbon in both the cases are primary, steric hindrance is out of question.

    Now, try forming the substrate, i.e. the carbocation, which is formed after the chloride atom leaves in both the cases.

    For the hint, more stable carbocation will be lax in proceeding in SN2 reaction than less stable one.
     
  5. Aug 5, 2012 #4
    Yes, i am talking about Steric. :smile:

    Hey, wait, i think there is no carbocation formation in the SN2 reaction mechanism.
     
  6. Aug 6, 2012 #5

    AGNuke

    User Avatar
    Gold Member

    A substrate is formed, when neither the attacking group is bonded completely, or the leaving group leaves it completely. This substrate is like a forming carbocation.

    This substrate has two ways to go - either trace back or advance. If the carbocation, which is "forming" is stable, the reaction will proceed forward, if not, it will trace back.

    It is true, that carbocation independently is not formed, but the substrate is nonetheless under scrutiny for determining the answer.

    This is like formation of substrate in Mercuration-Demercuration process, if you had read it.
     
  7. Aug 6, 2012 #6

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Methyl chloride is not primary. Chloroacetone is bulkier around its reaction center than methyl chloride so steric hindrance is different for these two substrates. But there is something special about chloroacetone that has to do with the stability of the transition state.

    We're talking SN2 here. There is no carbocation formed after the chloride leaves.

    You have this backwards.
     
  8. Aug 6, 2012 #7

    AGNuke

    User Avatar
    Gold Member

    Point taken.


    I never mentioned independent carbocation. I only said that to determine the answer, making the substrate as independent carbocation can help.

    I mentioned the substrate stage. And I also gave an example which I believe he had read (if he is also pursuing for JEE)



    Figures. I myself got doubts soon afterwards. Maybe I jotted my notes incorrectly.

    As a matter of fact, I believe CH3-(C=O)-CH2+ is less stable as electromeric effect of carbonyl group will be disastrous.
     
  9. Aug 6, 2012 #8

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Right, but if you take your analogy a little further, solvents that stabilize this theoretical 'independent cation' better should lead to faster reaction rates. This is counter to what is known about the reaction, so using analogies has its risks, especially for the first time student wading through all of this nomenclature, energy diagrams, steric discussions, etc. I prefer to discuss it in a more precise manner.

    By 'electromeric effect' I assume that you mean inductive effect. Yes, the inductive effect destabilizes any cationic character at the reaction center but more important is the adjacent pi electron system to the developing sp2-like transition state.
     
  10. Aug 6, 2012 #9
    My book says nothing about a positive charge being developed on carbon during the SN2 mechanism. I am using the book Organic Chemistry by Morrison and Boyd.

    Thanks for the reply chemisttree! I was waiting for you to jump in. :tongue2:
    Well, i was thinking about the same thing. It seems like that the stability has to do with the carbonyl group attached. If i draw the transition state,
    24df2id.png
    Nu is the attacking nucleophile.
    You say that it has to do with the adjacent pi electron system, i don't see what it can do here. I can only think of the inductive effect which stabilizes the negative charge developing on Nu and Cl.
     
    Last edited: Aug 7, 2012
  11. Aug 7, 2012 #10

    AGNuke

    User Avatar
    Gold Member

    The substrate which is formed, Carbon has partial positive charge. If not, how come two δ- groups are attached to a Single Carbon without carbon having δ+ charge? See water molecule for a change.

    SN2-TS.png

    According to excerpt I posted, Carbonyl group is an electron withdrawing group. So it seems like withdrawing electrons from electron deficient group is not a good idea, so the reaction tries to avoid this, thus the molecules goes into SN2 reaction.
     
    Last edited: Aug 7, 2012
  12. Aug 7, 2012 #11
    According to my book, no charge develops on the carbon. You can check the book at google books. Check the Organic Chemistry, 6/e by Morrison and Boyd and see page 222.
     
  13. Aug 7, 2012 #12

    AGNuke

    User Avatar
    Gold Member

    Damn. I need that book ASAP.

    It says that if the rate of attack is equal to rate of leaving, the carbon atom does not accumulate considerable polar charge.

    Now read Page 245
     
    Last edited: Aug 7, 2012
  14. Aug 8, 2012 #13

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You will note that the transition state you have illustrated here has three groups aligned roughly equidistant in a plane and the incoming and leaving groups aligned along an axis orthogonal to that plane. This is a description of an sp2 hybridized reaction center. The adjacent pi system (C=O) can conjugate with this reaction center. How might this conjugation affect the energy of the transition state? What does delocalization do to the energy of the transition state?

    BTW the difference between the rate of reaction of chloroacetone relative to methyl iodide is on the order of ten thousand.
     
  15. Aug 8, 2012 #14

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Remember we started with a neutral molecule and a negatively charged nucleophile. That negative charge of the nucleophile is smeared out between approaching and leaving group. The positive charge resides wholly on the counterion of the nucleophile, which you haven't shown. In the case where a neutral molecule nucleophile approaches, the corresponding positive charge develops on the attacking nucleophile. The substrate stays remarkably neutral.
     
    Last edited: Aug 8, 2012
  16. Aug 8, 2012 #15

    AGNuke

    User Avatar
    Gold Member

    Yes, that seems the case. What if the rate of attacking isn't equal to rate of leaving. Like in this case, due to steric reasons, attacking may be bit slower...?

    I studied some graphs of nucleophilic substitution rate of methyl, ethyl, sec. propyl and tert. butyl. They said that due to steric hindrance, sec.propyl and tert.butyl's substrate experiences charge, and they undergoes SN1 substitution, rather than SN2 substitution as in methyl and ethyl case.

    I was taught (yesterday only) that if we see something causing the difference in rate of attacking and leaving (steric hindrance) we must try forming carbocation. If it is stable, then SN1, else, SN2. Would be enough for questions asked in JEE.

    God knows what I am drinking right now... :frown:
     
  17. Aug 9, 2012 #16

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If the rate of attack and the rate of leaving were not the same we would have two cases.
    1. Rate of attack is faster than rate of leaving. Since we are bringing electrons with a nucleophile, the reaction center briefly contains too many electrons (?) and we are dealing with carbanions (???), 5 bonds to carbon and so forth and thus any discussion of carbocation stability is utter fantasy as is this possibility.

    2. Rate of attack is slower than rate of leaving. Since the bond breaking process is occuring faster than the bond making process, we are left with an electron deficient reaction center... a carbocation. It's stability governs the reaction order (substitution, nucleophilic, unimolecular) so we aren't really talking about SN2 reactions at all.
     
  18. Aug 9, 2012 #17

    AGNuke

    User Avatar
    Gold Member

    So, I conclude that it can deviate from SN2 mechanism.
     
  19. Aug 9, 2012 #18

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    SN1 reactions are defined as having the rate determining step being the generation of the charged intermediate. If the question were asking about SN1 reactions we would be discussing the stability of charged intermediates and leaving groups breaking their bond with the substrate faster than the attacking nucleophile forming a new bond.

    Good discussion but for a different problem.
     
  20. Aug 9, 2012 #19

    AGNuke

    User Avatar
    Gold Member

    Believe me, our exam loves to fiddle with our minds over such a small things. And since the exam is now subjective instead of objective, it is more difficult, with writing a sentence of wisdom. Everything must be crystal clear. Thanks for that.
     
  21. Aug 10, 2012 #20
    Woah! Seems like a lot of discussion happened here.

    I don't get it, what do you mean by conjugate with the reaction center? :frown:

    I am not sure but are you talking about the resonance effect here?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: SN2 reactivity
  1. SN2 reaction (Replies: 5)

  2. SN2 reaction (Replies: 1)

  3. Sn2 rxn (Replies: 1)

  4. E2 vs SN2 (Replies: 5)

Loading...