1. Not finding help here? Sign up for a free 30min 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!

E2 vs SN2

  1. Jul 16, 2013 #1
    1. The problem statement, all variables and given/known data
    Why would this reaction favor E2 over SN2? Since its a secondary halide reacting with a strong base/nucleophile, and all other conditions being equal isn't SN2 favored? Or is ethoxide somehow a stronger base than nucleophile? If so, how can you tell that it's a better base than nucleophile? Below is the question.

    What is the major product formed from the reaction of 2-chlorobutane in aqueous sodium ethoxide?
    A.

    2-ethoxybutane
    B.

    1-butene
    C.

    cis-2-butene
    D.

    trans-2-butene

    Correct Answer
    Explanation:
    D. 2-Chlorobutane, a secondary alkyl halide, will react with a strong base, such as ethoxide, via an E2 mechanism. Choice A can be eliminated because it is the result of a substitution reaction. Elimination reactions generally follow Zaitsev's rule, which states that the product with the most substituted double bond is formed (provided an unhindered base is used). 1-Butene has a terminal double bond, which is less substituted than the double bond in 2-butene, so choice B can be eliminated. The most stable geometric isomer will have the two largest groups on opposite sides of the double bond, so choice C can be eliminated.


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 17, 2013 #2

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    For secondary alkyl halides, any nucleophile that is a base stronger than acetate (pKa 4.8) will deprotonate rather than undergo substitution. What do you think this says about the acidity of secondary alkyl halides with alpha protons?
     
  4. Jul 18, 2013 #3
    Hi Chemisttree,
    I think I understand but I'm not sure. I'm confused on why you used pka of acetate, for comparison, to determine basicity. Are you just using this as a starting point converting to pkb and then looking at basicity? If so couldn't we just look at each pkb and pick the one with the smallest pkb to determine the stronger base. Is there a way to determine basicity, relative to nucleophilicity without using pkb/pka values? I read there's a trend where basicity is highest going towards the upper left hand side of the periodic table and a trend where nucleophilicity is highest going towards the bottom left hand side of the periodic table but they aren't explained and I can't find information on what these trends are based on, relative to each other. Would you happen to know what these trends are based on? About your question, this basically tells us that the alpha proton in this case is not very acidic so we need a strong base right?
     
    Last edited: Jul 18, 2013
  5. Jul 19, 2013 #4

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    We use either pKa or pKb as a measure of basisity. Of course when we use pKa, we assume the protonated form. pKa of acetate being 4.8 is actually that of the acid form... acetic acid. Kind of confusing to speak of base strength in terms of acidity but you will note that they are related by the formula pKa +pKb = 14.

    Is there a way to determine basisity relative to nucleophilicity without using pKa/pKb? Not really. You see, even a fairly weak base like acetate is a pretty good nucleophile. Pretty good towards the right alkyl halide, that is. Good nucleophile for primary alkyl halides without beta carbon branching, not so good toward secondary, tertiary or primary with beta carbon branching.

    Actually it was a bit of a trick question. If a base only a little stronger than acetate is capable of deprotonating alpha to the C-X bond you might be tempted to assume that something like chloroethane is a fairly strong acid and might exist in equilibrium as:
    CH3-CH2-X <------> CH-2-CH2-X + H+

    which doesn't happen... hence the "2" subscript for an E2 elimination. Plucking off that proton doesn't happen without the acetate and a particular arrangement of the C-Cl bond anti to the proton being removed. It is more than an acid/base thing.
     
  6. Jul 22, 2013 #5
     
  7. Jul 23, 2013 #6
    Actually never mind, I just realized the reason why is because it's a weak base.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted