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Substitution vs Elimination on halides

  1. Jun 24, 2013 #1
    If you add a strong base to a halide, you get mostly the alkene. If you add a weak base, especialy on primary halides not branched on the β carbon, the product is mostly the substituted. Why is that?

    1) The mechanism for the substitution reaction is the heterotytic break of the C-X (where X is the halide) bond, producing the carbocation and X-.
    The carbocation, though, is more stable as more alkyl groups are added to the α (or β) carbon. In other words, terciary halides and secondary halides produce a much more stable carbocation than primary halides. So why is the primary the only one that can be feasibly substituted?

    2) Why a stronger base facilitates the elimination?

    3) The reactivity order on substitutions is I>Br>Cl>F. Why?
     
    Last edited: Jun 24, 2013
  2. jcsd
  3. Jun 24, 2013 #2
    1) they are substituted in an Sn2 reaction because the carbocation is unstable and will not stay there, it needs to have an attachment to be stable

    2) bases abstract hydrogens, whereas nucleophiles do an attack on the carbon. You need to understand the difference between the two

    3) Look at leaving group ability and understand what a leaving group is.
     
    Last edited: Jun 24, 2013
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