Which halogenoalkane reacts fastest with sodium hydroxide?

A. 1-iodobutane B. 1-chlorobutane C. 2-chloro-2-methylpropane D. 2-iodo-2-methylpropane

Iodine has a larger radius, thus less electronegative making it a better leaving group. And since OH is a protic substance it favors Sn1, which is a tertiary, so the answer must be d... but it says that the answer is b.. how????

  • 2
    $\begingroup$ As others have noted below, this seems line an $S_{N}2$ reaction and iodine is an excellent leaving group, so the answer seems to be "A". Let us know if you hear back about why "B" is correct or if the answer key was in error. $\endgroup$
    – ron
    Apr 28, 2017 at 17:47

1 Answer 1


Yes iodine's good leaving group. the leaving ability is best described by the Gibbs energy change that occurs.

So since chlorine is a poor leaving group so that make the reaction slower as less energy is available. So I think that you are right so far.

  • 1
    $\begingroup$ Iodide is a much better leaving group than chloride. $\endgroup$ Feb 27, 2017 at 17:15
  • $\begingroup$ ohh! sorry about that I just thought the reverse case ; $\endgroup$ Feb 27, 2017 at 17:18
  • 3
    $\begingroup$ Also, this looks like a better candidate for $\mathrm{S}_{N}2$ chemistry... $\endgroup$
    – Zhe
    Feb 27, 2017 at 17:44

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