Which one of the following reactions does not involve the donation of an electron pair?

A) $\ce{H+ + CH3NH2 -> CH3NH3+}$

B) $\ce{AlCl3 + Cl- -> AlCl4-}$

C) $\ce{CH3Cl + CN- -> CH3CN + Cl-}$

D) $\ce{Cl2 + 2I- -> 2Cl- + I2}$

The answer is D, but I don't know why.

The equation could be split into two half equations:

$\ce{Cl2 +2e- -> 2Cl-}$

$\ce{2I- -> I2 + 2e-}$

But surely this shows the iodide ion is donating a pair of electrons to the chlorine atom?

I'm sure really what approach I should take for this question.


D donates 2 electrons, not an "electron pair." There is a subtle but important difference here because D is a reaction that takes place via the transfer of single electrons (though in the end, 2 are transferred in total).

So, A, B, and C are examples of Lewis acid-base neutralizations, but D is not. D is classical redox reaction.

  • $\begingroup$ How could one differentiate between the two (electron donor and electron pair donor) ? $\endgroup$ – doubleOrt Jan 18 at 7:45
  • $\begingroup$ Mechanistic studies, or in most cases, pattern recognition. $\endgroup$ – Zhe Jan 18 at 12:41

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