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Attempt Since CN is better nucleophile than both OMe and Cl, I believe that it should replace both. But none of the options match my answer. Where am I wrong? Please help.

  • 5
    $\begingroup$ MeO- is a really poor leaving group $\endgroup$ – Waylander Apr 25 '20 at 6:45
  • $\begingroup$ But compared to CN? $\endgroup$ – Aditya suresh Apr 25 '20 at 6:51
  • 2
    $\begingroup$ CN- is a better leaving group. The product here is b $\endgroup$ – Waylander Apr 25 '20 at 7:32
  • $\begingroup$ Nucleophilicity is an entirely different concept from leaving group ability. One does not imply the other. $\endgroup$ – Zhe Apr 30 '20 at 3:00

You need to reckon with how strong a base the nucleophile is compared with what you are trying to displace. Only if the nucleophile is the stronger base can you expect the reaction to go based on thermodynamics.

Thus, both of the following reactions can go:

$\ce{RCl + OCH3^- -> ROCH3 +Cl^-}$

$\ce{RCl + CN^- -> RCN +Cl^-}$

because both nucleophiles are stronger bases towards carbon than the chloride ion that would be displaced. You can then say that the second reaction will go faster with a better (kinetic) nucleophile. But if you have

$\ce{ROCH3 + CN^- -> RCN +OCH3^-}$

where you are trying to displace the strongly basic methoxide ion rather than adding it as a nucleophile like the first reaction above, then cyanide ion is too weakly basic to push through. Cyanide ion can be a good nucleophile only when displacing a weaker base or adding to an appropriate unsaturated compound (such as a carbonyl compound) without displacement.

Try it now.


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