The relative reactivities of acyl compounds towards nucleophilic substitution is acyl chloride> anhydride>ester>amide. I would like to know how this kinetic property is decided. Is it on the basis of better leaving grp or is there another explanation for it?,

  • $\begingroup$ Yes. It is often explained by the stability of the leaving group. Chloride is more stable than carboxylate, which is more stable than alkoxide, which is more stable than the amide anion. Thus, the order of reactivity is as such. $\endgroup$ Apr 14 '18 at 7:25
  • $\begingroup$ @Tan Does stability mean the stability of the leaving grp like Cl- Ion. Cl is more electronegative than the others .Is that why acyl chlorides are more reactive because of the stable leaving grp. Then what about the other 3? How are they stabilised? How is it 'their' relative orders are decided? $\endgroup$
    – Nihal N.L
    Apr 14 '18 at 7:47
  • $\begingroup$ How about arranging all leaving groups according to each of their relative strength as a base? Start with weekest base and end with strongest base. And then, compare with the order of reactivity (@Tan Yong Boon). $\endgroup$ Apr 14 '18 at 8:49

It is likely to be due to the different amounts of partial positive charge which exist on the carbonyl carbon.

We can consider for example the rate of reaction of a given amine (such as aniline) with the acid chloride, acid anhydride and an ester such as the methyl ester. The partial positive charge argument would explain this reactivity series.


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