JEE Main · 2005 · 08 May 2005 · Offline · General Organic Chemistry

Nucleophilicity here depends on how available and how localised the lone pair is. Cyanide leads the field because its charge sits on carbon, a less electronegative atom that holds its electrons loosely and offers them readily. Methoxide comes next, a strong base with its charge on a single oxygen; acetate is weaker because resonance smears its charge over two oxygens; and the sulphonate (tosylate) ion is the poorest, its negative charge spread thinly across the three oxygens of the $-\mathrm{SO}{}_3{}^{-}$ group and thus very reluctant to attack. The decreasing order runs cyanide, then methoxide, then acetate, then tosylate.