Question

For the following

(a) $I^{-}$ (b) $C{l}^{-}$ (c) $B{r}^{-}$
The increasing order of nucleophilicity would be

• A. $B{r}^{-}<C{l}^{-}<I^{-}$
• B. $I^{-}<B{r}^{-}<C{l}^{-}$
• C. $C{I}^{-}<B{r}^{-}<I^{-}$
• D. $I^{-}<C{l}^{-}<B{r}^{-}$

Answer 1

The correct option is C

$C{I}^{-}<B{r}^{-}<I^{-}$

The order of nucleophilicities of halides ions in water (default solvent) will be $C{l}^{-}<B{r}^{-}<I^{-}$ The larger the anion, the softer it is so there is less tendency to be a base. Also, as we go down this group (halogens), the lone pairs tend to have a higher energy (higher principal quantum number) and thus the electron pair of the nucleophile has a higher HOMO (highest occupied molecular orbital). This translates to being a better or a more-capable nucleophile.