Question

Order of decreasing basic strengths of halides is :

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

Answer 1

The correct option is B $F^{-}>C{l}^{-}>B{r}^{-}>I^{-}$

$\text{Basic strength}\propto \frac{1}{\text{Acidic strength of conjugate acid}}$
The conjugate acids of the respective species are :
$HF,HCl,HBr,HI$
Down the group the electronegativity decreases and the size increases.
Thus, it becomes easy to lose $H^{+}$ ion. This causes the increase in acidic strength down the group.
Conjugate acids strength order :
$HI>HBr>HCl>HF$

Hence, the basic strength order is:
$F^{-}>C{l}^{-}>B{r}^{-}>I^{-}$

Alternatively, in $I^{-}$ due to bigger size of $I$, the negative charge is more dispersed and stable, hence $I^{-}$ is weakest base.