Question

Arrange the increasing order of nucleophilicity. ${{\text{I}}^{\text{ - }}}$, ${\text{B}}{{\text{r}}^{\text{ - }}}$, ${{\text{F}}^{\text{ - }}}$
a.) ${{\text{I}}^{\text{ - }}}$< ${\text{B}}{{\text{r}}^{\text{ - }}}$< ${{\text{F}}^{\text{ - }}}$
b.) ${{\text{I}}^{\text{ - }}}$> ${\text{B}}{{\text{r}}^{\text{ - }}}$> ${{\text{F}}^{\text{ - }}}$
c.) ${{\text{I}}^{\text{ - }}}$= ${\text{B}}{{\text{r}}^{\text{ - }}}$> ${{\text{F}}^{\text{ - }}}$
d.) ${{\text{I}}^{\text{ - }}}$= ${\text{B}}{{\text{r}}^{\text{ - }}}$= ${{\text{F}}^{\text{ - }}}$

Answer 1

Hint: Nucleophilicity can be compared using the size of the given nucleophiles. If you know the concept of nucleophilicity, then the size comparison leads you to the answer.

Complete step by step answer:
First of all, you need to know what nucleophilicity is. So, nucleophilicity is the ability (and tendency) to donate electrons to an electrophile, an electron-deficient atom or molecule.
Here, Iodine has a larger size (size increases down the periodic table). Iodine can disperse the negative charge on a large space as compared to the smaller other elements in this group. Since, it has dispersed its negative charge on a large radius, it has a low tendency to donate electrons to an electrophile. Fluorine has a small size, therefore negative charge cannot be dispersed in this small radius. So, it can donate electrons more readily. Therefore, the correct order of increasing nucleophilicity is ${{\text{I}}^{\text{ - }}}$< ${\text{B}}{{\text{r}}^{\text{ - }}}$< ${{\text{F}}^{\text{ - }}}$. So, the correct option is (a).

Additional Information:
There are four factors that affect nucleophilicity, they are charge, electronegativity, solvent and Steric bulk.
Conjugate base is always a better nucleophile. Hydroxide ion is always a better nucleophile than water.

Note: Don’t be confused with basicity and nucleophilicity. First of all, remember that basicity is a subset of nucleophilicity. All nucleophiles are Lewis bases as they donate lone pairs of electrons. A base is just the name we give to a nucleophile when it’s forming a bond to a proton.