Q5 P. - Organic Chemistry (Mcmurry)

Question

Which substance in each of the following pairs is more reactive as a nucleophile? Explain.

(a) ${\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}{\mathbf{N}}^{\mathbf{-}}\mathbf{or}{\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}\mathbf{NH}$(b) ${\left(C{H}_3\right)}_{\mathbf{3}}\mathbf{B}\mathbf{ }\mathit{o}\mathit{r}\mathbf{ }{\left(C{H}_3\right)}_{\mathbf{3}}\mathbf{N}$(c) ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{ }\mathbf{or}\mathbf{ }{\mathbf{H}}_{\mathbf{2}}\mathbf{S}$ 

Step-by-Step Solution

Answer

(a) ${\left({\mathrm{CH}}_3\right)}_2{\mathrm{N}}^{-}$ Is more nucleophilic because it is more basic than ${\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}$ and because, a negatively charged nucleophile is more nucleophilic than a neutral nucleophile.

(b) ${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$ Is more nucleophilic than ${\left({\mathrm{CH}}_3\right)}_3\mathrm{B}$. ${\left({\mathrm{CH}}_3\right)}_3\mathrm{B}$ is non-nucleophilic because it has a lone electron pair.

(c) ${\mathrm{H}}_2\mathrm{S}$ is more nucleophilic than ${\mathrm{H}}_2\mathrm{O}$ because nucleophilicity increases in going down a column of the periodic table.

Nucleophilicity

Nucleophilicity is defined as the tendency of an electronegative chemical species or nucleophile to attack on the electrochemical species. It depends on many factors such as charge  and size of the nucleophile.

Identifying reactive nucleophiles from the pair of the following structure

(a) ${\left({\mathrm{CH}}_3\right)}_2{\mathrm{N}}^{-}\mathrm{or}{\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}$

${\left({\mathrm{CH}}_3\right)}_2{\mathrm{N}}^{-}$ Is more nucleophilic because it is more basic than ${\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}$, and because a negatively charged nucleophile is more nucleophilic than a neutral nucleophile.

(b) ${\left({\mathrm{CH}}_3\right)}_3\mathrm{B} \mathrm{or} {\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$

${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$ Is more nucleophilic than ${\left({\mathrm{CH}}_3\right)}_3\mathrm{B}$. ${\left({\mathrm{CH}}_3\right)}_3\mathrm{B}$ is non-nucleophilic because it has a lone electron pair.

Identifying reactive nucleophiles from the pair of the following structure

(C) ${\mathrm{H}}_2\mathrm{O} \mathrm{or} {\mathrm{H}}_2\mathrm{S}$

${\mathrm{H}}_2\mathrm{S}$ is more nucleophilic than ${\mathrm{H}}_2\mathrm{O}$ because nucleophilicity increases when we go down a column of the periodic table.