Question

Which of these is not a nucleophile? Explain why?
${H_2}O$ , $C{H_3}{O^ - }$ , $N{H_3}$ , $NH_4^ +$ .

Answer 1

A nucleophile is a species which has an excess of electrons and as the name suggests it is nucleus loving. For more structural representation a nucleophile either has a negative charge on it or a lone pair of electrons. Lone pairs of electrons if present in a species are used to donate on electrophilic sites for bond making.

Complete step-by-step answer:
We know that a nucleophile is a species which has an excess of electrons and as it has an excess of electrons it will attack on an electrophilic site having lesser electron density. Take the example of hydroxide ion ${}^ - OH$ it is a nucleophile because it is naturally present as such. It generally attacks the electron- deficient carbon of the carbonyl group.

So, among the options given let’s see them individually firstly we have ${H_2}O$ if you know that oxygen used to have a lone pair of electrons on it you called it as nucleophile and it is so. Yes, water is represented as and it will attack the electron deficient centre. Now, let’s see the next option which is $C{H_3}{O^ - }$ as we are seeing a negative charge on it, it will also act as a nucleophile and it will also attack the electron deficient centre.

We have $N{H_3}$ and $NH_4^ +$ both have nitrogen but the difference is that ammonia contains lone pairs of electrons on nitrogen while ammonium ion $NH_4^ +$ don’t have lone pairs of electrons for donation. Among all the options we get, , $C{H_3}{O^ - }$ and as nucleophiles.

Note: The attacking reagents nucleophile and electrophile have electron excess or electron deficiency. In case of electrophile, don’t consider a species which acts as electrophile by taking an atom for forming positive charge on it. Like ${H_3}{O^ + }$ it has a positive charge but it is not considered an electrophile because this positive charge comes on it by taking hydrogen from any species. So always consider species which are naturally present.