Question

Among the following the strongest nucleophile is:
A.\[{{\text{C}}_2}{{\text{H}}_5}{\text{SH}}\]
B.\[{\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ - }\]
C.\[{\text{C}}{{\text{H}}_3}{\text{N}}{{\text{H}}_2}\]
D.\[{\text{NCCH}}_2^ - \]

Answer 1

Hint: To answer this question we must have the knowledge about nucleophilic character. More is the electron density, more will be the nucleophilicity or donation power.

Complete step by step solution:
A substance is known as a base when it donates electrons to protium or \[{{\text{H}}^ + }\]and a substance is known as nucleophile when it donates electrons to carbocation. Both nucleophiles and bases are electron donating species.
A stronger nucleophile is that which can easily donate electrons to the carbocation or electron deficient species. It depends on various factors such as the negative charge has more electron density than the lone pair. Both can donate electrons but negative charge donates more. More is the electronegativity of the electron donating species, lesser will be its tendency to donate electrons. If the lone pair is in resonance then it will donate less because the charge will delocalize. So keeping in mind the above factors we will determine find out the strong nucleophile:
In option B and D the negative charge is delocalized and is highly stable by resonance with carbonyl carbon and hence are not available for donation, whereas in option A and C both have localized lone pairs of electrons. Now the role of electronegativity will come. We know the electronegativity of nitrogen is more than the electronegativity of sulphur. So nitrogen won’t easily give the electron and sulphur can give more easily and the sulphur site will be more nucleophilic.

So the correct option is A.

Note: The product \[{{\text{C}}_2}{{\text{H}}_5}{\text{SH}}\] is known as ethanethiol. The group \[{\text{ - SH}}\] is known as thiol. This is formed by just replacing the oxygen with sulphur in the alcohol functional group. As ethanol has hydrogen bonding, this ethanethiol does not have hydrogen bonding due to the presence of sulphur instead of oxygen.