Question

Which of the following is the strongest nucleophile?
(A) $O{{H}^{-}}$
(B) $C{{H}_{3}}{{O}^{-}}$
(C) $C{{H}_{3}}{{S}^{-}}$
(D) ${{C}_{2}}{{H}_{5}}{{O}^{-}}$

Answer 1

Hint: Recollect the meaning of nucleophile. Read about the concept of conjugate acids and bases. Write the conjugate reactions for each of the given species. Find out the weakest reacting product (acid/base) and that nucleophile forming weakest product will be the one which is strongest nucleophile. Keep in mind electronegativity and then solve this question.

Complete step by step solution:
-A nucleophile is a substance which is electron-rich and can donate its electrons to an electron-deficient species which is an electrophile. Nucleophile literally means (nucleus-loving) and electrophile means (electron-loving).
-Now, let’s brush up the conjugate acid-base concept. So, it says that a strong base will form a weak conjugate acid and a strong acid will form a weak conjugate base and vice-versa.
-Now, look at the question. We can see that all options given are bases. Let’s determine their conjugate acids to make the problem a little simpler.
 \[O{{H}^{-}}\xrightarrow{{{H}^{+}}}{{H}_{2}}O\]
\[C{{H}_{3}}{{O}^{-}}\xrightarrow{{{H}^{+}}}C{{H}_{3}}OH\]
$C{{H}_{3}}{{S}^{-}}\xrightarrow{{{H}^{+}}}C{{H}_{3}}SH$
${{C}_{2}}{{H}_{5}}{{O}^{-}}\xrightarrow{{{H}^{+}}}{{C}_{2}}{{H}_{5}}OH$
-So, we can say that methyl thiol is the most acidic because of less electronegativity of sulphur than oxygen and also larger size which make S-H bond longer and weaker which implies proton can be donated easily.
-We know, water is neutral and amphoteric but it still has a tendency to donate protons due to the presence of electronegative oxygen atoms. So, let’s compare ethanol and methanol. Methanol is a stronger acid than ethanol because of inductive effect (-I effect). In ethanol, it is easier to donate hydroxyl groups than hydrogen so it becomes a weaker base than methanol.
-Order of acidity can be given as: $C{{H}_{3}}SH>{{H}_{2}}O>C{{H}_{3}}OH>{{C}_{2}}{{H}_{5}}OH$
-So, we can ${{C}_{2}}{{H}_{5}}{{O}^{-}}$ conclude that ethanol is the weakest acid among all the other conjugate-acids. That implies, ethoxide ion is the strongest base because the conjugate base of ethanol will be a strong acid.


Therefore, ${{C}_{2}}{{H}_{5}}{{O}^{-}}$ is the strongest nucleophile.


Note: This question can also be solved using $p{{k}_{a}}$ values. Higher the $p{{k}_{a}}$, stronger is the base and weaker is the acid. While solving such application type questions, be crystal clear about basic concepts like inductive effect, electronegativity, conjugate acid-base concept, etc.