Animated Solution for Chemistry - Organic Chemistry: The increasing order of nucleophilicity of the following nucleophiles is (1) ${\text{CH}}_3{\text{CO}}_2^{-}$ (2) ${\text{H}}_2\text{O}$ (3) ${\text{CH}}_3{\text{SO}}_3^{-}$ (4) ${\text{OH}}^{-}$

A Tale of Basicity and Resonance

When evaluating the nucleophilicity of different species, the first thing you must look at is the donor atom—the atom that is actually handing over its electron pair. In our problem, we are comparing four species: acetate ($C{H}_{3}C{O}_2^{-}$), water ($H_{2}O$), methanesulfonate ($C{H}_{3}S{O}_3^{-}$), and hydroxide ($O{H}^{-}$).

Notice a pattern? In every single one of these molecules, the atom donating the electrons is Oxygen.

Nucleophilicity vs. Basicity

Here is a fundamental principle of organic chemistry: When the attacking atoms are the same, nucleophilicity runs parallel to basicity.

This means that the strongest base will also be the strongest nucleophile. But how do we determine which base is the strongest? We use the concept of conjugate acids. A stronger base always comes from a weaker conjugate acid. Therefore, our task simplifies to comparing the stability of these bases (or the strength of their conjugate acids).

Let's analyze the stability of our negatively charged ions. A more stable anion is less eager to donate its electrons, making it a weaker base.

Take the methanesulfonate ion ($C{H}_{3}S{O}_3^{-}$). The negative charge isn't just sitting on one oxygen atom; it is beautifully delocalized over three highly electronegative oxygen atoms through resonance. This extensive delocalization makes the ion incredibly stable. Because it is so stable, it is the weakest base among our anions.

Now, look at the acetate ion ($C{H}_{3}C{O}_2^{-}$). Here, the negative charge is shared between two oxygen atoms. It enjoys resonance stabilization, but not as much as the sulfonate ion. Consequently, it is less stable than sulfonate, making it a stronger base.

What about the hydroxide ion ($O{H}^{-}$)? The negative charge is entirely localized on a single oxygen atom. There is no resonance to help bear the burden. This localized charge makes hydroxide highly reactive and desperate to donate its electrons, crowning it the strongest base of the group.

We haven't forgotten about water ($H_{2}O$). Unlike the others, water is a neutral molecule. Neutral molecules are generally much weaker bases (and weaker nucleophiles) than their negatively charged counterparts.

Putting it all together, the increasing order of basicity is:

Since nucleophilicity parallels basicity in this scenario, the increasing order of nucleophilicity is exactly the same:

This perfectly matches option (b). Always remember to check the donor atom first—it dictates the rules of the game!