Animated Solution for Chemistry - Organic Chemistry: The decreasing order of nucleophilicity among the nucleophiles

When faced with a question asking to rank nucleophiles, it is easy to get overwhelmed by the different structures. In this problem, we are given four distinct nucleophiles: the acetate ion ($C{H}_{3}CO{O}^{-}$), the methoxide ion ($C{H}_3{O}^{-}$), the cyanide ion ($C{N}^{-}$), and the bulky p-toluenesulfonate ion, commonly known as the tosylate ion ($Ts{O}^{-}$).

Our objective is to arrange them in decreasing order of their nucleophilic strength. To do this, we need a reliable framework rather than just memorizing trends.

Nucleophilicity vs. Basicity

How do we compare their nucleophilic strengths? A very reliable rule of thumb in organic chemistry is that a stronger base is generally a stronger nucleophile, provided that steric hindrance isn't the primary differentiating factor.

But how do we determine which ion is the strongest base? We look at their conjugate acids! The fundamental principle of acid-base chemistry states that a weaker acid always produces a stronger conjugate base. Therefore, if we can rank the acidic strength of their conjugate acids, we can simply reverse that order to find the basic (and nucleophilic) strength.

Let's apply this concept by adding a proton ($H^{+}$) to each of these ions to form their respective conjugate acids:

1. Acetate ($C{H}_{3}CO{O}^{-}$) gains a proton to become acetic acid ($C{H}_{3}COOH$). 2. Methoxide ($C{H}_3{O}^{-}$) becomes methanol ($C{H}_{3}OH$). 3. Cyanide ($C{N}^{-}$) becomes hydrogen cyanide ($HCN$). 4. Tosylate ($Ts{O}^{-}$) becomes p-toluenesulfonic acid ($TsOH$).

Now we have four distinct acids to compare: $TsOH$, $C{H}_{3}COOH$, $HCN$, and $C{H}_{3}OH$.

Let's arrange these acids by their strength.

p-Toluenesulfonic acid ($TsOH$) is a sulfonic acid. It is highly acidic because its conjugate base ($Ts{O}^{-}$) is incredibly stable. The negative charge on the oxygen is delocalized over three highly electronegative oxygen atoms via resonance.

Acetic acid ($C{H}_{3}COOH$) is next. It is a carboxylic acid, and its conjugate base (acetate) is stabilized by resonance over two oxygen atoms. It is a weak acid, but stronger than the remaining two.

Hydrogen cyanide ($HCN$) is a weak inorganic acid with a $p{K}_a$ of around 9.2. The cyanide ion is relatively stable due to the $sp$ hybridized carbon, but it lacks the powerful resonance stabilization seen in oxygen-containing anions.

Finally, methanol ($C{H}_{3}OH$) is an extremely weak acid, even weaker than water. The methoxide ion has a localized negative charge on a single oxygen atom with no resonance stabilization, making it highly unstable and reactive.

Therefore, the order of acidic strength is:

Deducing Nucleophilicity

Since we have established the acidic strength order, the basic strength—and therefore the nucleophilicity—will be the exact reverse.

The weakest acid, methanol, gives the strongest conjugate base and the strongest nucleophile, methoxide. The strongest acid, $TsOH$, gives the weakest conjugate base and the poorest nucleophile, $Ts{O}^{-}$.

So the decreasing order of nucleophilicity is:

This perfectly matches the sequence (B) > (C) > (A) > (D), making option (b) the correct answer.

Key Takeaway: Whenever you need to compare nucleophiles, especially those with different attacking atoms, always form their conjugate acids. Remember that resonance stabilization of the negative charge makes an ion a great leaving group, but a terrible nucleophile!