Q.Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles:

Concept: Electrophile Nucleophile Concept

A nucleophile donates an electron pair (loves positive centres); an electrophile accepts an electron pair (loves negative centres).

Reasoning

1. (a) $\mathrm{HO^-}$ has a lone pair and a negative charge — it abstracts the acidic O–H proton of $\mathrm{CH_3COOH}$ (an acid–base reaction giving acetate).
2. (b) $\mathrm{CN^-}$ is negatively charged with a lone pair — it attacks the carbonyl carbon of $\mathrm{CH_3COCH_3}$. …

The Core Concept: Electrophiles vs. Nucleophiles

Before we touch a single equation, let's get the intuition straight. Think of chemical reactions as a dance of electrons.

A nucleophile ("nucleus-loving") is an electron-rich species. It has a lone pair, a negative charge, or a pi bond it can use to donate a pair of electrons to form a new bond. It is a Lewis base.

An electrophile ("electron-loving") is an electron-deficient species. It has a positive charge, an empty orbital, or a polar bond with a partial positive charge, so it accepts a pair of electrons from a nucleophile. It is a Lewis acid.

The golden rule: Nucleophiles attack electrophiles. In every reaction below, the bold reagent is either the attacker (nucleophile) or the attacked (electrophile). Let's identify which is which.

1. Equation (a): $\text{CH}_3\text{COOH} + \textbf{HO}^- \rightarrow \text{CH}_3\text{COO}^- + \text{H}_2\text{O}$

Here, acetic acid ($\text{CH}_3\text{COOH}$) reacts with the hydroxide ion.

The hydroxide ion ($\text{HO}^-$) carries a full negative charge and has three lone pairs on oxygen. It is electron-rich. It uses one of its lone pairs to grab a proton ($\text{H}^+$) from the carboxylic acid group.

Because $\text{HO}^-$ is donating an electron pair to form the O–H bond in water, it is acting as a nucleophile. The proton it attacks is the electrophile.

Conclusion for (a): $\textbf{HO}^-$ is a nucleophile.

2. Equation (b): $\text{CH}_3\text{COCH}_3 + \textbf{CN}^- \rightarrow (\text{CH}_3)_2\text{C(CN)(OH)}$

This is a classic nucleophilic addition to a carbonyl group. Acetone ($\text{CH}_3\text{COCH}_3$) has a carbonyl (C=O) bond. The oxygen is more electronegative, pulling electron density away from the carbon, making the carbonyl carbon partially positive ($\delta^+$) and thus electrophilic.

The cyanide ion ($\text{CN}^-$) has a full negative charge and a lone pair on carbon. It is electron-rich. It attacks the electrophilic carbonyl carbon, donating its electron pair to form a new C–C bond.