Q.Identify electrophilic centre in the following: CH₃CH=O, CH₃CN, CH₃I.

The key idea is that an electrophilic centre is an electron-deficient atom that can accept a pair of electrons — typically a carbon atom bonded to a more electronegative atom or group.

Step 1: In CH₃CH=O (acetaldehyde), the carbonyl carbon is double-bonded to oxygen (highly electronegative), so it carries a partial positive charge — this is the electrophilic centre.

Step 2: In CH₃CN (acetonitrile), the nitrile carbon is triple-bonded to nitrogen (more electronegative), making that carbon electron-deficient and thus electrophilic. …

Concept and Intuition: Electrophile Nucleophile Concept

An electrophile is a species that loves electrons (from Greek philos meaning loving). It is electron-deficient and seeks to accept a pair of electrons from a nucleophile (a nucleus-loving, electron-rich species). The electrophilic centre is the specific atom within a molecule that is most electron-poor and thus most likely to be attacked by a nucleophile.

How do we spot an electrophilic centre? Look for:

• A carbon atom bonded to a more electronegative atom (like O, N, or halogen) via a multiple bond or a polar single bond.
• A carbon with a partial positive charge ($\delta^+$) due to resonance or inductive effects.
• A positively charged atom (like a carbocation).

Let’s apply this to each molecule.

Step-by-Step Analysis

1. CH₃CH=O (Acetaldehyde)

The structure is:

$\text{CH}_3-\text{CH}= \text{O}$

The carbonyl group ($\text{C}= \text{O}$) has a highly polar double bond. Oxygen is much more electronegative than carbon (3.44 vs 2.55 on Pauling scale), so it pulls electron density away from the carbon. This gives the carbonyl carbon a significant partial positive charge ($\delta^+$).

Additionally, resonance structures show that the carbon can be represented with a positive charge:

The carbon in the carbonyl group is therefore electron-deficient and acts as the electrophilic centre. Nucleophiles attack this carbon in reactions like nucleophilic addition.

Electrophilic centre in CH₃CH=O: The carbonyl carbon ($\text{C}$ of $\text{C}= \text{O}$).

2. CH₃CN (Acetonitrile or Methyl Cyanide)

The structure is:

$\text{CH}_3-\text{C} \equiv \text{N}$

The nitrile group has a triple bond between carbon and nitrogen. Nitrogen is more electronegative than carbon (3.04 vs 2.55), so it pulls electron density along the triple bond. This leaves the nitrile carbon with a partial positive charge ($\delta^+$).

Resonance also contributes:

The carbon of the $\text{C} \equiv \text{N}$ group is electron-deficient and is the site where nucleophiles attack (e.g., in hydrolysis of nitriles to carboxylic acids).

Electrophilic centre in CH₃CN: The nitrile carbon ($\text{C}$ of $\text{C} \equiv \text{N}$).

3. CH₃I (Iodomethane or Methyl Iodide)

The structure is:

$\text{CH}_3 - \text{I}$ …