Electrophiles are electron seeking species. Which of the following groups contain only electrophiles? (Note: m

NCERT Exemplar · Q18

Q. Electrophiles are electron seeking species. Which of the following groups contain only electrophiles? (Note: more than one of the given options may be correct.)

(i) BF3, NH3, H2O

(ii) AlCl3, SO3, NO2^+

(iii) NO2^+, CH3^+, CH3—C^+=O

(iv) C2H5^-, •C2H5, C2H5^+

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Concept understanding — Brønsted Lowry Conjugate Pairs

From Intuition to Precision: Brønsted–Lowry Conjugate Pairs

Imagine you're at a party where people are passing around a single, very special coin. The coin represents a proton ( $\text{H}^+$ ). The game is simple: someone can give the coin to someone else, but only if the other person is willing to take it. You can't just throw the coin; you need a willing receiver.

In chemistry, acids and bases play exactly this game. An acid is the person who gives away the proton. A base is the person who accepts it. But here's the twist: the moment the acid gives away its proton, it transforms into something new — something that can now accept a proton back. That transformed form is called the conjugate base. Similarly, the base, after accepting the proton, becomes something that can donate it back — the conjugate acid.

This is the core of the Brønsted–Lowry theory: every acid-base reaction involves a pair of substances that are linked by the gain or loss of a single proton.

The Precise Statement

\text{Acid} \rightleftharpoons \text{Conjugate Base} + \text{H}^+

A conjugate acid-base pair consists of two species that differ by exactly one proton ( $\text{H}^+$ ). The acid has the proton; the conjugate base does not.

When an acid donates a proton, it becomes its conjugate base.

When a base accepts a proton, it becomes its conjugate acid.

Important

In any Brønsted–Lowry reaction, there are two conjugate pairs: one on the reactant side and one on the product side. They always appear together.

Seeing It in Action

Take the classic reaction between hydrochloric acid and water:

\text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+

Let's identify the pairs:

Pair 1: $\text{HCl}$ (acid) and $\text{Cl}^-$ (conjugate base). They differ by one $\text{H}^+$ .
Pair 2: $\text{H}_2\text{O}$ (base) and $\text{H}_3\text{O}^+$ (conjugate acid). They also differ by one $\text{H}^+$ .

Notice: water acted as a base here — it accepted the proton from HCl. But water can also act as an acid in other reactions. That's the beauty of the Brønsted–Lowry theory: a substance's role depends on the reaction, not on a fixed label.

Tip

To find the conjugate base of any acid, simply remove one $\text{H}^+$ and reduce the charge by +1.

To find the conjugate acid of any base, add one $\text{H}^+$ and increase the charge by +1.

A Quick Reference Table

Acid	Conjugate Base	Base	Conjugate Acid
$\text{HCl}$	$\text{Cl}^-$	$\text{NH}_3$	$\text{NH}_4^+$
$\text{H}_2\text{SO}_4$	$\text{HSO}_4^-$	$\text{H}_2\text{O}$	$\text{H}_3\text{O}^+$
$\text{NH}_4^+$	$\text{NH}_3$	$\text{OH}^-$	$\text{H}_2\text{O}$
$\text{H}_2\text{O}$	$\text{OH}^-$	$\text{CO}_3^{2-}$	$\text{HCO}_3^-$

Notice how water appears in both columns — it's amphoteric, meaning it can act as either an acid or a base depending on its partner.

Why This Matters …

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Electrophiles are electron-deficient species that accept electron pairs. Only groups (ii) and (iii) contain exclusively electrophiles: $\text{AlCl}_3$ , $\text{SO}_3$ , $\text{NO}_2^+$ in (ii); and $\text{NO}_2^+$ , $\text{CH}_3^+$ , $\text{CH}_3\text{–C}^+=\text{O}$ in (iii).

An electrophile is a species that seeks electrons because it has either a positive charge, an incomplete octet, or a highly polarised atom that can accommodate additional electron density. The defining feature is the ability to accept an electron pair from a nucleophile during bond formation.

The key is to recognise what makes a species electron-deficient:

Positive charge creates an obvious electron deficit.
Incomplete octets (fewer than eight valence electrons around a central atom) leave room for electron acceptance.
Highly polarised multiple bonds with electron-withdrawing atoms can act as electrophilic centres.

Conversely, species with lone pairs or negative charges are nucleophiles (electron donors), and free radicals, while reactive, are not classified as electrophiles because they seek a single electron, not an electron pair.

Let's examine each group:

Option (i): $\text{BF}_3$ , $\text{NH}_3$ , $\text{H}_2\text{O}$

$\text{BF}_3$ has boron with only six valence electrons—a classic Lewis acid and electrophile. However, $\text{NH}_3$ has a lone pair on nitrogen and acts as a nucleophile (it donates electrons). Similarly, $\text{H}_2\text{O}$ has two lone pairs on oxygen and is also a nucleophile. This group is mixed.
Option (ii): $\text{AlCl}_3$ , $\text{SO}_3$ , $\text{NO}_2^+$

$\text{AlCl}_3$ has aluminum with only six valence electrons around it—a strong Lewis acid and electrophile. $\text{SO}_3$ has sulphur bonded to three oxygens; the sulphur is highly electron-deficient due to the electron-withdrawing oxygens and can accept electron density (it acts as an electrophile in sulfonation reactions). $\text{NO}_2^+$ (the nitronium ion) carries a positive charge and is a powerful electrophile in nitration. All three are electrophiles.
Option (iii): $\text{NO}_2^+$ , $\text{CH}_3^+$ , $\text{CH}_3\text{–C}^+=\text{O}$ …

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Q.Electrophiles are electron seeking species. Which of the following groups contain only electrophiles? (Note: more than one of the given options may be correct.)

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