Why does SO3 act as an electrophile?

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Q. Why does SO3 act as an electrophile?

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Concept understanding — Electrophile Nucleophile Concept

The Electrophile–Nucleophile Concept: A First Look

Imagine you are in a crowded room. Some people are constantly reaching out to shake hands — they want to connect, to grab something. Others are holding their hands close, waiting for someone to come to them. In organic chemistry, molecules behave the same way.

The Intuition

Every chemical reaction is about electron movement. Some species are electron-poor — they want to accept electrons to become stable. Others are electron-rich — they have extra electrons and are happy to donate them.

Electrophile (from Greek philos = loving, electron = electron): "Electron-loving" — a species that loves electrons and seeks them out. It is electron-deficient (positive charge, partial positive charge, or an empty orbital).
Nucleophile (from Greek nucleus = nucleus, philos = loving): "Nucleus-loving" — a species that loves positive nuclei because it has excess electrons to donate. It is electron-rich (negative charge, lone pairs, or pi bonds).

Note

Think of an electrophile as a hungry guest at a party (wants food = electrons) and a nucleophile as a generous host (has food to give). The reaction happens when the host offers food to the hungry guest.

The Precise Statement

Electrophile: Any atom, ion, or molecule that accepts a pair of electrons to form a new covalent bond. It is Lewis acid (electron-pair acceptor).

Nucleophile: Any atom, ion, or molecule that donates a pair of electrons to form a new covalent bond. It is Lewis base (electron-pair donor).

How to Identify Them

Feature	Electrophile	Nucleophile
Charge	Often positive or neutral (with empty orbital)	Often negative or neutral (with lone pair)
Examples	$\ce{H+}$ , $\ce{AlCl3}$ , $\ce{BF3}$ , $\ce{CH3+}$ (carbocation), $\ce{CO2}$	$\ce{OH-}$ , $\ce{NH3}$ , $\ce{H2O}$ , $\ce{Cl-}$ , $\ce{CH3O-}$
Orbital	Empty orbital (can accept electrons)	Filled orbital (lone pair or pi bond)
Reaction type	Attacked by nucleophile	Attacks electrophile

The Arrow-Pushing Convention

In organic chemistry, we show electron movement with curved arrows:

Arrow starts at the nucleophile (where electrons come from) — usually a lone pair or a pi bond.
Arrow ends at the electrophile (where electrons go) — usually at a positive charge or an empty orbital.

Example: Reaction of hydroxide ion with methyl bromide

\ce{HO- + CH3Br -> HO-CH3 + Br-}

The arrow goes from the lone pair on $\ce{O}$ (nucleophile) to the carbon atom in $\ce{CH3Br}$ (electrophile, because $\ce{Br}$ pulls electron density away, making carbon partially positive). …

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The key idea is that electrophiles are electron-pair acceptors. $SO_3$ acts as an electrophile because its central sulphur atom is electron-deficient due to resonance and high oxidation state, making it strongly attracted to electron-rich species.

Why $SO_3$ is an Electrophile — The Concept

An electrophile (from Greek philos = loving) is literally an "electron lover" — a species that seeks out electron-rich centres to form a new bond. For a molecule to be a good electrophile, it must have either:

A positive charge (like $H^+$ or $NO_2^+$ ), or
An atom with an incomplete octet (like $BF_3$ ), or
An atom that can expand its octet and is electron-deficient due to resonance or high oxidation state.

$SO_3$ falls into the third category. Let's see why.

Structure of $SO_3$ Sulphur trioxide has a trigonal planar geometry with $S$ at the centre and three $O$ atoms at the vertices. The sulphur atom is $sp^2$ hybridized. But here's the crucial part: $SO_3$ is a resonance hybrid. One of its major contributing structures shows a double bond between $S$ and each $O$ , but another important structure has a formal positive charge on sulphur and a negative charge on one oxygen:

\text{Resonance: } \quad \overset{+}{S}(=O)_2-O^- \longleftrightarrow O=S(=O)_2

In the resonance form with a positive charge on sulphur, the sulphur atom has only 6 electrons in its valence shell (incomplete octet), making it highly electron-deficient. Even in the hybrid, the sulphur carries a partial positive charge ( $\delta^+$ ).

High oxidation state of sulphur

In $SO_3$ , sulphur is in its +6 oxidation state — the highest possible for sulphur. This means sulphur has lost almost all its valence electron density to the highly electronegative oxygen atoms. The sulphur atom is therefore strongly electron-poor and desperately wants to accept a pair of electrons to stabilise itself.
The electrophilic attack

When $SO_3$ encounters a nucleophile (like the $\pi$ electrons of a benzene ring in sulfonation), the electron-deficient sulphur atom accepts a lone pair from the nucleophile. This forms a new $\sigma$ bond, and the sulphur expands its octet to 10 electrons (using its available $3d$ orbitals). The reaction is:

$\text{SO}_3 + \text{Nu}^- \longrightarrow \text{Nu-SO}_3^-$ …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 14 on this concept.

AP EAPCET 2025Set eng-2025-05-24-FN1 markMCQ
$\mathrm{NH_4^+}, \mathrm{NH_3}, \mathrm{BF_3}, \mathrm{OH^-}, \mathrm{CH_3^+}, \mathrm{H^+}, \mathrm{CO}, \mathrm{C_2H_4}$
›Reveal solutionSolution
A Lewis base is an electron-pair (or $\pi$ -electron) donor. Counting the species that qualify gives 4: $\mathrm{NH_3}$ , $\mathrm{OH^-}$ , $\mathrm{CO}$ , $\mathrm{C_2H_4}$ .

Concept and Intuition

Lewis acid–base theory looks past H $^+$ /OH $^-$ chemistry to electron-pair donation and acceptance in general. A species is a Lewis base if it has an available lone pair (or a delocalised electron cloud, like a $\pi$ bond) that it can offer to an electron-deficient centre.

Step-by-Step Solution

$\mathrm{NH_4^+}$ : nitrogen's lone pair is already used to bond the fourth H; no lone pair remains, so it cannot donate — not a Lewis base.

$\mathrm{NH_3}$ : nitrogen retains one lone pair, freely donated (e.g. to $\mathrm{H^+}$ to form $\mathrm{NH_4^+}$ ) — Lewis base.

$\mathrm{BF_3}$ : boron has only 6 electrons around it (empty p-orbital) — it accepts electron pairs, a Lewis acid, not a base.

$\mathrm{OH^-}$ : oxygen carries three lone pairs and a negative charge, an excellent electron donor — Lewis base.

$\mathrm{CH_3^+}$ : an electron-deficient carbocation with an empty p-orbital — Lewis acid.

$\mathrm{H^+}$ : a bare proton with no electrons, the archetypal electron-pair acceptor — Lewis acid.

$\mathrm{CO}$ : carbon carries a lone pair (as does oxygen); it is well known to donate through carbon in metal carbonyls like $\mathrm{Ni(CO)_4}$ — Lewis base. …
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AP EAPCET 2024Set ap-2024-05-17-AN1 markMCQ
$\mathrm{CN^-}$
›Reveal solutionSolution
This tests recognition of ambident nucleophiles — species with two chemically distinguishable nucleophilic sites.

Concept and Intuition

An ambident nucleophile can attack an electrophile through either of two different atoms, giving two structurally different products. This requires the negative charge/lone pair to be delocalized over two non-equivalent atoms (e.g. C and N, or N and O) — not simply spread symmetrically over two identical atoms.

Step-by-Step Solution

$\mathrm{CN^-}$ (cyanide): the negative charge is delocalized between carbon and nitrogen. Attack through C gives a nitrile ( $R{-}CN$ ); attack through N gives an isonitrile ( $R{-}NC$ ). This is a classic ambident nucleophile.

$\mathrm{CH_3COO^-}$ (acetate): the negative charge is delocalized equally over the two oxygen atoms, which are chemically equivalent by resonance/symmetry — attack always occurs through oxygen giving the same type of bond, so acetate is not considered ambident. …
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AP EAPCET 2024Set eng-2024-05-20-FN1 markMCQ
$\mathrm{CH_3NH_2}$
›Reveal solutionSolution
Three of the four species ( $\mathrm{CH_3NH_2}$ , $\mathrm{C_2H_4}$ , $\mathrm{CH_3SH}$ ) are nucleophiles.

Concept and Intuition

A nucleophile ("nucleus-loving") is electron-rich and attacks electron-poor sites by donating either a non-bonding lone pair or the $\pi$ -electrons of a multiple bond. An electrophile is the opposite — electron-deficient and seeking electrons. So the test for each species is simply: does it have a readily available lone pair or $\pi$ -system to donate?

Step-by-Step Solution

$\mathrm{CH_3NH_2}$ (methylamine): nitrogen carries a lone pair $\Rightarrow$ nucleophile.

$\mathrm{CH_3CHO}$ (acetaldehyde): the carbonyl carbon is $\delta^+$ (electron-deficient) $\Rightarrow$ it behaves as an electrophile, not a nucleophile.

$\mathrm{C_2H_4}$ (ethene): the $\mathrm{C=C}$ $\pi$ -electrons are donated to electrophiles (e.g. in electrophilic addition) $\Rightarrow$ nucleophile.

$\mathrm{CH_3SH}$ (methanethiol): sulphur carries lone pairs $\Rightarrow$ nucleophile.

Count the nucleophiles: $\mathrm{CH_3NH_2}$ , $\mathrm{C_2H_4}$ , $\mathrm{CH_3SH}$ $=3$ .

Common Mistakes …
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AP EAPCET 2024Set eng-2024-05-22-AN1 markMCQ
$\mathrm{CH_3CH_2\!-\!I \longrightarrow A + B}$
›Reveal solutionSolution
The more electronegative atom in each bond keeps the electron pair on cleavage: iodine takes it from carbon (giving $\mathrm{CH_3CH_2^+}$ , $\mathrm{I^-}$ ), while carbon takes it from the metal copper (giving $\mathrm{CH_3CH_2^-}$ , $\mathrm{Cu^+}$ ).

Concept and Intuition

When a covalent bond between two atoms breaks so that both electrons go to one atom (heterolytic-style ionic products), the electron pair migrates to the more electronegative partner, which becomes negatively charged, while the less electronegative partner is left electron-deficient and positively charged. Carbon (electronegativity $\approx 2.5$ ) is more electronegative than a metal like copper, but less electronegative than a halogen like iodine ( $\approx 2.7$ ) — so carbon's charge sign flips depending on which partner it's bonded to.

Step-by-Step Solution

$\mathrm{CH_3CH_2-I}$ : iodine is more electronegative than the ethyl carbon, so it retains both electrons of the C–I bond on cleavage, becoming $\mathrm{I^-}$ (species B), leaving the ethyl group electron-deficient as $\mathrm{CH_3CH_2^+}$ (species A).

$\mathrm{CH_3CH_2-Cu}$ : here carbon is more electronegative than the metal copper, so carbon retains both electrons of the C–Cu bond, becoming $\mathrm{CH_3CH_2^-}$ (species C), leaving copper as $\mathrm{Cu^+}$ (species D). …
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AP EAPCET 2023Set ap-2023-05-22-FN1 markMCQ
$\mathrm{CH_2=CH_2}$
›Reveal solutionSolution
Matches each substrate to its characteristic reaction type based on its reactive site: alkene → electrophilic addition, carbonyl → nucleophilic addition, aromatic ring → electrophilic substitution, benzylic halide → nucleophilic substitution.

Concept and Intuition

The reaction type a molecule undergoes is dictated by where its electron density (or deficiency) sits. Alkenes have an electron-rich $\pi$ bond that attracts electrophiles, leading to electrophilic addition. Carbonyl carbons are electron-poor ( $\delta+$ ) due to the electronegative oxygen, so nucleophiles attack them, giving nucleophilic addition. Aromatic rings are electron-rich but "protect" their aromaticity by substituting a hydrogen rather than adding across a double bond, giving electrophilic substitution. Benzylic halides have a good leaving group (halide) on a carbon stabilized by the adjacent ring, favoring nucleophilic substitution.

Step-by-Step Solution

A. $\mathrm{CH_2=CH_2}$ (ethylene, alkene) $\rightarrow$ electrophilic addition $\rightarrow$ III.

B. $\mathrm{CH_3-CO-CH_3}$ (acetone, carbonyl) $\rightarrow$ nucleophilic addition $\rightarrow$ IV. …
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AP EAPCET 2023Set ap-2023-05-23-FN1 markMCQ
$\mathrm{CH_3^{+}, CO_2, NH_3, H_2O, SO_3, H^{+}, BF_3}$
›Reveal solutionSolution
Sorting the seven species into electrophiles vs nucleophiles by electron availability at the reactive centre shows 5 are electrophiles.

Concept and Intuition

An electrophile is 'electron-loving' — it is short of electrons (an empty orbital, a partial positive charge, or a highly polarized bond) and seeks electron density. A nucleophile is the opposite: it has a lone pair or negative charge to donate.

Step-by-Step Solution

$\mathrm{CH_3^{+}}$ : a carbocation with an empty p-orbital — electrophile.

$\mathrm{CO_2}$ : the carbon is doubly bonded to two electronegative oxygens, making it electron-poor — electrophile (attacked by nucleophiles like Grignard reagents).

$\mathrm{NH_3}$ : nitrogen has a lone pair readily donated — nucleophile, not an electrophile.

$\mathrm{H_2O}$ : oxygen has two lone pairs, primarily acts as a nucleophile/Lewis base.

$\mathrm{SO_3}$ : sulphur is electron-deficient (bonded to three electronegative O atoms) — electrophile.

$\mathrm{H^{+}}$ : a bare proton with no electrons at all — a classic electrophile.

$\mathrm{BF_3}$ : boron has only 6 electrons around it (incomplete octet) — a strong Lewis acid/electrophile. …
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AP EAPCET 2023Set eng-2023-05-17-AN1 markMCQ
$\mathrm{AlCl_3,\ NH_3,\ H^+,\ Co^{3+},\ {}^-OH,\ H^-Mg^{2+},\ BF_3,\ Cl^-}$
›Reveal solutionSolution
This tests identifying Lewis bases (electron-pair donors) vs Lewis acids (electron-pair acceptors) among a mixed list of species. Answer: 4 Lewis bases.

Concept and Intuition

A Lewis base donates an electron pair (has a lone pair or negative charge available to share); a Lewis acid accepts an electron pair (electron-deficient, like BF3/AlCl3, or a bare cation).

Step-by-Step Solution

$\mathrm{AlCl_3}$ : incomplete octet on Al, accepts electron pairs $\Rightarrow$ Lewis acid.

$\mathrm{NH_3}$ : lone pair on N, donates it $\Rightarrow$ Lewis base.

$\mathrm{H^+}$ : bare proton, accepts an electron pair $\Rightarrow$ Lewis acid.

$\mathrm{Co^{3+}}$ : electron-deficient transition-metal cation, accepts pairs $\Rightarrow$ Lewis acid.

$\mathrm{OH^-}$ : has lone pairs to donate $\Rightarrow$ Lewis base.

$\mathrm{H^-}$ (hydride ion): has a lone electron pair it can donate $\Rightarrow$ Lewis base.

$\mathrm{Mg^{2+}}$ : electron-deficient cation, accepts pairs $\Rightarrow$ Lewis acid. …
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AP EAPCET 2023Set eng-2023-05-18-AN1 markMCQ
$\mathrm{AlCl_3,\ NH_3,\ H^+,\ Co^{3+},\ {}^-OH,\ Mg^{2+},\ BF_3,\ Cl^-}$
›Reveal solutionSolution
Checking each species for an electron-pair-accepting capacity identifies 5 Lewis acids: $AlCl_3, H^+, Co^{3+}, Mg^{2+}, BF_3$ .

Concept and Intuition

A Lewis acid is any species that can accept a pair of electrons — typically cations (positive charge attracts electron density) or neutral molecules with an incomplete octet / empty orbital on the central atom. A Lewis base, by contrast, has a lone pair available to donate.

Step-by-Step Solution

$AlCl_3$ : Al has only 6 electrons around it (incomplete octet), so it readily accepts a lone pair — Lewis acid.

$NH_3$ : N has a lone pair to donate — Lewis base.

$H^+$ : a bare proton with an empty 1s orbital, strongly accepts electron pairs — Lewis acid.

$Co^{3+}$ : a metal cation with vacant d-orbitals, accepts electron pairs from ligands — Lewis acid.

$^-OH$ : has lone pairs and a negative charge, donates electrons — Lewis base.

$Mg^{2+}$ : a metal cation, accepts electron pairs — Lewis acid. …
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AP EAPCET 2023Set eng-2023-05-18-FN1 markMCQ
$Q. Which of the following is an ambident nucleophile? (A) Ammonia (B) Chloride ion (C) Water (D) Cyanide ion$
›Reveal solutionSolution
An ambident nucleophile attacks through either of two different atoms; cyanide ( $\mathrm{CN^-}$ ) can attack via C or N, unlike ammonia, chloride, or water.

Concept and Intuition

Most simple nucleophiles (like $\mathrm{Cl^-}$ , $\mathrm{NH_3}$ , $\mathrm{H_2O}$ ) donate an electron pair from just one atom. An ambident nucleophile has resonance delocalising negative charge (or lone pairs) over two different atoms, so it can bond through either site depending on conditions — the classic textbook examples are cyanide ( $\mathrm{:C\equiv N:^-}$ , attacking via C to give a nitrile, or via N to give an isonitrile) and nitrite ( $\mathrm{NO_2^-}$ ).

Step-by-Step Solution

Ammonia — single nucleophilic atom (N). Not ambident.

Chloride ion — single atom (Cl). Not ambident. …
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AP EAPCET 2022Set eng-2022-07-06-AN1 markMCQ
$PH_3$
›Reveal solutionSolution
Chlorine disproportionates in NaOH; the products depend on temperature/concentration — cold dilute gives chloride + hypochlorite, hot concentrated gives chloride + chlorate.

Concept and Intuition

$Cl_2$ has an oxidation state of 0. In base, it disproportionates: some Cl atoms are reduced to $Cl^-$ ( $-1$ ) while others are oxidized. The extent of oxidation depends on conditions — cold/dilute base favors the milder $+1$ (hypochlorite) product, while hot/concentrated base drives it further to $+5$ (chlorate), because hypochlorite itself disproportionates further at higher temperature.

Step-by-Step Solution

Cold, dilute: $Cl_2 + 2NaOH \rightarrow NaCl + NaOCl + H_2O$ . Here A = NaCl, B = NaOCl. …
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AP EAPCET 2022Set eng-2022-07-07-AN1 markMCQ
$Q. With reference to the carbonyl group which statement(s) is/are correct?$
a) Carbonyl Carbon is electrophilic
b) Carbonyl Carbon is nucleophilic
c) Carbonyl Oxygen is electrophilic
d) Carbonyl Oxygen is nucleophilic (A) a, d (B) b, c (C) b only (D) c only
›Reveal solutionSolution
Because oxygen is more electronegative than carbon, the C=O bond is polarized so that carbon is electrophilic and oxygen is nucleophilic — matching statements (a) and (d).

Concept and Intuition

The carbonyl group's reactivity in nucleophilic addition reactions (aldehydes, ketones) stems directly from this polarization: nucleophiles attack the electron-deficient carbonyl carbon, while the oxygen (bearing lone pairs and partial negative charge) can itself act as a nucleophile/base or a site for electrophilic attack (e.g., protonation) in acid-catalyzed mechanisms.

Step-by-Step Solution

Draw the resonance/polarization of C=O: the more electronegative oxygen pulls electron density toward itself, leaving carbon relatively electron-poor.

Carbon, being electron deficient, is attacked by nucleophiles — i.e., carbon is electrophilic. (statement a: TRUE) …
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AP EAPCET 2021Set eng-2021-08-25-AN1 markMCQ
$CN^-$
›Reveal solutionSolution
An electrophile is an electron-pair acceptor; $CN^-$ is instead an electron-pair donor (nucleophile), unlike $BF_3$ , $NO_2^+$ , and $AlCl_3$ .

Concept and Intuition

Electrophiles ("electron lovers") are species that seek electron pairs — they are either positively charged or have an incomplete octet, making them electron-deficient. Nucleophiles ("nucleus lovers") are the opposite: electron-rich species that donate a lone pair. $CN^-$ , bearing a negative charge and a readily available lone pair on carbon, is a textbook nucleophile (used in $S_N2$ substitutions to form nitriles). $BF_3$ and $AlCl_3$ are classic Lewis acids (incomplete octet on B/Al), and $NO_2^+$ (nitronium ion) is a cationic electrophile central to aromatic nitration.

Step-by-Step Solution

Check each species' electronic character: $CN^-$ — anion with a lone pair to donate → nucleophile.

$BF_3$ — boron has only 6 electrons, incomplete octet → strong Lewis acid/electrophile. …
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Q.Why does SO3 act as an electrophile?

Why SO3SO_3SO3​ is an Electrophile — The Concept

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Why $SO_3$ is an Electrophile — The Concept

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