The trap: Students treat electrophiles and nucleophiles as mirror categories distinguished only by charge. They forget that the defining feature is the electron-density site — not whether the species is positive or negative overall. A neutral molecule with an electron-deficient atom (like BF₃ or AlCl₃) is an electrophile; a neutral molecule with a lone pair available for donation (like NH₃ or H₂O) is a nucleophile.
Core definitions (NCERT Class 11 Chemistry, Chapter 8, page 273):
An electrophile ("electron-loving") is any species that accepts an electron pair to form a new covalent bond. It has either a positive charge or an electron-deficient atom. Examples: H⁺, NO₂⁺, BF₃, carbocations, AlCl₃.
A nucleophile ("nucleus-loving") is any species that donates an electron pair to form a new covalent bond. It has either a negative charge or a lone pair available for donation. Examples: OH⁻, CN⁻, NH₃, H₂O, R-NH₂.
Key distinction for NEET:
The inductive effect matters here. When comparing nucleophilic or electrophilic strength, students confuse the through-sigma-bond inductive pathway with through-pi resonance effects. Inductive effect operates through sigma bonds and weakens with distance; resonance operates through the pi system and is often dominant. A species can be simultaneously an electrophile at one site and bear a partial negative elsewhere — what matters is which atom participates in bond formation.
Watch-out: NEET often gives neutral species and asks whether they're electrophilic or nucleophilic. Focus on the reactive atom: does it have a vacant orbital or electron deficiency (electrophile) or a donatable lone pair (nucleophile)?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is an electrophile?
Answer: B. BF₃ has an incomplete octet on boron (only 6 electrons) and readily accepts an electron pair, making it an electrophile (NCERT Class 11 Chemistry, Chapter 8, page 273).
Why A is wrong: NH₃ has a lone pair on nitrogen available for donation — it is a nucleophile, not an electrophile.
Why C is wrong: OH⁻ carries a negative charge and has lone pairs available for donation — it acts as a nucleophile.
Why D is wrong: CH₃⁻ (carbanion) has an unshared electron pair and negative charge — it is a nucleophile.
Which of the following species can act as both an electrophile and a nucleophile?
Answer: C. H₂O has lone pairs (nucleophilic — donates to H⁺) and also has O-H bonds that are electrophilic toward strong bases. It can act both ways in Lewis acid-base terms.
Why A is wrong: BF₃ has an empty p-orbital on boron but no available lone pair for donation — it is only an electrophile.
Why B is wrong: Na⁺ can accept electrons but has no lone pair to donate — it acts only as an electrophile (Lewis acid).
Why D is wrong: CH₃⁺ is a carbocation with an empty p-orbital — it can only accept electrons, acting solely as an electrophile.
The electrophilic character of a carbonyl carbon (C=O) is primarily due to:
Answer: D. Oxygen is more electronegative than carbon, so in the C=O bond, electron density of the pi bond is pulled toward oxygen, leaving the carbon electron-deficient and electrophilic. This is the primary cause of carbonyl electrophilicity (NCERT lists the carbonyl carbon as an electrophile: NCERT Class 11 Chemistry, Chapter 8, page 273).
Why A is wrong: Resonance structures show oxygen gaining negative charge and carbon gaining positive charge — this supports electrophilicity but the primary driving force is electronegativity difference in the pi bond, not resonance making oxygen positive.
Why B is wrong: Inductive effect through sigma bonds contributes but is weaker than the polarization of the pi bond by electronegativity. Students confuse through-sigma inductive pull with through-pi polarization (trap: inductive vs resonance confusion).
Why C is wrong: Hyperconjugation involves alpha C-H bonds and is relevant to carbocation stability, not to carbonyl electrophilicity.
Among the following, which is the strongest nucleophile in a protic solvent?
Answer: C. In protic solvents, nucleophilicity increases down a group because larger ions are less solvated (weaker ion-dipole interactions), making their lone pairs more available. Order: I⁻ > Br⁻ > Cl⁻ > F⁻ in protic solvents.
Why A is wrong: F⁻ is the strongest base but in protic solvents it is heavily solvated due to its small size and high charge density — this reduces its nucleophilicity. Students confuse basicity with nucleophilicity (trap: inductive vs resonance-type confusion about different reactivity scales).
Why B is wrong: Cl⁻ is intermediate — less solvated than F⁻ but more solvated than I⁻.
Why D is wrong: Br⁻ is a good nucleophile but I⁻ is less solvated (larger, more polarisable) and therefore stronger in protic solvents.
Which of the following correctly distinguishes an electrophile from a nucleophile?
Answer: A. By definition, electrophiles are electron-pair acceptors and nucleophiles are electron-pair donors in bond formation (NCERT Class 11 Chemistry, Chapter 8, page 273).
Why B is wrong: Charge is not the defining criterion. Neutral species like BF₃ (electrophile) and NH₃ (nucleophile) exist. The defining feature is electron-pair acceptance or donation.
Why C is wrong: This is the exact inversion of the definitions. Electrophiles accept, nucleophiles donate.
Why D is wrong: This is backwards. Electrophiles have empty orbitals (to accept); nucleophiles have lone pairs (to donate).
AlCl₃ acts as an electrophile in Friedel-Crafts reactions because:
Answer: B. In AlCl₃, aluminium has only 6 electrons in its valence shell (incomplete octet), leaving a vacant p-orbital that accepts an electron pair from the substrate — defining electrophilic character (NCERT gives the same incomplete-octet reasoning for BF₃: NCERT Class 11 Chemistry, Chapter 8, page 273).
Why A is wrong: Aluminium in AlCl₃ does not have a lone pair available — its electron deficiency is exactly what makes it electrophilic, not nucleophilic.
Why C is wrong: Aluminium does NOT have a complete octet in AlCl₃; it has only 6 valence electrons. Claiming a complete octet contradicts its Lewis-acid behaviour.
Why D is wrong: While Cl does have lone pairs, back-donation from Cl to Al is limited and does not fill the vacant orbital sufficiently — AlCl₃ remains a strong Lewis acid/electrophile in practice.
Consider the following species: (i) NO₂⁺, (ii) CN⁻, (iii) CH₂=CH₂, (iv) NH₃. How many of these can act as nucleophiles?
Answer: A. CN⁻ (lone pair + negative charge), CH₂=CH₂ (pi electrons available for donation), and NH₃ (lone pair on N) are all nucleophiles — they can donate electron density. NO₂⁺ is an electrophile (electron-deficient nitrogen). Three species act as nucleophiles (definitions: NCERT Class 11 Chemistry, Chapter 8, page 273).
Why B is wrong: B is wrong because 2 undercounts: CN⁻ and NH₃ are obvious nucleophiles, but CH₂=CH₂ also donates its π electrons, so it is a nucleophile too.
Why C is wrong: Only counting CN⁻ as a nucleophile misses that neutral species with lone pairs (NH₃) or pi-electron density (ethylene) also donate electrons.
Why D is wrong: All four cannot be nucleophiles because NO₂⁺ is the nitronium ion — a classic electrophile used in aromatic nitration.
The inductive effect differs from the resonance effect in that the inductive effect:
Answer: D. The inductive effect is transmitted through sigma bonds and diminishes significantly with each successive carbon (becomes negligible beyond 3-4 bonds). Resonance, by contrast, operates through the pi system and requires conjugation (NCERT Class 11 Chemistry, Chapter 8, page 276). This distinction is a common confusion point — students treat inductive as long-range like resonance.
Why A is wrong: This describes resonance (through pi, though resonance does not increase with distance — it requires conjugation). The description is also factually incorrect about 'increases with distance' for any effect (trap: confusing properties of inductive vs resonance).
Why B is wrong: Conjugation is required for resonance, not inductive effect. Inductive effect needs only a polar bond and sigma-bond framework.
Why C is wrong: Inductive effect is generally weaker than resonance when both operate simultaneously. When resonance and inductive effects oppose each other, resonance usually dominates.
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Given
The following species are presented: (a) CH₃OH, (b) BF₃, (c) NO₂⁺, (d) C₂H₅NH₂.
Classify each as electrophile, nucleophile, or dual-character (can act as both, at different sites). Note: do NOT call these “ambident” — in IUPAC usage an ambident nucleophile is one with TWO different donor atoms, such as CN⁻ (C or N), NO₂⁻ (N or O) or SCN⁻ (S or N), and NEET uses the term in that sense.
Required
Identify the electron-pair donor/acceptor character of each species.
Concept
An electrophile has an electron-deficient centre (empty orbital, positive charge, or incomplete octet). A nucleophile has a lone pair or pi electrons available for donation. Some species carry both features at different sites.
Formula/Rule
Classification rule: Check the reactive atom — vacant orbital → electrophile; available lone pair → nucleophile; both features at different sites → dual character.
Substitution/Analysis
• CH₃OH: Oxygen has two lone pairs → can donate → nucleophilic. However, the O-H bond is also polarised (H is electrophilic). Ambident.• BF₃: Boron has only 6 valence electrons, vacant p-orbital → electrophile.• NO₂⁺: Nitrogen is electron-deficient (positive charge, needs electrons) → electrophile.• C₂H₅NH₂: Nitrogen has a lone pair → nucleophile.
Calculation
No arithmetic needed — this is classification by electronic structure analysis.
Final answer
• CH₃OH → principally a nucleophile, through the oxygen lone pair. Its electrophilic site, when one is invoked, is the δ+ CARBON bonded to oxygen — not the hydroxyl hydrogen, whose loss is Brønsted acidity rather than electrophilic attack. NCERT treats CH₃OH as a nucleophile.• BF₃ → electrophile• NO₂⁺ → electrophile• C₂H₅NH₂ → nucleophile
Common trap
Students classify all neutral molecules as "neither electrophile nor nucleophile." Neutrality does not preclude electrophilic or nucleophilic character — the relevant criterion is whether the reactive atom has a vacant orbital or donatable lone pair. BF₃ is neutral yet strongly electrophilic. NH₃ is neutral yet nucleophilic.
Similar NEET-style question
"Among BeCl₂, CCl₄, NH₃, and H₂O, identify the species that act as Lewis acids (electrophiles)." Answer: BeCl₂ (Be has incomplete octet — only 4 valence electrons in the molecule, vacant p-orbital).
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Electrophile (E⁺): electron-pair acceptor (Lewis acid). Examples: H⁺, Cl⁺, NO₂⁺, BF₃. Nucleophile (Nu⁻): electron-pair donor (Lewis base). Examples: OH⁻, CN⁻, NH₃, H₂O.
-- NCERT Class 11 Chemistry, Ch. 8, p. 272More in Organic Chemistry — Some Basic Principles: 9 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
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