BPSC 70th CCE Re-exam Prelims General Studies · Question 98 of 142

Correct — B, Na⁺. An electrophile is an electron-loving species: electron-deficient, and able to accept a pair of electrons to form a new covalent bond. The definition has two halves and Na⁺ fails the second. Take the options in turn. (a) H⁺ is the archetype — a bare proton with no electrons at all, which accepts a lone pair from water to give H₃O⁺ or from ammonia to give NH₄⁺; because H⁺ plainly is an electrophile, option (d) 'All of the above' collapses immediately. (c) BF₃ is the standard neutral electrophile of the syllabus: boron in it has only six electrons around it, an incomplete octet with an empty 2p orbital, so it accepts a lone pair and forms the adduct F₃B←NH₃. That leaves (b). Na⁺ is positively charged, which is why it looks electrophilic, but it has already reached the neon configuration — a full, stable octet with no low-lying vacancy to receive a pair into. It attracts anions electrostatically and stays an ion; it does not form a covalent bond by taking an electron pair. So alkali-metal cations are the standard exception: charge alone does not make an electrophile, an accessible empty orbital does. One printed difficulty has to be flagged rather than hidden. Option (c) is printed in the booklet as BF₃ with a superscript minus, confirmed at 600 dpi. Read literally that anion would carry an extra electron and be electron-rich, so it too would not be an electrophile and the question would have two answers with no 'more than one of the above' offered. Our derivation takes the coherent reading — that the charge is a typesetting slip and the species intended is neutral BF₃ — because that is the only reading on which the item has a single answer.

• (a)H⁺ — The clearest electrophile there is, so it cannot be the odd one out. A proton has no electrons whatever, which makes it maximally electron-deficient: it attacks anything with a lone pair or a π bond. Every acid-catalysed organic reaction begins this way, and so does the simplest acid–base chemistry, where H⁺ from an acid takes a lone pair from water to become the hydronium ion. Its presence in the list is what kills option (d).
• (c)BF₃⁻ — The genuinely difficult option, and the reason our confidence here is medium rather than high. Read as neutral BF₃ — which is what the chemistry of the question requires — it is the textbook neutral electrophile: boron with a sextet, an empty p orbital and a strong appetite for a lone pair, the standard Lewis acid alongside AlCl₃. Read as printed, with the superscript minus, it would be an electron-rich species and therefore also not an electrophile, which would give the question two answers. The item is sound only on the first reading.
• (d)All of the above — An 'all of the above' option is correct only when every listed item satisfies the stem, and here the stem is negative, so it would require that none of the three is an electrophile. H⁺ alone defeats it — a bare proton is the definitive electron-pair acceptor. Test this kind of option against the single clearest item on the list before considering the others; it usually falls in one step.

Organic reactions are classified by which partner brings the electrons. A nucleophile is electron-rich and donates a pair — hydroxide, ammonia, a halide ion, an alkene's π cloud. An electrophile is electron-poor and accepts that pair, forming the new covalent bond in the process. The categories map almost exactly onto the Lewis definitions of base and acid: a Lewis base donates a lone pair, a Lewis acid accepts one. Electrophiles come in three structural types. First, positively charged species with a vacancy — H⁺, NO₂⁺, the carbocation R₃C⁺, Br⁺ generated in bromination. Second, neutral molecules with an incomplete octet — BF₃ and AlCl₃, where the central atom has only six valence electrons. Third, neutral molecules with a strongly polarised bond, where one atom carries a partial positive charge — the carbon of a carbonyl group, the carbon of an alkyl halide. What unites all three is not the charge but the vacancy: an empty or accessible orbital into which a lone pair can be donated. That is why a cation with a completed noble-gas shell, such as Na⁺ or K⁺, is left out of the class in the conventional organic-chemistry treatment even though it is positive.

The route through this question is the definition, applied to the vacancy rather than to the sign of the charge. Ask of each species: is there somewhere for an electron pair to go? H⁺ has nothing but vacancy. BF₃ has an empty 2p orbital on boron. Na⁺ has none — the 3s electron it lost leaves it with the closed [Ne] shell, and the next orbital available is far too high in energy to be used in ordinary chemistry. This is a convention with an edge to it, and it is worth knowing the edge: in the broadest Lewis sense, small, highly charged metal cations do act as acids in water, which is why a copper sulphate solution turns acidic while a sodium chloride solution does not. The difference is one of degree — Cu²⁺ polarises the water molecules bound to it strongly enough to release H⁺, Na⁺ does not — and school and undergraduate organic chemistry draws the line so that alkali-metal cations sit outside the class. The question is asked in that framework, and within it the answer is clean. What is not clean is the printed minus on option (c), which is the Commission's own and must not be edited away: it is recorded here so that a reader who notices it knows the difficulty is real and not a misprint in our transcription.

• An electrophile is an electron-deficient species that accepts a pair of electrons to form a covalent bond; it is the reaction-mechanism name for a Lewis acid, while a nucleophile is the Lewis base that donates the pair.
• H⁺ is the archetypal electrophile — no electrons at all — and forms H₃O⁺ with water and NH₄⁺ with ammonia by accepting a lone pair.
• In BF₃ the boron atom has only six valence electrons and an empty 2p orbital, making it a neutral electrophile and a classic Lewis acid; it forms the adduct F₃B←NH₃ with ammonia.
• Na⁺ has the neon configuration — a complete octet with no accessible empty orbital — so it interacts ionically and is conventionally excluded from the class of electrophiles despite its positive charge.
• The booklet prints option (c) as BF₃ with a superscript minus, confirmed by a 600 dpi reading of the page; on a literal reading that anion would be electron-rich and the question would have two correct answers, with no 'more than one of the above' option offered.

• Equating a positive charge with electrophilic character. Na⁺ and K⁺ are cations with complete noble-gas shells and are conventionally outside the class; the test is an accessible empty orbital.
• Assuming a neutral molecule cannot be an electrophile. BF₃ and AlCl₃ are neutral and are the standard examples, because boron and aluminium sit on six electrons rather than eight.
• Skipping past 'All of the above'. On a negative stem it asserts that none of the three is an electrophile, which one clear counter-example — here H⁺ — is enough to disprove.

BPSC asks chemistry as a single definition tested against four species, and this re-examination of January 2025 groups several such one-liners together in its science block. UPSC has not set a bare electrophile question in its General Studies paper; it reaches the same chemistry sideways, through salt hydrolysis, ionic bonding and periodic trends, so the definition has to be carried as an explanation of behaviour rather than as a label.

• practice — not a real PYQ

  Which of the following acts as a Lewis acid ?

  • (a)NH₃
  • (b)BF₃
  • (c)H₂O
  • (d)Cl⁻

  Answer(b) BF₃ — boron has only six valence electrons and an empty 2p orbital, so it accepts a lone pair. Ammonia, water and the chloride ion all have lone pairs to donate and are therefore Lewis bases.

• practice — not a real PYQ

  An electrophile is best described as a species which

  • (a)Donates a pair of electrons to form a covalent bond
  • (b)Accepts a pair of electrons to form a covalent bond
  • (c)Always carries a negative charge
  • (d)Always contains an unpaired electron

  Answer(b) Accepts a pair of electrons to form a covalent bond — the reaction-mechanism name for a Lewis acid. A species that donates the pair is a nucleophile, and a species with an unpaired electron is a free radical.