Which of the following is not an electrophile ?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2025 Q98 — Science & Technology PYQ with Answer

State PCS 2025 Q98 — Science & Technology PYQ with Answer