MPSC 2021 Prelims Paper-I · Question 70 of 100

Correct — option (3), NaBH4, sodium borohydride. The stem sets three conditions at once, and only one reagent in the list meets all three: it must be a reducing agent, it must be the mildest of those offered, and it must attack the carbonyl group while leaving a nitro group, a carboxyl group, a carbon-carbon double bond and an ester group untouched. Sodium borohydride is the standard answer to exactly that specification. It is a source of hydride, and it delivers hydride to the carbon of a carbonyl group, converting an aldehyde into a primary alcohol and a ketone into a secondary alcohol. What makes it selective is that boron is less electropositive than aluminium, so the boron-hydrogen bonds of the borohydride ion are less polar and release hydride far less readily than the aluminium-hydrogen bonds of lithium aluminium hydride. That reduced reactivity is a virtue here. It means the reagent is energetic enough for the strongly polarised carbon-oxygen double bond of an aldehyde or a ketone, where the carbon already carries a partial positive charge and invites attack, but not energetic enough for the far less reactive carbonyl carbon of an ester or of a carboxylic acid, where the neighbouring oxygen atom feeds electron density back into the carbonyl and blunts it. Nitro groups and isolated carbon-carbon double bonds are left alone as well, since neither presents the polarised, electron-poor carbon that hydride attacks. Sodium borohydride is also mild in the practical sense: it is a stable solid, it can be used in alcohol or even in water, and it needs no elaborate precautions, whereas the stronger hydride reagent must be handled in scrupulously dry ether because it reacts violently with any water present. That combination of selectivity and convenience is why sodium borohydride is the reagent of choice whenever a chemist wants to reduce a carbonyl group in a molecule that contains other reducible functions and wants those other functions preserved. A useful way to hold the comparison is to arrange the four reagents by what they attack. Sodium borohydride touches aldehydes and ketones only. Lithium aluminium hydride touches those and esters, acids, amides and nitriles as well. Hydrogen over a metal catalyst attacks carbon-carbon multiple bonds and can reduce nitro groups. Sodium in liquid ammonia is a dissolving-metal reagent that works on aromatic rings and on alkynes. Only the first of the four is confined to the carbonyl group, which is what the stem asks for. The formulae are printed with subscript digits in the booklet and are written inline here.

• (1)LiAlH4 — Lithium aluminium hydride is the opposite of what the stem asks for: it is the most powerful of the common hydride reducing agents, not the mildest. Aluminium is markedly more electropositive than boron, so the aluminium-hydrogen bonds are strongly polarised and the reagent gives up hydride very readily. It reduces aldehydes and ketones, but it also reduces esters to alcohols, carboxylic acids to primary alcohols, acid chlorides, amides and nitriles, and it opens epoxides, so in a molecule of the kind described in the stem it would demolish the ester and the carboxyl group along with the carbonyl. It is also chemically fierce in the laboratory sense, reacting violently with water and with alcohols, so it has to be used in dry ether under anhydrous conditions. Everything that makes it valuable when a chemist wants complete reduction makes it useless when selectivity is the object.
• (2)Na-NH3 — Sodium in liquid ammonia is not a hydride reagent at all; it is a dissolving-metal reducing system, in which the metal dissolves to give solvated electrons that are transferred one at a time to the substrate. Its characteristic uses are quite different from carbonyl reduction. It reduces an alkyne to the trans or E alkene, which is the standard complement to catalytic hydrogenation over a poisoned catalyst that gives the cis isomer, and it reduces an aromatic ring to a non-conjugated cyclohexadiene in the reaction known as the Birch reduction. Neither of those is selective for a carbonyl group, and neither leaves an aromatic or unsaturated system untouched — which is precisely what the stem demands. The option is offered because 'Na' looks like a reagent related to sodium borohydride, but the sodium in the borohydride is only the counter-ion; the reducing species there is the borohydride ion itself.
• (4)H2-Ni — Hydrogen gas over a nickel catalyst is catalytic hydrogenation, and its most characteristic substrate is exactly the one the stem says must be preserved — the carbon-carbon double bond. Hydrogen is adsorbed on the metal surface and added across a multiple bond, which is how vegetable oils are hardened into vanaspati and how alkenes are converted to alkanes. Nitro groups are also reducible under hydrogenation conditions, giving amines. So a molecule containing a double bond, a nitro group and a carbonyl group, treated with hydrogen over nickel, would be attacked at the wrong places, and the reagent fails the selectivity test the question is built on. It is worth remembering the pairing that examiners like here: catalytic hydrogenation of an alkyne over a poisoned catalyst gives the cis alkene, while sodium in liquid ammonia gives the trans alkene.

Reduction in organic chemistry means the addition of hydrogen or the removal of oxygen, and the practical question is always which reagent attacks which functional group. The two hydride reagents are the workhorses for carbonyl compounds. Sodium borohydride is mild and selective: it reduces aldehydes to primary alcohols and ketones to secondary alcohols and leaves esters, carboxylic acids, amides, nitriles, nitro groups and isolated carbon-carbon double bonds alone, and because it is comparatively unreactive towards protic solvents it can be used in alcohol or water. Lithium aluminium hydride is powerful and unselective: besides aldehydes and ketones it reduces esters and carboxylic acids to primary alcohols, amides and nitriles to amines, and acid chlorides and epoxides as well, and it must be used in dry ether because it reacts violently with water. The difference in strength follows from the electronegativity of the central atom, aluminium being more electropositive than boron so that its hydrides are more hydridic. Catalytic hydrogenation, hydrogen gas over finely divided nickel, palladium or platinum, is the standard method for adding hydrogen across carbon-carbon double and triple bonds, and is the basis of the hardening of vegetable oils; over a poisoned catalyst it stops at the cis alkene. Dissolving-metal reduction, sodium in liquid ammonia, supplies solvated electrons and converts alkynes to trans alkenes and aromatic rings to cyclohexadienes in the Birch reduction. Choosing among them is a matter of matching the reagent's reach to the functional groups present in the molecule.

Organic chemistry appears in MPSC's science section in a compact and predictable form: named reagents and what they do, name reactions, functional groups and their tests, and the industrial or everyday application of a transformation. Questions on reducing agents are a favourite because they can be set as a straightforward recall item or, as here, as a specification problem in which the stem lists the conditions and the candidate has to find the reagent that satisfies all of them. The word doing the work in this stem is 'only' — the reagent must reduce the carbonyl group and nothing else — and a candidate who reads the stem quickly and looks merely for a reducing agent will find all four options defensible, since every one of them reduces something. The habit worth building is to store each reagent with a list of what it does not touch as well as what it does, because selectivity is what examiners test. It is also worth noting that this stem is a fill-in-the-blank, with the blank at the end of the English sentence and at the start of the Marathi one; the two columns are laid out differently, and nothing follows from the position of the blank.

• Sodium borohydride, NaBH4, is a mild and selective reducing agent: it reduces aldehydes to primary alcohols and ketones to secondary alcohols while leaving esters, carboxylic acids, amides, nitriles, nitro groups and isolated carbon-carbon double bonds untouched.
• Lithium aluminium hydride, LiAlH4, is a far stronger reducing agent that reduces aldehydes, ketones, esters, carboxylic acids, acid chlorides, amides and nitriles alike, and it reacts violently with water, so it must be used in dry ether.
• The difference in strength arises because aluminium is more electropositive than boron, so the aluminium-hydrogen bonds are more polar and release hydride ion much more readily than the boron-hydrogen bonds.
• Catalytic hydrogenation with hydrogen over nickel, palladium or platinum adds hydrogen across carbon-carbon double and triple bonds and is the basis of the hardening of vegetable oils; it can also reduce nitro groups to amines.
• Sodium in liquid ammonia is a dissolving-metal reducing system that supplies solvated electrons; it converts alkynes to trans alkenes and reduces aromatic rings to cyclohexadienes in the Birch reduction.

• Reading the stem as asking merely for a reducing agent, when the word 'only' makes selectivity the whole point and every option reduces something
• Treating the two hydride reagents as interchangeable, when one reduces esters and carboxylic acids and the other deliberately does not
• Assuming that catalytic hydrogenation is gentle because it uses ordinary hydrogen gas, when its characteristic action is on carbon-carbon multiple bonds
• Reading 'Na-NH3' as a relative of sodium borohydride, when the sodium in the borohydride is only a counter-ion and the two reagents work by entirely different mechanisms

MPSC's organic chemistry questions cluster around reagents, name reactions and functional-group behaviour, and they are usually one-liners: which reagent converts a named compound into another, which reagent will not touch a stated group, which catalyst is used in a named industrial process. Reduction is asked more often than most topics because it connects to industry through the hardening of oils and to biology through the reducing sugars. The examiner's standard device is the specification stem used here, listing the groups that must survive, and it rewards a candidate who has learnt each reagent as a pair of lists — what it reduces and what it spares. Expect also the complementary questions on oxidising agents such as acidified potassium permanganate and potassium dichromate, and on the distinguishing tests for aldehydes and ketones, since these are the other half of the same chapter.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which reagent will reduce a ketone to a secondary alcohol without affecting an ester group present in the same molecule ?

  • (a)Lithium aluminium hydride
  • (b)Sodium borohydride
  • (c)Hydrogen over a nickel catalyst
  • (d)Acidified potassium dichromate

  Answer(b) Sodium borohydride — it is a mild source of hydride, reactive enough for the strongly polarised carbonyl carbon of an aldehyde or a ketone but not for the far less reactive carbonyl carbon of an ester, where the adjoining oxygen feeds electron density into the carbonyl group. Lithium aluminium hydride would reduce the ester as well, since it is a much stronger hydride donor. Hydrogen over nickel acts chiefly on carbon-carbon multiple bonds, and acidified potassium dichromate is an oxidising agent, not a reducing one.

• practice — not a real PYQ

  The hardening of vegetable oils to make vanaspati is an example of

  • (a)catalytic hydrogenation of carbon-carbon double bonds
  • (b)reduction of ester groups by lithium aluminium hydride
  • (c)oxidation by acidified potassium permanganate
  • (d)the Birch reduction using sodium in liquid ammonia

  Answer(a) Catalytic hydrogenation of carbon-carbon double bonds — vegetable oils contain unsaturated fatty acid chains, and passing hydrogen through the oil in the presence of finely divided nickel adds hydrogen across those double bonds, converting the liquid oil into a semi-solid fat with a higher melting point. The reaction leaves the ester linkages of the glyceride untouched, which is why the product is still a fat. Lithium aluminium hydride would attack those ester groups instead, permanganate is an oxidising agent, and the Birch reduction acts on aromatic rings.