The mildest reducing agent which reduces only carbonyl group in presence of nitro, carboxyl, double bond and ester group is _________.

Question

UnlockIAS · Accepted Answer

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.

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

Concept: 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.

State PCS 2021 Q70 — Science & Technology PYQ with Answer

State PCS 2021 Q70 — Science & Technology PYQ with Answer