JEE Chemistry Reduction Oxidation and Hydrolysis Notes Set 01

Reduction: Catalytic Hydrogenation

Reduction here means adding hydrogen across a multiple bond (hydrogenation) or replacing an atom or group by hydrogen (hydrogenolysis). Catalytic hydrogenation uses H2 on a finely divided metal surface. The catalyst gives a lower-energy path: the substrate and hydrogen are chemisorbed on the metal and hydrogen atoms are delivered to the same face of the multiple bond, so the addition is syn. The reaction is exothermic. If H2 and D2 are mixed over platinum they scramble quickly to HD, which is why the product then carries both H and D.

Carboxylic acids, esters and amides are not reduced by catalytic hydrogenation.

1. Heterogeneous catalysts (insoluble): Raney nickel, Pd on charcoal, platinum or its oxide. Homogeneous catalyst (soluble): Wilkinson's catalyst, \( \ce{RhCl(PPh3)3} \); both hydrogens still add from the same side.
2. Lindlar's catalyst (Pd on CaCO3 poisoned with quinoline) and nickel boride Ni2B (P-2 catalyst, from nickel acetate and NaBH4) stop at the alkene stage and give the cis (syn addition) alkene from an alkyne.
3. Rosenmund catalyst (Pd on BaSO4 poisoned with quinoline or sulphur) reduces an acid chloride to the aldehyde, and also partly reduces alkynes to alkenes.

Reduction: Dissolving Metals and Related Reductions

Here the metal is a source of electrons: the substrate accepts an electron to form an anion (radical anion), which then takes a proton from the acidic source.

Reduction: Metal Hydrides

Oxidation: Alkanes, Alkenes and Alkynes

1. Alkanes resist KMnO4 and K2Cr2O7, except that an alkane with a tertiary hydrogen is oxidised to the tertiary alcohol: \( \ce{(CH3)3CH -> (CH3)3COH} \). Catalytic air oxidation gives methanol (Cu, 523 K, high pressure), methanal (MoO3) or acetic acid from ethane (manganese acetate); limited oxygen burning gives carbon black.
2. Cold dilute alkaline KMnO4 (Baeyer's reagent) or OsO4 with NaHSO3 or H2O2 add two OH groups to the same face of an alkene (syn addition) to give a vicinal diol; an alkyne gives a diketone. The purple colour of Baeyer's reagent disappears and brown MnO2 appears, which is the test for unsaturation.
3. Peroxyacids (peroxyformic, peracetic, perbenzoic and m-chloroperbenzoic acid, MCPBA) convert alkenes to epoxides; the more substituted (more nucleophilic) double bond reacts first. Opening the epoxide with H3O+ gives the trans (anti) diol, the opposite stereochemistry to OsO4.
4. Hot or acidified KMnO4 breaks the C=C or C#C bond: a terminal =CH2 becomes CO2, =CHR becomes RCOOH, =CR2 becomes a ketone; a terminal alkyne ≡CH becomes CO2 and an internal alkyne gives two acid molecules.
5. Ozonolysis at about -78 C: with Zn and water (reductive) =CH2 gives HCHO, =CHR an aldehyde and =CR2 a ketone; with H2O2 (oxidative) aldehydes are taken further to acids. An internal alkyne gives a 1,2-diketone under reductive work-up and acids under oxidative work-up.
6. Worked use of the data: a hydrocarbon C16H26 (degree of unsaturation 4) giving only hexanoic acid and succinic acid on ozonolysis must be the diyne \( \ce{CH3(CH2)4-C#C-CH2CH2-C#C-(CH2)4CH3} \).

Oxidation: Alcohols

1. PCC is pyridinium chlorochromate, \( \ce{C5H5NH^+ CrO3Cl^-} \), used in non-aqueous solvent; PDC is pyridinium dichromate; Jones reagent is dilute chromic acid in acetone; Collins reagent is CrO3 with pyridine in CH2Cl2.
2. Manganese dioxide selectively oxidises allylic and benzylic alcohols to aldehydes or ketones. Oppenauer oxidation turns a secondary alcohol into a ketone using a ketone and aluminium tert-butoxide (the reverse of the MPV reduction).
3. Rate order: primary > secondary > tertiary alcohol; aldehyde > alcohol. In chromic acid oxidation a chromate ester forms, and the slow step breaks the C-H bond, so R-CH2OH is oxidised faster than R-CD2OH (a kinetic isotope effect). In water the aldehyde forms a gem-diol and is oxidised further to the acid.

Oxidation: Aldehydes, Ketones and Diols

1. Aldehydes are easily oxidised to acids with the same number of carbons; ketones need vigorous conditions and break on the side of the smaller alkyl group (Popoff's rule), so butanone gives two molecules of acetic acid.
2. Five tests separate aldehydes from ketones: Tollens' reagent (silver mirror), Fehling's solution (red Cu2O; benzaldehyde does not respond), Benedict's solution, mercuric chloride (grey Hg2Cl2 and Hg) and Schiff's reagent (pink colour restored). Both aldehydes and ketones give a 2,4-DNP derivative.
3. SeO2 oxidises a CH2 or CH3 group next to a carbonyl to C=O or CHO.
4. Baeyer-Villiger oxidation: a peracid (or H2O2 with a Lewis acid) inserts an oxygen atom next to the carbonyl, converting a ketone to an ester and a cyclic ketone to a lactone.
5. Periodic acid HIO4 (and lead tetraacetate) cleaves a vicinal diol into two carbonyl compounds through a cyclic ester, so the two OH groups must be able to be syn; only cis cyclic 1,2-diols react. Alpha-hydroxy carbonyl and 1,2-dicarbonyl compounds are also cleaved.

Hydrolysis

Hydrolysis splits a molecule with water into a part that takes OH and a part that takes H. It is endothermic and differs from hydration, where water is only added without fragmentation.