VKC Notes · Synthetic uses of reagents · Civil Services P-II · IFoS P-II · Geo-Scientist P-III

11.1 Reagents for Oxidation, Reduction and Hydroboration

★★★ Asked in every paper, 2021–2025: 56 parts, 448 marks (Civil Services 26 parts / 265 m · IFoS 15 / 95 m · Geo-Scientist 15 / 88 m), plus 7 Geo-Scientist Prelims questions. The single biggest topic of Civil Services Paper II.

Syllabus lines. Civil Services and IFoS: “Synthetic uses of reagents: OsO₄, HIO₄, CrO₃, Pb(OAc)₄, SeO₂, NBS, B₂H₆, Na–liquid NH₃, LiAlH₄, NaBH₄, n-BuLi, MCPBA”. Geo-Scientist: “Oxidation and reduction” and additions to C=C (hydroboration, epoxidation). Carbon–carbon bond-forming reactions are in the carbonyl notes.

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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes

11.1.1 Hydride reductions: what reduces what ★★★

Reagent	Aldehyde, ketone	Ester, lactone	Acid	Amide, nitrile	Acid chloride	Solvent
NaBH₄	✓ alcohol	✗ (slow)	✗	✗	✓ alcohol	MeOH, EtOH, H₂O
LiAlH₄	✓	✓ two alcohols	✓ 1° alcohol	✓ amine	✓	dry ether, THF only
DIBAL-H (−78 °C)	✓	→ aldehyde	–	nitrile → aldehyde	–	toluene, hexane
BH₃·THF	✓	slow	✓ fastest	✓ amine	slow	THF

Fig. 11.1.1 How a hydride reduces a ketone.

Draw it in 60 s: draw the H–BH₃⁻ bond and propanone; arrow from the B–H bond to the carbonyl C, second arrow from C=O to O; then the alkoxide and MeOH → propan-2-ol.

LiAlH₄ is far more reactive: Al–H is more polar than B–H and Li⁺ activates the C=O. It reacts violently with water and alcohols, so it needs ether or THF; NaBH₄ survives in alcohols and water CSE 2021 Q7(b) 10m.SN
BH₃ is an electrophilic (Lewis-acid) reductant: it attacks the electron-rich carboxylic acid oxygen first, so it reduces COOH in the presence of an ester. A hydroxy ester then lactonises; a stereocentre not touched keeps its configuration CSE 2025 Q6(a)(i) 10m CSE 2021 Q8(a)(i).SN
Conjugated C=O: NaBH₄ and LiAlH₄ (in the cold) add 1,2: cinnamaldehyde → cinnamyl alcohol IFoS 2023 Q6(a) 10m. NaBH₄/CeCl₃ (Luche) makes 1,2 addition certain; excess hot LiAlH₄ can also reduce a styrene-type C=C.SN
Acid chloride + NaBH₄ → aldehyde → 1° alcohol (butanoyl chloride → butan-1-ol) CSE 2022 Q7(b)(i).
Stereochemistry: hydride adds from the less hindered face (steric approach control). Camphor: the syn-C7 methyl blocks the exo face, H⁻ comes endo, and 90 % isoborneol (OH exo) forms; norcamphor gives the opposite CSE 2023 Q5(d) 10m. Bulky L-Selectride gives the axial alcohol from 4-t-butylcyclohexanone; LiAlH₄ the equatorial.SN

Also asked: CSE 2024 Q7(b) N-Boc piperidine aldehyde → LiAlH₄ → hydroboration → Jones · CSE 2023 Q6(b)(i) NaBH₄, acetylide, Na/NH₃ in one scheme.

11.1.2 Birch reduction and dissolving metals ★★★

Fig. 11.1.2 Birch reduction: where the hydrogens go, and the products.

Draw it in 60 s: CO₂H: the ipso and para carbons become CH₂/CH, leaving two C=C not conjugated; OMe: the OMe carbon keeps its C=C, and H₃O⁺ turns the enol ether into cyclohex-2-enone.

(11.1.1)

Na or Li in liquid NH₃ with an alcohol: e⁻ → radical anion → protonation → e⁻ → protonation. The product is the non-conjugated 1,4-diene.SN
Electron-withdrawing group (CO₂H, CO₂⁻): the substituted (ipso) and para carbons are reduced: benzoic acid → cyclohexa-2,5-diene-1-carboxylic acid CSE 2021 Q5(c) 10m Geo 2022 Q6(c) 4m.SN
Electron-donating group (OMe, alkyl): the substituted carbon stays on a C=C: anisole → 1-methoxycyclohexa-1,4-diene; with acid this enol ether hydrolyses to cyclohex-3-enone and then cyclohex-2-enone Geo 2023 Q8(a) IFoS 2021 Q1(f) 5m IFoS 2024 Q5(c) 5m.SN
The C-4 carbanion can be trapped inside the molecule: 4-(4-bromobutyl)benzoic acid → a spiro diene acid Geo 2024 Q9(d)(i) 5m.
Alkynes: Na/NH₃ → trans (E) alkene (the vinyl radical anion takes the trans shape); H₂, Lindlar (Pd/CaCO₃, Pb, quinoline) → cis (Z). Isolated C=C are untouched by Na/NH₃ CSE 2024 Q5(b) 10m CSE 2022 Q7(b)(ii) Geo Pre 2021 Q101.SN

11.1.3 C=O to CH₂, and Meerwein–Ponndorf–Verley ★★

Wolff–Kishner (N₂H₄, KOH, ethylene glycol, 200 °C; Huang–Minlon): hydrazone → loss of N₂ → carbanion → CH₂. Basic, so acid-sensitive groups and the carbon skeleton survive (camphenilone → 2,2-dimethylnorbornane, no rearrangement) Geo 2023 Q6(b) 5m CSE 2024 Q1(d) 10m.SN
Clemmensen (Zn–Hg, conc. HCl): for acid-stable, base-sensitive compounds; aryl ketones best.SN
A β-halo ketone hydrazone loses N₂ and the carbanion closes a cyclopropane; a 1,2-dione is reduced at both ends IFoS 2024 Q1(h) 5m.
MPV (Al(O-i-Pr)₃, i-PrOH): hydride from the isopropoxide to the C=O through a six-membered transition state; only C=O is reduced (C=C, NO₂ survive). Benzophenone → benzhydrol. The reverse, with acetone as acceptor, is the Oppenauer oxidation Geo 2024 Q10(a)(ii).SN

Fig. 11.1.3 SeO₂, Pb(OAc)₄ and Wolff–Kishner on one page.

Draw it in 60 s: SeO₂ turns the CH₃ next to C=O into CHO; Pb(OAc)₄ closes butan-1-ol to THF; N₂H₄/KOH turns C=O into CH₂.

11.1.4 Hydroboration ★★★

Fig. 11.1.4 Hydroboration–oxidation of 1-methylcyclohexene.

Draw it in 60 s: arrow from the C=C to B and from B–H to the more substituted C (one step, syn); put BR₂ and H cis; then the peroxide adds to B and the alkyl moves from B to O with retention.

(11.1.2)

B–H adds syn, in one four-centre step, with B on the less substituted carbon (steric, and δ+ develops on the more substituted carbon). No carbocation, so no rearrangement CSE 2021 Q8(a)(ii) 5m.SN
H₂O₂/OH⁻ replaces B by OH with retention. 1-Methylcyclohexene → trans-2-methylcyclohexanol (H and OH added cis) Geo 2025 Q6(e) 4m. Norbornene → exo-norborneol (exo face) Geo 2024 Q8(a)(ii).SN
Compare: oxymercuration–demercuration (Hg(OAc)₂, H₂O; NaBH₄) gives the Markovnikov alcohol without rearrangement; acid hydration gives Markovnikov with possible rearrangement IFoS 2021 Q7(a)(ii) Geo 2021 Q9(b) Geo Pre 2024 Q103.SN
Organoboranes: CO insertion (with H₂O₂) → tertiary alcohols; with LiAlH(OMe)₃ → aldehydes; with water → ketones. Heating moves boron to the chain end (isomerisation) Geo 2023 Q9(b) IFoS 2024 Q7(a)(i) IFoS 2024 Q7(a)(ii).

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