Organic chemistry study guide · Reaction Mechanism MCAT Tutor
Organic chemistry redox: PCC vs Jones, NaBH₄ vs LiAlH₄, Lindlar & hydride reductions
Redox in undergraduate orgo is mostly reagent matching: which alcohol oxidation stops at the aldehyde, which hydride reduces an ester, and whether an alkyne becomes a cis or trans alkene. The OrgoMech library groups nine core redox reactions — the same family scored in the Daily Drill, including the Lindlar (cis) vs dissolving metal (trans) trap.
Written for undergraduate orgo, pre-med, and MCAT organic chemistry review — United States exam-style framing.
Short lesson
Oxidizing alcohols — PCC vs Jones (chromic acid)
PCC (pyridinium chlorochromate) in dichloromethane oxidizes primary alcohols to aldehydes and secondary alcohols to ketones without adding water — the aldehyde often survives.
Jones reagent (CrO₃/H₂SO₄, aqueous chromic acid) is stronger: primary alcohols go to carboxylic acids; secondary alcohols to ketones. Tertiary alcohols generally resist oxidation without C–C cleavage.
Exam tip: if the answer line expects a carboxylic acid from a primary alcohol, think Jones/aqueous Cr(VI); if it expects an aldehyde, think PCC or Swern-type mild oxidation.
Reducing carbonyls — NaBH₄ vs LiAlH₄
NaBH₄ is selective: reduces aldehydes and ketones to alcohols under protic or alcoholic workup. It does not normally reduce esters, amides, or carboxylic acids in standard undergraduate conditions.
LiAlH₄ is stronger: reduces aldehydes, ketones, acids, and esters to alcohols (after careful aqueous workup). Amide reduction depth varies by course.
Match hydride to functional group in the starting material and the oxidation level you need in the product — especially when a molecule has both a ketone and an ester.
Alkene formation and hydrogenation
Catalytic hydrogenation (H₂, Pd/C or Pt) reduces alkenes to alkanes — syn addition of H₂ across the π bond.
From alkynes: Lindlar catalyst (partial poisoned Pd) gives cis (syn) alkene; dissolving metal reduction (Na/NH₃ or Li/NH₃) gives trans alkene via radical anion chemistry.
Daily Drill trap: Lindlar → cis; dissolving metal → trans — do not swap when a stereochemical alkene is drawn.
Other high-yield redox patterns
Cold, dilute KMnO₄ on an alkene gives a syn dihydroxylation (vicinal diol) — related to OsO₄ chemistry in addition hubs.
Strong oxidation of alkyl benzenes with hot KMnO₄ can give benzoic acid (side-chain cleavage to COOH) when the benzylic position has hydrogen — ties to aromatic synthesis order.
Link carbonyl reductions to Grignard additions (carbon increases) vs hydrides (only changes oxidation state at C=O).
Common traps on exams
Using NaBH₄ on an ester
Ester → alcohol needs LiAlH₄ (or DIBAL at controlled temperature in advanced courses). NaBH₄ on a methyl ester is a common wrong answer.
PCC vs Jones on 1-butanol
PCC → butanal; Jones → butanoic acid. The reagent line is the entire question.
Lindlar vs Na/NH₃ on an alkyne
Lindlar gives cis-alkene; dissolving metal gives trans. Stereochemistry of the alkene is part of the answer.
Hydrogenating an alkyne with H₂/Pd without poison
Unpoisoned Pd reduces alkyne all the way to alkane. Lindlar stops at cis-alkene.
Practice-style prompts
Try these on paper, then check product and mechanism in OrgoMech.
- 1-Pentanol + PCC vs Jones — name the organic product from each (no over-thinking).
- Cyclohexanone + NaBH₄ then H₃O⁺ — what alcohol class forms?
- Can LiAlH₄ reduce methyl benzoate? What alcohol do you get after workup?
- 3-Hexyne + Lindlar vs 3-hexyne + Na/NH₃ — draw cis vs trans product.
- Cyclohexene + cold dilute KMnO₄ — what functional group forms on the alkene?
Student questions
Common questions undergraduates ask about this topic before exams.
- What is the difference between PCC and Jones reagent?
- PCC oxidizes primary alcohols to aldehydes and secondary alcohols to ketones under anhydrous conditions. Jones (aqueous chromic acid) oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones.
- What is the difference between NaBH₄ and LiAlH₄?
- NaBH₄ reduces aldehydes and ketones to alcohols but generally not esters or acids. LiAlH₄ is stronger and can reduce esters and carboxylic acids to alcohols after workup.
- What does Lindlar reduction do to an alkyne?
- Lindlar catalyst (poisoned Pd) partially hydrogenates an alkyne to a cis alkene — syn addition of hydrogen to the triple bond.
- What is dissolving metal reduction of an alkyne?
- Sodium or lithium in liquid ammonia reduces an alkyne to a trans alkene. Compare with Lindlar (cis) on the same starting alkyne in exam traps.
- Does catalytic hydrogenation (H₂/Pd) reduce alkynes to cis alkenes?
- Unpoisoned Pd/C typically reduces alkynes all the way to alkanes unless a poisoned catalyst like Lindlar is specified.
- What does cold dilute KMnO₄ do to an alkene?
- Syn dihydroxylation to a vicinal diol — a common redox/addition crossover question on finals.
- How is redox tested in OrgoMech?
- The app library lists nine redox reactions; the Daily Drill scores the Redox family and uses traps such as Lindlar (cis) vs dissolving metal (trans).
Further reading (external)
Open textbooks and reference sites for the same topics covered above. We link out to established science education publishers — not affiliate pages.
- Oxidation and reduction reactions (LibreTexts) — LibreTexts
- OpenStax — Oxidation and reduction — OpenStax
- PubChem — NIH NLM
Continue on this site
Drill this topic in OrgoMech
Predict the major product for your structure, then order the curved-arrow mechanism — the same skills tested on homework, finals, and the MCAT orgo section. Predict the major product, then order the curved arrows. Organic chemistry practice for reactions, mechanisms, and the days left before your final.