Aldehydes and Ketones — Reduction and Oxidation

• Fehling's and Tollens' tests to distinguish aldehydes from ketones.
• Clemmensen reduction (Zn-Hg/HCl) to reduce C=O to −CH₂−.
• Wolff-Kishner reduction (NH₂NH₂/KOH/Δ) as an alternative deoxygenation.
• Oxidation of aldehydes with various reagents.

Level 1 — Fehling's and Tollens' tests (oxidation)

Tollens' test (silver mirror test): Aldehyde + Ag(NH₃)₂⁺ (Tollens' reagent) → RCOOH + Ag mirror on walls of test tube. Ketones: NO reaction. Even aromatic aldehydes (benzaldehyde) give positive Tollens' test.

Fehling's test: Aldehyde + Fehling's solution (Cu²⁺/tartrate alkaline) → RCOO⁻ + Cu₂O↓ (brick-red precipitate). Aliphatic aldehydes: POSITIVE. Ketones: NEGATIVE. Exception: Benzaldehyde gives NEGATIVE Fehling's test (aromatic aldehyde, less reactive toward Cu²⁺).

Schiff's test: Aldehyde + Schiff's reagent (decolourised fuchsin) → pink/magenta colour. Ketones: negative (or very slow).

Summary:

Level 2 — Clemmensen and Wolff-Kishner reductions

Clemmensen reduction: R−CO−R' + Zn(Hg)/conc. HCl/reflux → R−CH₂−R' (C=O → CH₂). Acidic conditions (HCl). Used when molecule is acid-stable. Cannot use if molecule contains acid-sensitive groups.

Wolff-Kishner reduction: R−CO−R' + NH₂NH₂ (hydrazine) → hydrazone R−C(=NNH₂)−R' → (KOH/Δ or ethylene glycol) → R−CH₂−R'. Basic conditions. Used when molecule is acid-sensitive. Kishner: base catalysis.

Both reduce C=O to CH₂ — choose based on stability:

Full reduction to alcohol: NaBH₄ (mild, selective — reduces aldehydes/ketones, NOT carboxylic acids/esters) or LiAlH₄ (strong — reduces all carbonyl groups).

JEE tip: Clemmensen = HCl (acid); Wolff-Kishner = KOH (base). One has NH₂NH₂ (Wolff-Kishner). Both give the same product (alkane equivalent). Use for synthesising alkylbenzenes from aryl ketones (Friedel-Crafts acylation followed by Wolff-Kishner is a classic two-step synthesis).

NCERT spotlight — Synthesis sequence

Benzene → (Friedel-Crafts, RCOCl/AlCl₃) → aryl ketone → (Clemmensen reduction, Zn-Hg/HCl) → alkylbenzene. This is the standard JEE two-step synthesis. Direct Friedel-Crafts alkylation gives rearrangement; acylation + reduction avoids rearrangement.

Oxidation with KMnO₄: Aldehyde → carboxylic acid (KMnO₄/H⁺ oxidises all aldehydes). Methyl ketone → acetic acid + other acid fragment (oxidative cleavage). Ketones generally resist mild oxidation.

An organic compound A gives a silver mirror with Tollens' reagent but no red precipitate with Fehling's solution. Identify the type of compound and give an example.

Step 1 — Tollens' positive → compound is an aldehyde (not ketone).
Step 2 — Fehling's negative → NOT an aliphatic aldehyde.
Step 3 — Aromatic aldehydes: positive Tollens', negative Fehling's.
Step 4 — Example: benzaldehyde (C₆H₅CHO).
Step 5 — Reason for Fehling's negative: aromatic ring delocalises C=O slightly; 
         Cu²⁺ (weaker oxidant than Ag⁺ complex) cannot oxidise benzaldehyde under these conditions.
Step 6 — Tollens' (Ag⁺ complex) is a stronger oxidant → can oxidise benzaldehyde. ✓
Answer: Compound A is an aromatic aldehyde, e.g. benzaldehyde.

Applications — food testing and organic synthesis

Fehling's test: traditionally used to test for reducing sugars (glucose, fructose) in urine (diabetes screening before modern glucometers). Tollens' test: silver mirror process — manufacturing of mirrors and silvered glass. Wolff-Kishner: pharmaceutical synthesis of drugs from keto intermediates.

• What observation will you see when acetaldehyde is added to Fehling's solution?
• Give the product of Clemmensen reduction of cyclohexanone.
• Why is Wolff-Kishner preferred over Clemmensen for acid-sensitive molecules?

Open the Practice tab for graded questions on Reduction and Oxidation.

• Use the platform-native live simulation or PhET-style tool for this topic (number line, Venn, physics playground, molecule builder, sensor dashboard, etc.).
• Mirror / body / home activity: physically do the concept (count objects, measure, role-play) and photograph or describe for portfolio.
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.