Oxidation & Reduction Reactions

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Oxidation & Reduction Reactions — Key Facts for Makerere University (Uganda) Core concept: Oxidation involves loss of electrons (or gain of oxygen/loss of hydrogen); reduction involves gain of electrons (or loss of oxygen/gain of hydrogen). In organic chemistry, these terms are applied to changes in oxidation state of carbon High-yield points: Oxidizing and reducing agents in organic chemistry; oxidation of alcohols (1°, 2°, 3°); reduction of carbonyls; oxidation states of carbon; redox reactions in organic synthesis ⚡ Exam tip: Remember that oxidation of carbon increases its number of bonds to oxygen or decreases bonds to hydrogen; reduction does the opposite

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Oxidation & Reduction Reactions — Makerere University (Uganda) Study Guide

1. Oxidation States in Organic Chemistry

Definition

Oxidation state (or oxidation number) of carbon is the apparent charge on carbon if all bonds to atoms of different electronegativity are assigned to the more electronegative atom.

Calculating Oxidation State of Carbon

Rules:

1. Each bond to H or metal (less electronegative than C) = −1
2. Each bond to C, N, S, halogens = 0 (equal electronegativity)
3. Each bond to O or halogen (more electronegative than C) = +1
4. Each bond to N with higher electronegativity contribution considered similarly

Examples:

Compound	Oxidation State of Carbon	Calculation
CH₄	−4	4 × (−1) = −4
CH₃Cl	−2	3(−1) + 0 = −2
CH₂Cl₂	0	2(−1) + 2(0) = 0
CHCl₃	+2	(−1) + 3(0) = +2
CCl₄	+4	4(0) = +4 (no bonds to less electronegative)
H–CHO	+1	2(0) + 1(+1) + 1(−1) = +1
CH₃OH	−2	3(−1) + 1(+1) = −2
HCOOH	+2	2(0) + 2(+1) = +2
CO₂	+4	2(+1) = +4

Oxidation increases oxidation state; reduction decreases it.

Summary of Oxidation State Changes

Oxidation (lose e⁻, O, or H):
CH₄ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄
  −4     −2       0       +2      +4

Or with O:
CH₄ → CH₃OH → HCHO → HCOOH → CO₂
  −4     −2      +1      +2      +4

Reduction (gain e⁻, H, or lose O):
CO₂ → HCOOH → HCHO → CH₃OH → CH₄
  +4     +2      +1      −2      −4

2. Oxidation of Alcohols

Oxidizing Agents

1. Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄): Orange Cr(VI) → Green Cr(III)
2. Acidified potassium permanganate (KMnO₄/H₂SO₄): Purple → Brown MnO₂
3. Tollens’ reagent (Ag(NH₃)₂⁺): For aldehydes only
4. Pyridinium chlorochromate (PCC): Mild oxidant, stops at aldehyde for 1° alcohols

Oxidation Products by Alcohol Type

Primary Alcohols (1°): 1° alcohol → ALDEHYDE → CARBOXYLIC ACID

• Mild oxidant (PCC) → aldehyde (stops)
• Strong oxidant (K₂Cr₂O₇ or KMnO₄) → carboxylic acid

Example — Ethanol oxidation: CH₃CH₂OH → (K₂Cr₂O₇/H⁺) → CH₃CHO → (further oxidation) → CH₃COOH

• Cr₂O₇²⁻ (orange) + 3e⁻ → 2Cr³⁺ (green) — color change indicates oxidation

Secondary Alcohols (2°): 2° alcohol → KETONE (no further oxidation under normal conditions)

• Oxidation stops at ketone because the carbon already has 4 bonds and cannot form a stable carboxylic acid without breaking C–C bond

Example — Isopropanol oxidation: (CH₃)₂CHOH → (K₂Cr₂O₇/H⁺) → (CH₃)₂C=O (acetone) + H₂O

Tertiary Alcohols (3°): 3° alcohol → NO REACTION under mild oxidizing conditions

• No hydrogen on the carbon bearing –OH (cannot lose H from same carbon)
• Carbon already has 4 bonds and cannot form stable oxidation product without breaking C–C bond

⚠️ IMPORTANT: 3° alcohols DO NOT oxidize under conditions that oxidize 1° and 2° alcohols. This is a key distinguishing feature.

Test for Alcohols — Lucas Test (ZnCl₂/HCl)

• 1° alcohol: No reaction at room temp (slow)
• 2° alcohol: Cloudiness in 5-10 min (moderate)
• 3° alcohol: Immediate cloudiness/turbidity (fast)
• Reaction: Alcohol + HCl → alkyl chloride (SN1 mechanism, carbocation formation)

3. Oxidation of Aldehydes

Aldehydes are easily oxidized to carboxylic acids.

Common Oxidizing Agents

1. Tollens’ Reagent (Ag(NH₃)₂⁺): Ag⁺ → Ag⁰ (silver mirror) — aldehydes only
2. Fehling’s Solution (Cu²⁺/tartrate): Cu²⁺ → Cu₂O (red precipitate) — aldehydes only
3. Benedict’s Solution (Cu²⁺/citrate): Similar to Fehling’s
4. Acidified KMnO₄ or K₂Cr₂O₇: Carboxylic acid
5. Silver oxide (Ag₂O) with NaOH: Carboxylic acid

Tollens’ Test

Reaction: R–CHO + 2Ag(NH₃)₂⁺ + H₂O → R–COO⁻NH₄⁺ + 2Ag⁰ + 3NH₃ + NH₄⁺ (Silver mirror on test tube)

Positive test: Aldehydes ONLY (form silver mirror) Negative test: Ketones, alcohols, alkanes (no silver mirror)

⚠️ Ketones do not react with Tollens’ reagent under normal conditions (except α-hydroxy ketones which tautomerize to aldehydes).

Fehling’s Test

Reaction: R–CHO + 2Cu²⁺ + 2H₂O → R–COO⁻ + Cu₂O (brick-red precipitate) + 4H⁺ Positive test: Aldehydes (form red Cu₂O precipitate) Negative test: Ketones (except α-hydroxy ketones)

Distinguishing Aldehydes from Ketones

Test	Aldehyde	Ketone
Tollens’ reagent	Silver mirror	No reaction
Fehling’s solution	Red precipitate	No reaction
Acidified K₂Cr₂O₇	Oxidizes to acid	Oxidizes to ketone (2° only)
2,4-DNP	Aldehyde 2,4-DNP derivative	Ketone 2,4-DNP derivative

4. Oxidation of Alkenes

Cold Dilute KMnO₄ (Syn Dihydroxylation)

C=C + KMnO₄ (cold, dilute) → vicinal diol (glycol)

• Syn-addition (both OH groups add to same face)
• Purple KMnO₄ decolorizes (brown MnO₂ precipitate)

Mechanism: Cyclic manganate ester intermediate forms and is hydrolyzed.

Examples:

• Ethene + cold dilute KMnO₄ → ethane-1,2-diol (ethylene glycol)
• Cyclohexene → cyclohexane-1,2-diol (cis-diol)

Hot Concentrated KMnO₄ (Oxidative Cleavage)

C=C + hot conc. KMnO₄ → CO₂ (from C=C with H’s) or carboxylic acids

• R–CH=CH₂ → CO₂ + CO₂ (complete cleavage to CO₂)
• R–CH=CH–R’ → R–COOH + R’–COOH (two carboxylic acids)
• R–CO–R’ (ketones form if internal alkene with same groups)

Symmetrical alkenes → two identical carboxylic acids or CO₂.

Example: CH₃–CH=CH–CH₃ + hot KMnO₄ → 2CH₃COOH (acetic acid) Example: CH₂=CH–CH₂–CH₃ + hot KMnO₄ → CO₂ + CH₃CH₂COOH (propionic acid)

Ozonolysis (O₃ followed by Zn/H₂O or Me₂S)

Alkene + O₃ → ozonide → reductive workup → carbonyl compounds

• O₃ + Zn/H₂O: Aldehydes and/or ketones
• O₃ + H₂O₂: Carboxylic acids (oxidative workup)

Mechanism: Ozone adds across C=C to give ozonide, which is cleaved.

Examples:

• Ethene → 2HCHO (formaldehyde)
• Propene → HCHO + CH₃CHO (acetaldehyde)
• 2-methylpropene → HCHO + (CH₃)₂C=O (acetone)

Uses: Determining alkene structure by identifying carbonyl products.

5. Reduction of Carbonyl Compounds

Reducing Agents

1. NaBH₄ (Sodium borohydride): Mild; reduces aldehydes and ketones to alcohols; does NOT reduce esters, carboxylic acids, or amides
2. LiAlH₄ (Lithium aluminium hydride): Strong; reduces aldehydes, ketones, esters, carboxylic acids, amides; requires dry conditions (reacts violently with water)
3. H₂/Ni, Pt, or Pd: Catalytic hydrogenation; reduces C=C and C=O; aromatic rings NOT reduced under mild conditions
4. Sn/HCl or Fe/HCl: Reduces nitro groups to amines (in aniline synthesis)
5. Hydrazine (N₂H₄) + KOH: Wolff-Kishner reduction (reduces C=O to CH₂)

Reduction Products by Carbonyl Type

Aldehydes → Primary Alcohols: R–CHO + NaBH₄ → R–CH₂OH Example: Acetaldehyde → ethanol

Ketones → Secondary Alcohols: R–CO–R’ + NaBH₄ → R–CH(OH)–R’ Example: Acetone → isopropanol

Esters → Two Alcohols (requires LiAlH₄): R–COOR’ + 2[H] → R–CH₂OH + R’OH Example: Ethyl acetate → ethanol + acetic acid (reduced to ethanol)

Carboxylic Acids → Primary Alcohols (requires LiAlH₄): R–COOH + 4[H] → R–CH₂OH + H₂O Example: Acetic acid → ethanol

⚠️ NaBH₄ cannot reduce carboxylic acids or esters.

Catalytic Hydrogenation

C=C Reduction:

• H₂/Pt or H₂/Ni → alkane
• H₂/Pd-CaCO₃ (Lindlar’s catalyst) → cis-alkene
• Na/NH₃ (dissolving metal reduction) → trans-alkene

C=O Reduction:

• H₂ does NOT reduce C=O under normal catalytic hydrogenation conditions (requires high pressure or special catalysts)
• LiAlH₄ is the standard reagent for reducing carbonyls

6. Oxidation States Summary — Key Conversions

In Synthetic Sequences

Alkane → (free radical halogenation) → alkyl halide
Alkyl halide → (hydrolysis) → alcohol (1° for 1° RX)
Alcohol → (oxidation) → aldehyde → (oxidation) → carboxylic acid
Carboxylic acid → (LiAlH₄) → alcohol (1°)
Alcohol → (oxidation) → ketone (2° only)
Ketone → (reduction) → alcohol (2°)
Aldehyde → (reduction) → alcohol (1°)

Redox in Terms of Oxidation State Change

Reaction	Change in Oxidation State
Alkane → Alcohol	C: −4 to −2 (oxidation by 2)
Alcohol → Aldehyde	C: −2 to +1 (oxidation by 3)
Aldehyde → Acid	C: +1 to +2 (oxidation by 1)
Alcohol → Ketone	C: −2 to −1 (oxidation by 1)
Ketone → Alcohol	C: −1 to −2 (reduction by 1)
Acid → Alcohol	C: +2 to −2 (reduction by 4)

7. Oxidation of Alkylbenzenes

Side-chain Oxidation

With KMnO₄ (hot, alkaline) or chromic acid:

• C₆H₅–CH₃ → C₆H₅–COOH (benzoic acid)
• C₆H₅–CH₂–CH₃ → C₆H₅–COOH (side chain completely oxidized to COOH)
• Any alkyl chain on benzene oxidizes completely to –COOH (regardless of length)

Note: The benzene ring is resistant to oxidation under these conditions.

Oxidation of Aromatic Amines

Aniline (C₆H₅–NH₂) → (oxidation) → complex mixtures including quinones.

8. Reduction of Nitro Compounds

Reduction of Nitro to Amine

Mechanism: R–NO₂ → R–NH₂ (via nitroso and hydroxylamine intermediates)

Reagents:

1. Sn/HCl or SnCl₂/HCl (classical method)
2. Fe/HCl (industrial)
3. H₂/Pd or H₂/Pt (catalytic hydrogenation)
4. Zn/NH₄Cl (reduction with metal in neutral conditions — for acid-sensitive compounds)

TNT (2,4,6-trinitrotoluene) can be reduced to TNA (2,4,6-triamino-1-methylbenzene).

Reduction in Different Media

Acidic conditions: R–NO₂ → R–NH₃⁺ → R–NH₂ Neutral conditions: Zn/NH₄Cl → R–NHOH → R–NH₂ Basic conditions: Zn/NaOH → azoxy compounds → azo → hydrazine compounds

9. Self-Indicating Oxidation-Reduction Reactions

Many oxidations produce visible changes:

Oxidant	Color Change	Observation
K₂Cr₂O₇/H⁺	Orange → Green	Color disappears
KMnO₄/H⁺	Purple → Brown MnO₂	Precipitate forms
KMnO₄ (cold dilute)	Purple → Colorless	Decolorizes
Bromine water	Brown → Colorless	Decolorizes (alkenes)
Ag₂O/NaOH	Black Ag₂O → Ag⁰	Silver mirror forms

10. Exam-Style Questions & Tips

Common exam question patterns at Makerere:

1. “By what oxidation state change does [alcohol] become [aldehyde/carboxylic acid]?”
2. “Using [oxidizing agent], what product(s) are formed from [substrate]?”
3. “Explain how you would distinguish between [compound A] and [compound B] using chemical tests”
4. “In the oxidation of [compound] with [agent], what is the role of the oxidizing agent?”
5. “When [alkene] is treated with hot KMnO₄, what products are formed?”
6. “Calculate the oxidation state of carbon in [compound]”

⚡ Exam tips:

• If asked to name an oxidizing or reducing agent in a reaction, write the FULL reagent (e.g., “acidified potassium dichromate” not just “dichromate”)
• When asked about oxidation state, calculate it explicitly showing your working
• Remember: NaBH₄ reduces aldehydes and ketones but NOT esters; LiAlH₄ is stronger and reduces esters and carboxylic acids
• Distinguishing tests (Tollens’, Fehling’s) are for ALDEHYDES ONLY — ketones generally don’t respond

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11. Electrochemical Methods

Redox Potentials and Organic Reactions

Standard reduction potentials can predict whether a redox reaction will occur spontaneously.

Examples (E° in V):

• F₂/F⁻: +2.87 (strongest oxidizing agent)
• MnO₄⁻/Mn²⁺ (acidic): +1.51
• Cr₂O₇²⁻/Cr³⁺ (acidic): +1.33
• Ag⁺/Ag: +0.80
• I₂/I⁻: +0.54
• O₂/H₂O: +1.23

A species with a higher (more positive) reduction potential will oxidize one with a lower potential.

Balancing Redox Equations

Half-reaction method for organic redox:

1. Assign oxidation states
2. Write oxidation and reduction half-reactions
3. Balance electrons
4. Add half-reactions
5. Simplify

Example: Oxidation of ethanol by dichromate: CH₃CH₂OH → CH₃COOH (C: −2 to +2, change = 4 electrons lost) Cr₂O₇²⁻ → Cr³⁺ (Cr: +6 to +3, change = 3 electrons gained per Cr) Multiply ethanol half-reaction by 3 and dichromate by 4 to balance electrons: 3CH₃CH₂OH + 4Cr₂O₇²⁻ → 3CH₃COOH + 8Cr³⁺ + 11H₂O (balanced with H⁺ and H₂O)

12. Biological Redox Reactions

NAD⁺/NADH

Nicotinamide adenine dinucleotide is a biological oxidizing/reducing agent.

• NAD⁺ + 2H → NADH + H⁺ (reduction of NAD⁺)
• NAD⁺ is an oxidizing agent in metabolism
• NADH is a reducing agent (e.g., in oxidative phosphorylation)

FAD/FADH₂

Flavin adenine dinucleotide — another biological redox cofactor.

Biological Oxidations

• Alcohol dehydrogenase: ethanol → acetaldehyde (uses NAD⁺ as oxidant)
• This is why drinking alcohol increases NADH/NAD⁺ ratio, affecting metabolism

13. Specificity of Oxidizing Agents

Mild vs Strong Oxidation

Substrate	Mild Oxidation	Strong Oxidation
1° alcohol	Aldehyde (PCC)	Carboxylic acid (KMnO₄/K₂Cr₂O₇)
2° alcohol	Ketone	Ketone
3° alcohol	No reaction	No reaction
Aldehyde	—	Carboxylic acid
Alkene (C=C)	Vicinal diol (cold KMnO₄)	Cleavage products (hot KMnO₄/O₃)

14. Industrial Redox Processes

Contact Process (Sulfuric Acid Production)

2SO₂ + O₂ → 2SO₃ (V₂O₅ catalyst) — catalytic oxidation of SO₂

Ostwald Process (Nitric Acid Production)

NH₃ + 2O₂ → HNO₃ + H₂O (Pt/Rh catalyst, high temperature)

Haber-Bosch Process (Ammonia Synthesis)

N₂ + 3H₂ → 2NH₃ (Fe catalyst, high P, high T) — catalytic reduction of N₂

Oxidation of Ethylene to Ethylene Oxide

2CH₂=CH₂ + O₂ → 2(CH₂–CH₂)O (Ag catalyst) — partial oxidation

Oxidation of Ethylene to Acetaldehyde (Wacker Process)

CH₂=CH₂ + [O] → CH₃CHO (PdCl₂/CuCl₂ aqueous solution)

Practice Problems

Q1: Calculate the oxidation state of carbon in: (a) CCl₄ (b) CH₃Br (c) CH₃OCH₃ (d) CO (e) C₂H₄

Q2: When but-2-ene is treated with cold dilute KMnO₄, what is the product? Draw its structure. What would hot concentrated KMnO₄ give?

Q3: Describe chemical tests to distinguish between: (a) Propan-1-ol and propan-2-ol (b) Propanal and propanone (c) Ethanol and ethanoic acid (d) Benzoic acid and phenol

Q4: Write equations showing the oxidation of: (a) Propan-1-ol to propanoic acid (b) Cyclohexanol to cyclohexanone (c) 2-methylpropan-1-ol to 2-methylpropanoic acid

Q5: Why does NaBH₄ reduce aldehydes and ketones but not esters or carboxylic acids?

Q6: When 1-methylcyclohexene undergoes oxidative cleavage with hot KMnO₄, what organic products are formed?

Common Mistakes to Avoid

1. Confusing oxidation and reduction: Oxidation = LEO (Loss of Electrons); Reduction = GER (Gain of Electrons). OR: Oxidation = more bonds to O (or fewer bonds to H); Reduction = fewer bonds to O (or more bonds to H).
2. Forgetting that Tollens’ and Fehling’s are for aldehydes only: Ketones don’t react (with rare exceptions).
3. Thinking hot KMnO₄ oxidation of alkenes gives diols: No — hot concentrated gives cleavage products; cold dilute gives diols.
4. Confusing NaBH₄ and LiAlH₄ capabilities: NaBH₄ is milder and doesn’t reduce carboxylic acids or esters; LiAlH₄ is stronger and does.
5. Forgetting that oxidation of 1° alcohol to aldehyde is a 6-electron change: 1° alcohol (C=−2) → aldehyde (C=+1) = 3-electron change per carbon; but you need to consider the full half-reaction.

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