CBSE Class 12 Chemistry — Most Important Reactions for Board Exams

If there's one thing that separates a 90+ scorer in Chemistry from the rest, it's knowing your reactions cold. CBSE board exams test reactions directly — in named reaction questions, conversion problems, mechanism-based questions, and even in reasoning-type answers.

This guide covers the most important reactions from Class 12 Chemistry, organised chapter-wise. Bookmark this page, make flashcards, and revise these reactions until writing them becomes muscle memory.

Why Reactions Matter So Much in CBSE Chemistry Boards

Chemistry board exam papers typically carry 15-20 marks worth of questions that directly involve writing reactions. But the impact goes beyond that — understanding reactions helps you answer reasoning questions, complete conversion chains, and explain mechanisms. A student who knows reactions well can comfortably score 55-60 out of 70 in Chemistry.

The CBSE marking scheme awards full marks for correctly balanced equations with proper conditions (temperature, catalyst, solvent). Missing the reaction conditions or not balancing the equation costs you marks even if the products are correct.

Organic Chemistry — Named Reactions and Conversions

Organic Chemistry is the most reaction-heavy section. These named reactions appear almost every year.

Reactions of Haloalkanes and Haloarenes (Chapter 10)

Wurtz Reaction: 2R-X + 2Na (dry ether) → R-R + 2NaX Used to prepare higher alkanes. Remember: only works well for symmetrical alkanes.

Fittig Reaction: 2Ar-X + 2Na (dry ether) → Ar-Ar + 2NaX Similar to Wurtz but for aryl halides.

Wurtz-Fittig Reaction: Ar-X + R-X + 2Na (dry ether) → Ar-R + 2NaX Combines aryl and alkyl halides.

Finkelstein Reaction: R-X + NaI (acetone) → R-I + NaX Converts chlorides/bromides to iodides. The driving force is precipitation of NaCl/NaBr in acetone.

Swarts Reaction: R-X + AgF → R-F + AgX Used to prepare fluoroalkanes.

Reactions of Alcohols, Phenols, and Ethers (Chapter 11)

Williamson Ether Synthesis: R-ONa + R'-X → R-O-R' + NaX The most important method for preparing ethers. Remember: use primary alkyl halides to avoid elimination.

Kolbe's Reaction: Phenol + NaOH → Sodium phenoxide + CO2 (125°C, 4-7 atm) → Salicylic acid Phenol is converted to salicylic acid. This is an electrophilic substitution at the ortho position.

Reimer-Tiemann Reaction: Phenol + CHCl3 + NaOH → Salicylaldehyde Introduces an aldehyde group at the ortho position of phenol. With CCl4 instead of CHCl3, you get salicylic acid.

Reactions of Aldehydes and Ketones (Chapter 12)

Aldol Condensation: Two molecules of an aldehyde (or ketone) with alpha-hydrogen undergo base-catalysed condensation to form beta-hydroxy aldehyde (aldol), which on heating gives alpha-beta unsaturated aldehyde.

Cannizzaro Reaction: Aldehydes without alpha-hydrogen undergo self-oxidation and reduction in the presence of concentrated NaOH. One molecule is oxidised to acid, the other is reduced to alcohol.

Clemmensen Reduction: R-CO-R' + Zn-Hg/conc. HCl → R-CH2-R' Reduces carbonyl group to CH2 in acidic conditions.

Wolff-Kishner Reduction: R-CO-R' + NH2NH2/KOH (ethylene glycol) → R-CH2-R' Same result as Clemmensen but in basic conditions. Choose based on whether the substrate is acid-sensitive or base-sensitive.

Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids with alpha-hydrogen are brominated at the alpha-carbon using Br2/P (red phosphorus).

Reactions of Amines (Chapter 13)

Sandmeyer Reaction: Diazonium salt + CuCl/HCl → Aryl chloride Diazonium salt + CuBr/HBr → Aryl bromide Diazonium salt + CuCN/KCN → Aryl cyanide

Gattermann Reaction: Similar to Sandmeyer but uses Cu powder instead of CuX/HX.

Carbylamine Reaction (Isocyanide Test): Primary amine + CHCl3 + KOH → Isocyanide (foul smell) This is a test for primary amines. The offensive smell of isocyanide is the positive indicator.

Hoffmann Bromamide Degradation: R-CO-NH2 + Br2 + NaOH → R-NH2 + Na2CO3 Converts an amide to a primary amine with one fewer carbon atom.

Diazotisation: Ar-NH2 + NaNO2 + HCl (0-5°C) → Ar-N2+Cl- Temperature control is critical — above 5°C, the diazonium salt decomposes.

Coupling Reaction

Diazonium salt + Phenol (in alkaline medium) → Azo dye (orange) Diazonium salt + Aniline (in acidic medium) → Azo dye (yellow) This is an electrophilic substitution reaction and is important for dye chemistry questions.

Inorganic Chemistry — Key Reactions

The p-Block Elements (Chapters 7-8)

While Inorganic Chemistry is more about properties and trends, certain reactions are frequently tested:

• Preparation of ozone: 3O2 → 2O3 (UV radiation / silent electric discharge)
• Reaction of ozone with KI: 2KI + O3 + H2O → 2KOH + I2 + O2
• Preparation of sulphuric acid (Contact Process): key steps and conditions
• Preparation of nitric acid (Ostwald Process): catalytic oxidation of ammonia

The d- and f-Block Elements (Chapter 8)

• Preparation of KMnO4 from pyrolusite ore (fusion with KOH + oxidation)
• Preparation of K2Cr2O7 from chromite ore
• Reactions showing the oxidising nature of KMnO4 in acidic, basic, and neutral media

Physical Chemistry — Reactions to Remember

Electrochemistry (Chapter 3)

• Nernst equation application for cell EMF calculations
• Electrode reactions at anode and cathode during electrolysis of molten NaCl, aqueous NaCl, and aqueous CuSO4

Chemical Kinetics (Chapter 4)

• First-order reaction examples: decomposition of N2O5, radioactive decay
• Integrated rate law derivation and its application

Tips to Memorise Chemistry Reactions Effectively

Write, Don't Just Read

The single most effective way to memorise reactions is to write them out by hand. Reading a reaction and writing it activate different parts of your brain. Write each important reaction at least 5 times over the course of your preparation.

Group Similar Reactions Together

Notice patterns. Sandmeyer and Gattermann reactions are nearly identical — the difference is Cu powder vs. CuX. Clemmensen and Wolff-Kishner achieve the same result in different conditions. Grouping similar reactions helps you remember differences.

Use the Reaction Conditions as Memory Anchors

Often, students remember the reactants and products but forget the conditions. The conditions are what CBSE specifically tests. Make a separate list of just reaction conditions and quiz yourself.

Practice Conversion Chains

CBSE loves conversion problems: "Convert ethanol to ethanoic acid" or "Convert aniline to chlorobenzene." These require you to chain multiple reactions together. Practice at least 20-30 conversion chains from previous year papers.

Use AI for Instant Doubt Clearing

When you're practising reactions and get stuck on a mechanism or can't figure out why a particular product forms, use EaseLearn AI to get instant explanations. Just photograph the reaction or question with your phone camera, and the AI breaks down the mechanism step by step. It's free and available 24/7 — perfect for those late-night revision sessions.

How to Score Full Marks in Reaction-Based Questions

1. Always balance the equation
2. Write reaction conditions above or below the arrow (catalyst, temperature, solvent, pressure)
3. For named reactions, write the name of the reaction alongside the equation
4. In mechanism questions, show electron movement with curved arrows
5. For conversion problems, write each intermediate step — don't skip reactions even if you think the answer is obvious

Frequently Asked Questions

How many named reactions are important for CBSE Class 12 Chemistry boards?

Approximately 15-20 named reactions are regularly tested. The most frequently appearing ones are Sandmeyer, Wurtz, Aldol Condensation, Cannizzaro, Williamson Synthesis, Kolbe's, Reimer-Tiemann, Hoffmann Bromamide, and Carbylamine reactions. Focus on these first, then cover the rest.

Do I need to learn reaction mechanisms for CBSE boards?

Yes, CBSE can ask mechanisms for reactions like SN1, SN2, electrophilic addition, and electrophilic aromatic substitution. Understanding mechanisms also helps you predict products in unfamiliar reactions, which is useful for HOTS (Higher Order Thinking Skills) questions.

Is NCERT enough for Chemistry reactions in board exams?

For board exams, NCERT is absolutely sufficient. Every reaction that appears in the board paper is from NCERT. However, for competitive exams like NEET and JEE, you may need additional practice from reference books for application-based problems.

How do I remember reaction conditions for so many reactions?

Create a dedicated "conditions chart" — a single sheet listing each reaction with only its conditions (catalyst, temperature, solvent). Revise this sheet daily during the last month. You can also use flashcard apps or AI tools like EaseLearn to quiz yourself on conditions specifically.

What if I forget a reaction during the exam?

If you forget the exact reaction, try to write what you remember — the reactants, the type of reaction, and the general product. CBSE follows a step-marking scheme, so partial answers still earn marks. Never leave a reaction-based question blank.

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