Hydrocarbons Solutions | KalviGavani

How to Use This Page

Welcome to the Hydrocarbons exercise solutions for Class 11 Chemistry (Tamil Nadu State Board). Here is how to get the most from this page:

1. Attempt every question on your own first — do not jump straight to the answers. 2. Compare your answer with the solution provided. If they differ, trace through the explanation step by step. 3. Re-work mistakes — write the corrected answer in your own words; this builds deeper understanding. 4. Use the Key Concepts section for a quick recap before exams.

Key Concepts (Quick Recap)

Hydrocarbon Classes — Comparison Table

Preparation Methods — Summary

Key Reactions of Alkanes

Key Reactions of Alkenes

Key Reactions of Alkynes

Markovnikov's Rule vs Anti-Markovnikov Addition

I. Choose the Best Answer (MCQs)

• The CH₂= side gives HCHO (formaldehyde)
• The CH₃CH= side gives CH₃CHO (acetaldehyde)

II. Fill in the Blanks

1. The general formula of alkenes is CₙH₂ₙ.

2. Alkanes are also called paraffins because of their low reactivity.

3. The hybridisation of carbon in ethene is sp².

4. When ethyne is passed through red-hot iron tubes at 873 K, it trimerises to benzene.

5. The Wurtz reaction is used to prepare alkanes with even number of carbon atoms.

6. Markovnikov's rule states that H adds to the carbon with more hydrogen atoms.

7. The anti-Markovnikov addition of HBr to alkenes in the presence of peroxide is called the Kharasch effect.

8. Ozonolysis of ethene gives formaldehyde (HCHO).

9. Methane is prepared by the decarboxylation of sodium acetate (CH₃COONa) with soda lime.

10. The bond angle in ethyne (C₂H₂) is 180°.

11. Ethene undergoes polymerisation to form polythene (polyethylene).

12. The colour of bromine water is decolourised by alkenes and alkynes (unsaturated hydrocarbons).

13. The product of complete combustion of methane is CO₂ and H₂O.

14. Lindlar's catalyst is used to convert alkynes to cis-alkenes.

15. The IUPAC name of CH₃C≡CH is prop-1-yne.

III. True or False (with Corrections)

IV. Match the Following

Set 1: Hydrocarbon and Preparation

1. Methane → (c) Decarboxylation of sodium acetate 2. Ethene → (a) Dehydration of ethanol 3. Ethyne → (b) Calcium carbide + water 4. Ethane → (d) Wurtz reaction 5. Propene → (e) Dehydrohalogenation of propyl halide

Set 2: Reaction and Product

1. CH₄ + Cl₂ →(UV)→ (c) CH₃Cl + HCl 2. C₂H₂ + H₂O →(HgSO₄/H⁺)→ (a) CH₃CHO 3. CH₃CH=CH₂ + HBr → (b) CH₃CHBrCH₃ 4. CH₂=CH₂ + Br₂ → (d) CH₂Br–CH₂Br 5. C₂H₂ + 2H₂ →(Ni)→ (e) C₂H₆

V. Answer Briefly (2-mark)

VI. Answer in Detail (5-mark)

Preparation of Alkanes

1. From Alkenes (Hydrogenation/Catalytic Reduction): Alkenes react with hydrogen in the presence of a catalyst (Ni, Pd, or Pt) to form alkanes.

2. From Carboxylic Acids (Decarboxylation): Sodium salt of a carboxylic acid is heated with soda lime (NaOH + CaO) to remove CO₂.

3. Wurtz Reaction: An alkyl halide reacts with sodium metal in dry ether to form a higher alkane.

Limitation: This reaction gives alkanes with an even number of carbons and can produce a mixture if two different alkyl halides are used.

4. From Grignard Reagent: A Grignard reagent (RMgX) reacts with water or a carbonyl compound.

Physical Properties of Alkanes

• State: C₁–C₄ are gases, C₅–C₁₇ are liquids, C₁₈+ are waxy solids
• Boiling point: Increases with molecular mass (more van der Waals forces)
• Solubility: Insoluble in water; soluble in organic solvents
• Density: Less than water (float on water)

Chemical Reactions of Alkanes

1. Combustion (Burning): Alkanes burn in excess oxygen to give CO₂ and H₂O.

Incomplete combustion gives carbon monoxide or carbon (soot):

2. Halogenation (Free Radical Substitution): Alkanes react with halogens in the presence of UV light.

The reaction follows a free radical mechanism (initiation, propagation, termination).

3. Nitration: Alkanes react with nitric acid at high temperature to form nitroalkanes.

4. Pyrolysis/Cracking: Higher alkanes decompose on heating without air to give smaller molecules.

1. Addition of Hydrogen (Hydrogenation)

This is used in the hydrogenation of vegetable oils (liquid) to form vanaspati ghee (solid).

2. Addition of Halogens (Halogenation)

This reaction is used as a test for unsaturation — bromine water is decolourised from reddish-brown to colourless.

3. Addition of Hydrogen Halides (Hydrohalogenation)

With symmetrical alkene (ethene):

With unsymmetrical alkene (propene) — Markovnikov's Rule:

Markovnikov's Rule: "When an unsymmetrical reagent adds to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogen atoms."

Mechanism (Electrophilic Addition):

Step 1: H⁺ from HBr attacks the double bond, forming a carbocation intermediate.

Step 2: Br⁻ attacks the carbocation.

The Markovnikov product is favoured because the more stable secondary carbocation forms preferentially.

4. Anti-Markovnikov Addition (Peroxide Effect / Kharasch Effect)

In the presence of organic peroxides (R–O–O–R), HBr adds to unsymmetrical alkenes in an anti-Markovnikov fashion:

This occurs because the reaction follows a free radical mechanism instead of an electrophilic addition:

The Br· radical adds to the terminal carbon (the one with more H atoms), generating the more stable radical at the secondary carbon. Then HBr provides the H atom.

Important note: The peroxide effect is observed ONLY with HBr, not with HCl or HI. This is because:

• HCl bond is too strong (radical cannot abstract H)
• HI bond is too weak (iodine radicals are unstable)

5. Addition of Water (Hydration)

For unsymmetrical alkenes, Markovnikov's rule applies:

6. Ozonolysis

Ozonolysis is used to determine the position of the double bond.

Preparation of Alkynes

Physical Properties of Alkynes

• State: Ethyne is a colourless gas with a sweet smell
• Boiling point: Lower than corresponding alkenes and alkanes
• Solubility: Slightly soluble in water; soluble in organic solvents
• Flame: Burns with a very sooty flame (luminous) due to high carbon content

Chemical Reactions of Alkynes

Common Mistakes to Avoid

1. Confusing general formulas: Alkanes = CₙH₂ₙ₊₂, Alkenes = CₙH₂ₙ, Alkynes = CₙH₂ₙ₋₂. The pattern is: each degree of unsaturation removes 2 hydrogen atoms from the alkane formula.

2. Applying Markovnikov's rule incorrectly: Remember — H adds to the carbon with MORE H atoms (not fewer). Draw the carbocation intermediate to verify: the more stable carbocation is always formed.

3. Forgetting the peroxide effect applies ONLY to HBr: The anti-Markovnikov addition with peroxides works only with HBr. It does NOT work with HCl or HI. This is a very common exam trap.

4. Incorrect ozonolysis products: Remember that with Zn/H₂O (reductive workup), alkenes give aldehydes or ketones. With H₂O₂ (oxidative workup), aldehydes are further oxidised to carboxylic acids.

5. Confusing Lindlar's catalyst with regular hydrogenation: Lindlar's catalyst (Pd/BaSO₄, poisoned) converts alkynes to CIS-alkenes only. Regular Ni catalyst converts alkynes all the way to alkanes.

6. Wrong products in Wurtz reaction: The Wurtz reaction always gives an alkane with an EVEN number of carbon atoms (R–R). Using two different halides gives a mixture of three products.

7. Forgetting that alkynes can undergo two successive additions: Because alkynes have two pi bonds, they can add two molecules of H₂, HBr, or Br₂. Do not stop at the first addition unless specified (e.g., Lindlar's catalyst).

8. Misidentifying the acidic hydrogen in terminal alkynes: Only the terminal H in C≡C–H is acidic (due to sp hybridisation, 50% s-character). Internal alkynes (C≡C with no terminal H) are NOT acidic.

Exam Checklist

• [ ] I can write the general formulas of alkanes, alkenes, and alkynes
• [ ] I can explain the preparation methods for each class of hydrocarbon
• [ ] I can state and apply Markovnikov's rule with examples
• [ ] I can explain the peroxide effect (anti-Markovnikov addition) and why it works only with HBr
• [ ] I can predict ozonolysis products for given alkenes and alkynes
• [ ] I can write the reactions of alkanes: combustion, halogenation, nitration, pyrolysis
• [ ] I can write the addition reactions of alkenes: H₂, X₂, HX, H₂O, O₃
• [ ] I can describe the reactions of alkynes: hydrogenation, halogenation, hydration, ozonolysis
• [ ] I can explain the acidic nature of terminal alkynes and their reactions with Na, NaNH₂, and Tollen's reagent
• [ ] I can differentiate alkanes, alkenes, and alkynes by their chemical tests (Br₂ water, Baeyer's reagent, Tollen's reagent)