Class 12 Chemistry – High-Value Question Bank
Section A: Multiple Choice Questions (MCQs)
1. The general formula of haloalkanes is:
A) R–OH
B) R–X
C) R–COOH
D) Ar–OH
Answer: B) R–X
2. In haloalkanes, the carbon attached to halogen is generally:
A) sp hybridised
B) sp² hybridised
C) sp³ hybridised
D) dsp² hybridised
Answer: C) sp³ hybridised
3. Which of the following is a haloarene?
A) CH₃Cl
B) C₂H₅Br
C) C₆H₅Cl
D) CH₂Cl₂
Answer: C) C₆H₅Cl
4. The order of C–X bond strength is:
A) C–I > C–Br > C–Cl > C–F
B) C–F > C–Cl > C–Br > C–I
C) C–Cl > C–F > C–I > C–Br
D) C–Br > C–I > C–F > C–Cl
Answer: B
5. The most reactive alkyl halide towards nucleophilic substitution is:
A) CH₃Cl
B) CH₃Br
C) CH₃I
D) CH₃F
Answer: C) CH₃I
6. The reagent used in Finkelstein reaction is:
A) NaCl
B) NaBr
C) NaI
D) AgNO₃
Answer: C) NaI
7. Finkelstein reaction is carried out in:
A) Water
B) Alcohol
C) Dry acetone
D) Benzene
Answer: C) Dry acetone
8. Swarts reaction is used for preparing:
A) Alkyl chlorides
B) Alkyl fluorides
C) Alkyl iodides
D) Alcohols
Answer: B
9. The best reagent for converting alcohol into alkyl chloride is:
A) NaCl
B) SOCl₂
C) NaOH
D) KMnO₄
Answer: B
10. Lucas reagent contains:
A) HCl + ZnCl₂
B) H₂SO₄ + Zn
C) NaOH + ZnCl₂
D) HNO₃ + Zn
Answer: A
11. The reaction mechanism preferred by tertiary haloalkanes is:
A) SN2
B) SN1
C) Addition
D) Oxidation
Answer: B
12. The order of SN1 reactivity is:
A) 1° > 2° > 3°
B) 3° > 2° > 1°
C) CH₃ > 3° > 2°
D) 2° > 3° > 1°
Answer: B
13. SN2 reaction involves:
A) Carbocation formation
B) One-step mechanism
C) Two intermediates
D) Free radical formation
Answer: B
14. SN2 reaction causes:
A) Racemisation
B) Retention
C) Inversion of configuration
D) No stereochemical change
Answer: C
15. Alcoholic KOH with haloalkane gives:
A) Alcohol
B) Alkene
C) Ether
D) Acid
Answer: B
16. Magnesium reacts with alkyl halides in dry ether to form:
A) Alcohol
B) Alkene
C) Grignard reagent
D) Ketone
Answer: C
17. Wurtz reaction gives:
A) Alcohols
B) Symmetrical alkanes
C) Haloarenes
D) Amines
Answer: B
18. Fittig reaction produces:
A) Alkane
B) Biphenyl
C) Alcohol
D) Ether
Answer: B
19. Wurtz–Fittig reaction produces:
A) Benzene
B) Alkyl benzene
C) Biphenyl
D) Phenol
Answer: B
20. Chlorobenzene is less reactive than chloroethane because:
A) Chlorine is smaller
B) C–Cl bond has partial double bond character
C) Benzene ring is unstable
D) Chlorine is less electronegative
Answer: B
Section B: Assertion–Reason Questions
Choose:
A) Both A and R are true and R explains A
B) Both true but R does not explain A
C) A true, R false
D) A false, R true
1.
Assertion: Alkyl iodides are more reactive than alkyl chlorides.
Reason: C–I bond is weaker than C–Cl bond.
Answer: A
2.
Assertion: Chlorobenzene does not easily undergo SN1 reaction.
Reason: Phenyl carbocation is unstable.
Answer: A
3.
Assertion: Halogens are deactivating groups in benzene.
Reason: Halogens show –I effect.
Answer: B
4.
Assertion: Halogens are ortho-para directing groups.
Reason: Halogens donate electrons by resonance.
Answer: A
5.
Assertion: SOCl₂ is preferred for preparing alkyl chlorides.
Reason: The by-products are gases.
Answer: A
Section C: Fill in the Blanks
1. Haloalkanes contain a C–X bond with carbon having ______ hybridisation.
Answer: sp³
2. Haloarenes contain a halogen attached directly to a ______ carbon.
Answer: sp²
3. The order of reactivity of alkyl halides is RI > RBr > ______ > RF.
Answer: RCl
4. Finkelstein reaction uses sodium ______.
Answer: iodide
5. Swarts reaction is used for preparation of alkyl ______.
Answer: fluorides
6. Grignard reagent has the general formula ______.
Answer: R–MgX
7. SN1 reaction involves formation of a ______ intermediate.
Answer: carbocation
8. SN2 reaction occurs by ______ attack.
Answer: backside
9. Alcoholic KOH promotes ______ reaction.
Answer: elimination
10. Freons are responsible for ______ layer depletion.
Answer: ozone
11. DDT is a ______ biodegradable compound.
Answer: non
12. Sandmeyer reaction uses copper ______.
Answer: salts
Section D: Match the Following
| Column A | Column B |
|---|---|
| Finkelstein reaction | Alkyl iodide |
| Swarts reaction | Alkyl fluoride |
| Wurtz reaction | Alkane |
| Fittig reaction | Biphenyl |
| Grignard reagent | R–MgX |
Answers:
- Finkelstein → Alkyl iodide
- Swarts → Alkyl fluoride
- Wurtz → Alkane
- Fittig → Biphenyl
- Grignard → R–MgX
Section E: One Word / Very Short Answer
1. Name the reaction used to prepare alkyl iodides.
Answer: Finkelstein reaction
2. Name the reaction used to prepare alkyl fluorides.
Answer: Swarts reaction
3. What is the product of Wurtz reaction?
Answer: Alkane
4. Which reagent converts alcohol into alkyl chloride?
Answer: SOCl₂
5. Which compound causes ozone depletion?
Answer: Freons
6. What type of reaction is shown by haloalkanes with nucleophiles?
Answer: Nucleophilic substitution
Section F: Short Answer Questions
1. Why are haloalkanes polar?
Answer:
Because halogens are more electronegative than carbon. The C–X bond becomes polar with partial charges:
Cδ⁺–Xδ⁻
2. Why is chlorobenzene less reactive than chloroethane?
Answer:
Because the C–Cl bond in chlorobenzene has partial double bond character due to resonance, making it stronger.
3. Why is dry ether used in Grignard reagent preparation?
Answer:
Because Grignard reagents react with water and get destroyed.
4. Why does branching decrease boiling point?
Answer:
Branching decreases surface area, reducing intermolecular forces.
5. Why are haloalkanes insoluble in water?
Answer:
They cannot form hydrogen bonds effectively with water.
Section G: Important Reaction-Based Questions
1. Complete the reaction:
CH₃CH₂Br + KOH(aq) →
Answer:
CH₃CH₂OH + KBr
2. Complete:
CH₃CH₂Br + KOH(alc) →
Answer:
CH₂=CH₂ + KBr + H₂O
3. Complete:
C₂H₅Br + Mg (dry ether) →
Answer:
C₂H₅MgBr
4. Complete:
2CH₃Br + 2Na →
Answer:
C₂H₆ + 2NaBr
5. Complete:
C₆H₅Cl + NaOH (high temperature, pressure) →
Answer:
C₆H₅OH + NaCl
Section H: Long Answer / Board-Level Questions
1. Explain SN1 mechanism with example.
Answer points:
- Two-step mechanism.
- Formation of carbocation.
- Nucleophile attacks carbocation.
- Favoured by tertiary haloalkanes.
- Gives racemisation.
2. Explain SN2 mechanism.
Answer points:
- One-step process.
- Backside attack by nucleophile.
- Simultaneous breaking and formation of bonds.
- Gives inversion of configuration.
3. Explain why haloarenes are less reactive than haloalkanes.
Answer points:
- Resonance gives partial double bond character.
- Stronger C–X bond.
- sp² carbon.
- SN1 and SN2 reactions are difficult.
4. Explain preparation and uses of chloroform.
Answer points:
- Prepared by chlorination of methane.
- Used as solvent.
- Stored with ethanol because it forms poisonous phosgene in air.
5. Explain preparation of haloarenes from diazonium salts.
Include:
- Sandmeyer reaction
- Gattermann reaction
- Balz–Schiemann reaction
Part 1 – CBSE Competency-Based / Case-Based Questions
Case Study 1: Reactivity of Haloalkanes
A student compares three haloalkanes:
- CH₃Cl
- C₂H₅Br
- (CH₃)₃CBr
He observes that they show different rates of nucleophilic substitution.
Questions:
1. Which compound will undergo SN1 reaction fastest?
A) CH₃Cl
B) C₂H₅Br
C) (CH₃)₃CBr
D) All react equally
Answer: C) (CH₃)₃CBr
2. The reason for faster SN1 reaction of tertiary haloalkanes is:
A) Stronger C–X bond
B) Stable carbocation formation
C) Smaller molecular size
D) Absence of nucleophile
Answer: B
3. Which mechanism will CH₃Br preferably follow?
Answer: SN2
4. Write the order of SN1 reactivity.
Answer:
3° > 2° > 1° > CH₃
Case Study 2: Preparation of Haloalkanes
An organic chemist converts ethanol into chloroethane using different reagents.
Questions:
1. Which reagent gives the best yield of chloroethane?
A) NaCl
B) SOCl₂
C) NaOH
D) H₂O
Answer: B
2. Why is SOCl₂ preferred?
Answer:
Because the by-products SO₂ and HCl are gases and escape, leaving pure alkyl chloride.
3. Name the reaction:
R–Cl + NaI → R–I + NaCl
Answer: Finkelstein reaction
Case Study 3: Haloarenes
Chlorobenzene is treated with sodium hydroxide at high temperature and pressure.
Questions:
1. Product formed is:
A) Benzene
B) Phenol
C) Aniline
D) Toluene
Answer: B
2. Why is chlorobenzene less reactive than chloroethane?
Answer:
Because the C–Cl bond in chlorobenzene has partial double bond character due to resonance.
3. Which group increases the reactivity of chlorobenzene towards nucleophilic substitution?
A) CH₃
B) NO₂
C) OH
D) NH₂
Answer: B
Part 2 – Advanced Assertion–Reason Questions
Choose:
A) Both A and R are true and R explains A
B) Both true but R does not explain A
C) A true, R false
D) A false, R true
1.
Assertion: Alkyl iodides react faster than alkyl chlorides.
Reason: C–I bond is weaker than C–Cl bond.
Answer: A
2.
Assertion: Chlorobenzene undergoes electrophilic substitution reactions.
Reason: The benzene ring is still available for electrophilic attack.
Answer: A
3.
Assertion: Halogens deactivate benzene ring.
Reason: Halogens withdraw electrons through inductive effect.
Answer: A
4.
Assertion: Halogens direct incoming groups to ortho and para positions.
Reason: Lone pair on halogen participates in resonance.
Answer: A
5.
Assertion: Tertiary haloalkanes undergo SN1 reactions easily.
Reason: Tertiary carbocations are more stable.
Answer: A
6.
Assertion: SN2 reactions are faster for methyl halides.
Reason: Methyl halides have minimum steric hindrance.
Answer: A
7.
Assertion: Grignard reagents cannot be prepared in moist ether.
Reason: They react with water.
Answer: A
8.
Assertion: Wurtz reaction is not suitable for preparing unsymmetrical alkanes.
Reason: A mixture of products may be formed.
Answer: A
9.
Assertion: Chlorobenzene does not easily undergo SN1 reaction.
Reason: Phenyl carbocation is unstable.
Answer: A
10.
Assertion: DDT remains in the environment for a long time.
Reason: It is non-biodegradable.
Answer: A
Part 3 – Reaction Conversion Practice
A. Haloalkane Conversions
1. Ethanol → Chloroethane
Reaction:
CH₃CH₂OH
↓ SOCl₂
CH₃CH₂Cl
2. Chloroethane → Ethanol
Reaction:
CH₃CH₂Cl
↓ aq. KOH
CH₃CH₂OH
3. Bromoethane → Ethene
Reaction:
CH₃CH₂Br
↓ alcoholic KOH
CH₂=CH₂
4. Bromoethane → Ethyl magnesium bromide
Reaction:
C₂H₅Br + Mg
↓ dry ether
C₂H₅MgBr
5. Bromoethane → Butane
Reaction:
2C₂H₅Br + 2Na
↓ dry ether
C₄H₁₀ + 2NaBr
6. Chloroethane → Ethyl iodide
Reaction:
C₂H₅Cl + NaI
↓ dry acetone
C₂H₅I + NaCl
B. Haloarene Conversions
7. Benzene → Chlorobenzene
Reaction:
C₆H₆ + Cl₂
↓ FeCl₃
C₆H₅Cl
8. Chlorobenzene → Phenol
Reaction:
C₆H₅Cl
↓ NaOH, high temperature and pressure
C₆H₅OH
9. Aniline → Chlorobenzene
Steps:
Aniline
↓ NaNO₂ + HCl (0–5°C)
Diazonium salt
↓ CuCl
Chlorobenzene
10. Aniline → Fluorobenzene
Steps:
Aniline
↓ Diazotisation
Diazonium salt
↓ HBF₄ + heat
Fluorobenzene
Part 4 – Important Conversion Questions (Practice)
1. Convert propene into 2-bromopropane.
Answer:
Propene + HBr → 2-bromopropane
2. Convert propene into 1-bromopropane.
Answer:
Propene + HBr/peroxide → 1-bromopropane
3. Convert chlorobenzene into biphenyl.
Answer:
2C₆H₅Cl + 2Na
↓ dry ether
C₆H₅–C₆H₅ + 2NaCl
4. Convert chlorobenzene into toluene.
Answer:
C₆H₅Cl + CH₃Cl + 2Na
↓ dry ether
C₆H₅CH₃ + 2NaCl
5. Convert alkyl halide into nitrile.
Answer:
R–X + KCN → R–CN + KX
6. Convert alkyl halide into isocyanide.
Answer:
R–X + AgCN → R–NC + AgX
Part 5 – High-Level Board Thinking Questions
1. Arrange in increasing SN2 reactivity:
tert-butyl bromide, methyl bromide, ethyl bromide
Answer:
tert-butyl bromide < ethyl bromide < methyl bromide
2. Arrange in increasing SN1 reactivity:
ethyl chloride, tert-butyl chloride, isopropyl chloride
Answer:
ethyl chloride < isopropyl chloride < tert-butyl chloride
3. Why does p-nitrochlorobenzene react faster than chlorobenzene?
Answer:
The nitro group withdraws electrons and stabilises the intermediate formed during nucleophilic substitution.
4. Why is vinyl chloride less reactive than ethyl chloride?
Answer:
The C–Cl bond in vinyl chloride is attached to an sp² carbon and has partial double bond character.
5. Why are Freons harmful?
Answer:
They release chlorine radicals that destroy ozone molecules.