How to Use This Page
Welcome to the Haloalkanes and Haloarenes 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)
SN1 vs SN2 Comparison
| Feature | SN1 | SN2 |
|---|---|---|
| Full name | Unimolecular Nucleophilic Substitution | Bimolecular Nucleophilic Substitution |
| Rate law | Rate = k[RX] (first order) | Rate = k[RX][Nu⁻] (second order) |
| Steps | Two steps (carbocation intermediate) | One step (concerted) |
| Substrate preference | 3° > 2° > 1° (stable carbocation) | 1° > 2° > 3° (less steric hindrance) |
| Stereochemistry | Racemisation (partial) | Inversion of configuration (Walden inversion) |
| Nucleophile | Weak nucleophile sufficient | Strong nucleophile needed |
| Solvent | Polar protic (H₂O, ROH) | Polar aprotic (DMSO, DMF, acetone) |
| Leaving group | Better LG favours SN1 | Better LG favours SN2 |
| Intermediate | Carbocation | No intermediate (transition state) |
| Rearrangement | Possible (carbocation rearrangement) | Not possible |
E1 vs E2 Comparison
| Feature | E1 | E2 |
|---|---|---|
| Full name | Unimolecular Elimination | Bimolecular Elimination |
| Rate law | Rate = k[RX] (first order) | Rate = k[RX][Base] (second order) |
| Steps | Two steps (carbocation intermediate) | One step (concerted) |
| Substrate preference | 3° > 2° > 1° | 3° > 2° > 1° (but strong base needed) |
| Base | Weak base sufficient | Strong base required |
| Product | More substituted alkene (Saytzeff) | More substituted alkene (Saytzeff) |
| Competition with | SN1 | SN2 |
| Solvent | Polar protic | Polar aprotic or protic with strong base |
Haloalkane vs Haloarene Reactivity
| Feature | Haloalkanes | Haloarenes |
|---|---|---|
| C–X bond | Pure single bond | Partial double-bond character |
| Reason | No resonance | Lone pair on X resonates with benzene ring |
| Bond length | Longer | Shorter |
| Reactivity in SN | High | Low |
| Why less reactive? | — | Resonance stabilisation + shorter C–X bond |
| Example | CH₃Cl (reactive) | C₆H₅Cl (unreactive in SN) |
Grignard Reagent — Key Reactions
| Reaction | Equation | Product |
|---|---|---|
| With H₂O | RMgX + H₂O → RH + Mg(OH)X | Alkane |
| With CO₂ | RMgX + CO₂ → RCOOMgX →(H₃O⁺)→ RCOOH | Carboxylic acid |
| With HCHO | RMgX + HCHO → RCH₂OMgX →(H₃O⁺)→ RCH₂OH | Primary alcohol |
| With RCHO | RMgX + RCHO → R₂CHOMgX →(H₃O⁺)→ R₂CHOH | Secondary alcohol |
| With R₂CO | RMgX + R₂CO → R₃COMgX →(H₃O⁺)→ R₃COH | Tertiary alcohol |
| With RCOR | RMgX + RCOR → R₂C(OMgX)R →(H₃O⁺)→ R₂C(OH)R | Tertiary alcohol |
Classification of Haloalkanes
| Type | Structure | Example |
|---|---|---|
| Primary (1°) | C–C–X (X on a carbon bonded to one C) | CH₃CH₂Cl |
| Secondary (2°) | C–C(X)–C (X on a carbon bonded to two C) | (CH₃)₂CHCl |
| Tertiary (3°) | C–C(X)–C with three C attached | (CH₃)₃CCl |
| Allylic | C=C–C–X | CH₂=CH–CH₂Cl |
| Benzylic | C₆H₅–C–X | C₆H₅CH₂Cl |
| Vinylic | C=C–X | CH₂=CHCl |
I. Choose the Best Answer (MCQs)
II. Fill in the Blanks
1. The IUPAC name of CH₃CH₂CH₂Cl is 1-chloropropane.
2. In SN2 mechanism, the nucleophile attacks from the back side (opposite to the leaving group).
3. The reactivity order of alkyl halides for SN1 mechanism is 3° > 2° > 1°.
4. A Grignard reagent has the general formula RMgX.
5. The carbon atom bonded to the halogen in a tertiary alkyl halide is attached to three other carbon atoms.
6. In haloarenes, the C–Cl bond has partial double-bond character due to resonance.
7. The product of the reaction between a Grignard reagent and CO₂ followed by hydrolysis is a carboxylic acid.
8. In the E2 mechanism, the base removes a proton that is anti-periplanar to the leaving group.
9. The elimination reaction that follows Saytzeff's rule produces the more substituted alkene as the major product.
10. The bond dissociation energy of the C–X bond decreases in the order: C–F > C–Cl > C–Br > C–I.
11. The common name of CHCl₃ is chloroform.
12. The reagent used to convert an alcohol to an alkyl chloride is SOCl₂ (thionyl chloride) or PCl₅.
13. The IUPAC name of C₆H₅Cl is chlorobenzene.
14. DDT stands for DichloroDiphenylTrichloroethane.
15. Boiling point of alkyl halides increases with increasing molecular weight.
III. True or False (with Corrections)
IV. Match the Following
Set 1: Reaction Types and Features
| Column A | Column B |
|---|---|
| 1. SN1 | (a) Inversion of configuration |
| 2. SN2 | (b) Racemisation |
| 3. E1 | (c) Two-step elimination |
| 4. E2 | (d) One-step elimination |
| 5. Grignard | (e) RMgX |
1. SN1 → (b) Racemisation 2. SN2 → (a) Inversion of configuration 3. E1 → (c) Two-step elimination 4. E2 → (d) One-step elimination 5. Grignard → (e) RMgX
Set 2: Haloalkane Classification
| Column A | Column B |
|---|---|
| 1. CH₃CH₂Cl | (a) Tertiary halide |
| 2. (CH₃)₂CHCl | (b) Primary halide |
| 3. (CH₃)₃CCl | (c) Secondary halide |
| 4. CH₂=CHCH₂Cl | (d) Allylic halide |
| 5. C₆H₅CH₂Cl | (e) Benzylic halide |
1. CH₃CH₂Cl → (b) Primary halide 2. (CH₃)₂CHCl → (c) Secondary halide 3. (CH₃)₃CCl → (a) Tertiary halide 4. CH₂=CHCH₂Cl → (d) Allylic halide 5. C₆H₅CH₂Cl → (e) Benzylic halide
V. Answer Briefly (2-mark)
(ii) (CH₃)₂CHCl — The longest chain has 3 carbons (propane) with Cl on C-2.
VI. Answer in Detail (5-mark)
SN2 Mechanism (Bimolecular Nucleophilic Substitution)
The SN2 mechanism is a one-step, concerted process where the nucleophile attacks the substrate from the back side (opposite to the leaving group) while the leaving group departs simultaneously.
Rate law: Rate = k[RX][Nu⁻] (second order)
Mechanism — Example: Hydrolysis of methyl bromide
The nucleophile (OH⁻) attacks the carbon from the side opposite to Br. In the transition state, both OH and Br are partially bonded to carbon. As the OH–C bond forms fully, the C–Br bond breaks completely.
Stereochemistry: Complete inversion of configuration (Walden inversion). If the starting material is R-configured, the product is S-configured.
Factors favouring SN2:
- Primary (1°) substrates (less steric hindrance)
- Strong nucleophiles (OH⁻, CN⁻, CH₃O⁻)
- Polar aprotic solvents (DMSO, acetone, DMF)
- Good leaving groups (I⁻ > Br⁻ > Cl⁻ > F⁻)
SN1 Mechanism (Unimolecular Nucleophilic Substitution)
The SN1 mechanism is a two-step process. In the first step, the leaving group departs to form a carbocation intermediate. In the second step, the nucleophile attacks the carbocation.
Rate law: Rate = k[RX] (first order)
Step 1 — Slow step (rate-determining): Ionisation
Step 2 — Fast step: Nucleophilic attack
Stereochemistry: Racemisation (partial). The carbocation is planar (sp²), so the nucleophile can attack from either side. If the starting material is optically active, the product is a mixture of retention and inversion configurations, with a slight preference for inversion.
Factors favouring SN1:
- Tertiary (3°) substrates (form stable carbocations)
- Weak nucleophiles (H₂O, ROH)
- Polar protic solvents (H₂O, ROH — stabilise the carbocation)
- Good leaving groups (same as SN2)
Comparison Table
| Feature | SN1 | SN2 |
|---|---|---|
| Kinetics | First order | Second order |
| Steps | Two | One |
| Intermediate | Carbocation | None |
| Substrate | 3° > 2° > 1° | 1° > 2° > 3° |
| Stereochemistry | Racemisation | Inversion |
| Rearrangement | Possible | Not possible |
| Solvent | Polar protic | Polar aprotic |
| Nucleophile | Weak | Strong |
| Catalysis | Lewis acid can help | Not needed |
Preparation of Grignard Reagents
Grignard reagents (RMgX) are prepared by reacting an alkyl or aryl halide with magnesium metal in dry ether. The ether acts as both a solvent and a stabiliser for the Grignard reagent.
Examples:
Important conditions:
- The ether must be absolutely dry — even trace water destroys the Grignard reagent
- The reaction is carried out under anhydrous conditions
- Iodides and bromides are more reactive than chlorides
- The magnesium must be freshly cleaned
Properties of Grignard Reagents
1. Highly reactive — the C–Mg bond is polar with significant covalent character 2. Strong nucleophiles — the carbon bonded to Mg is electron-rich (carbanion-like) 3. Strong bases — they react with any acidic hydrogen 4. Moisture sensitive — they react with water to form alkanes
Important Reactions of Grignard Reagents
E1 Mechanism (Unimolecular Elimination)
The E1 mechanism is a two-step process. In the first step, the leaving group departs to form a carbocation intermediate. In the second step, a base removes a β-hydrogen to form a double bond.
Rate law: Rate = k[RX] (first order)
Step 1 — Slow (rate-determining):
Step 2 — Fast (proton removal):
Example: Dehydration of tertiary alcohols
Characteristics of E1:
- Favoured by tertiary substrates (stable carbocation)
- Competes with SN1 (both involve carbocation)
- Polar protic solvents favour E1
- Weak base is sufficient
- Carbocation rearrangement is possible
E2 Mechanism (Bimolecular Elimination)
The E2 mechanism is a one-step, concerted process. The base removes a β-hydrogen while the leaving group departs simultaneously, forming a double bond.
Rate law: Rate = k[RX][Base] (second order)
Mechanism:
Example: Dehydrohalogenation of 2-bromopropane
Example with Saytzeff's rule:
Characteristics of E2:
- Favoured by strong bases (KOH in alcohol, NaOEt)
- Anti-periplanar geometry required (H and X must be on opposite sides)
- Competes with SN2
- No carbocation intermediate (no rearrangement possible)
- Polar aprotic solvents with strong base favour E2
Comparison: Substitution vs Elimination
| Feature | Substitution (SN) | Elimination (E) |
|---|---|---|
| Product | New group replaces old group | Double bond formed |
| Saturation | No change | Decreases (unsaturation increases) |
| Base/Nu needed | Moderate | Strong base needed |
| Favoured by | Polar solvents, weak base | High temperature, strong base |
| SN1/E1 competition | Low temp, polar protic solvent | Higher temp promotes E1 over SN1 |
| SN2/E2 competition | Polar aprotic solvent, good Nu | Strong bulky base, high temp |
Factors favouring elimination over substitution: 1. High temperature — elimination needs more energy 2. Strong base — e.g., KOH in ethanol, NaOEt 3. Bulky base — e.g., t-BuOK favours elimination 4. Tertiary substrate — more β-hydrogens available 5. Concentrated base — higher base concentration favours E2
Common Mistakes to Avoid
1. Confusing SN1 and SN2 substrate preferences: SN1 favours tertiary substrates (stable carbocation), while SN2 favours primary substrates (less steric hindrance). Do NOT mix these up.
2. Incorrect stereochemical outcome: SN2 gives inversion of configuration (Walden inversion), not retention. SN1 gives racemisation, not inversion. Remember the "umbrella flip" analogy for SN2.
3. Forgetting that Grignard reagents react with water: This is a very common exam trap. Grignard reagents MUST be prepared in dry ether. Even trace moisture will destroy them, converting RMgX to RH.
4. Confusing E1 with SN1 products: Both E1 and SN1 involve a carbocation intermediate. Conditions that favour one may also produce the other. High temperature favours E1; low temperature favours SN1.
5. Applying Saytzeff's rule incorrectly: The more substituted alkene is the major product, not the less substituted one. "More substituted" means more alkyl groups on the double-bonded carbons.
6. Thinking haloarenes undergo SN reactions easily: Haloarenes are much LESS reactive than haloalkanes in nucleophilic substitution due to resonance. Do not treat them as equivalent.
7. Confusing allylic and vinylic halides: Allylic halides (halogen on carbon adjacent to C=C) are MORE reactive than alkyl halides. Vinylic halides (halogen on the carbon of C=C) are LESS reactive. Know the difference.
8. Forgetting the anti-periplanar requirement in E2: E2 elimination requires the H and X to be anti-periplanar (180° dihedral angle). This is why certain stereoisomers give different E2 products.
Exam Checklist
- [ ] I can define and distinguish SN1 and SN2 mechanisms with examples
- [ ] I can explain the stereochemical outcomes: inversion (SN2) vs racemisation (SN1)
- [ ] I can list the factors favouring SN1 vs SN2 (substrate, solvent, nucleophile)
- [ ] I can define and distinguish E1 and E2 mechanisms
- [ ] I can state and apply Saytzeff's rule for elimination products
- [ ] I can explain why haloarenes are less reactive than haloalkanes (resonance)
- [ ] I can prepare a Grignard reagent and write its key reactions
- [ ] I can classify haloalkanes as primary, secondary, tertiary, allylic, benzylic, and vinylic
- [ ] I can write IUPAC names of haloalkanes and haloarenes
- [ ] I can compare substitution and elimination reactions and state the factors favouring each