Organic Chemistry – Chapter 17 Halogenoalkanes | Interactive Revision

17.1 Introduction to Halogenoalkanes

⚛️ Definition & General Formula

  • Halogenoalkanes (Alkyl Halides): Compounds where one hydrogen atom of an alkane is replaced by a halogen atom
  • General Formula: R-X where R = alkyl group, X = F, Cl, Br, I
  • Also called: Alkyl halides, halogen derivatives of alkanes
  • Uses: Non-polar solvents, synthesis of many organic compounds
📝 Key Point: Halogenoalkanes are classified as mono, di, tri, or poly haloalkanes based on number of halogen atoms

🏷️ Types of Halogenoalkanes

Primary (1°)
Halogen attached to primary carbon
→
Secondary (2°)
Halogen attached to secondary carbon
→
Tertiary (3°)
Halogen attached to tertiary carbon
TypeCarbon TypeStructureExample
Primary Attached to 0 or 1 alkyl group R-CH₂-X CH₃-CH₂-Cl (Chloroethane)
Secondary Attached to 2 alkyl groups R₂CH-X (CH₃)₂CH-Cl (2-Chloropropane)
Tertiary Attached to 3 alkyl groups R₃C-X (CH₃)₃C-Cl (2-Chloro-2-methylpropane)

17.2 Physical Properties

🔥 Melting & Boiling Points

  • Higher MP/BP than corresponding alkanes
  • Reason: Polarity of C-X bond (halogen more electronegative than carbon)
  • C-X bond polar: Cδ⁺ – Xδ⁻

🔬 Molecular Structure

R – X (sp³ hybridized)
C-X bond: Polar covalent
Electronegativity: F(3.98) > Cl(3.16) > Br(2.96) > I(2.66)
💡 Polarity Order: C-F > C-Cl > C-Br > C-I (Based on electronegativity difference)

17.3 Preparation of Alkyl Halides

⚗️ 1. From Alcohols

Using Various Reagents

R-OH + HCl (ZnCl₂) → R-Cl + H₂O

R-OH + SOCl₂ (pyridine) → R-Cl + SO₂ + HCl

R-OH + PCl₃ → 3R-Cl + H₃PO₃

R-OH + PCl₅ → R-Cl + POCl₃ + HCl

⚡ 2. From Alkanes (Free Radical Substitution)

CH₃-CH₃ + Cl₂ → CH₃-CH₂-Cl + HCl (UV light)

Limitation: Not pure – forms mixtures of mono, di, tri substituted products

🔗 3. From Alkenes

a) Hydrohalogenation CH₂=CH₂ + HBr → CH₃-CH₂-Br (Markovnikov’s rule) b) Halogenation CH₂=CH₂ + Br₂ → Br-CH₂-CH₂-Br (1,2-dibromoethane)
🎯 Best Method: Reaction of alcohols with SOCl₂, PX₃, or PX₅ gives pure alkyl halides

17.4 Reactivity of Alkyl Halides

⚡ Factors Controlling Reactivity

  • 1. Bond Polarity: Electronegativity difference creates Cδ⁺ – Xδ⁻
  • 2. Bond Energy: Energy required to break C-X bond

📊 Electronegativity Values

AtomElectronegativityAtomElectronegativity
F3.98I2.66
Cl3.16H2.20
Br2.96C2.55

⚖️ Reactivity Orders

By Bond Polarity
R-F > R-Cl > R-Br > R-I
⇄
By Bond Energy
R-I > R-Br > R-Cl > R-F
Overall Reactivity: R-I > R-Br > R-Cl > R-F
Note: C-F bond very strong → alkyl fluorides unreactive under ordinary conditions

17.5 Nucleophilic Substitution (Sₙ) Reactions

🔄 General Reaction

R-X + Nu⁻ → R-Nu + X⁻

🎯 Key Terms

TermDefinitionExamples
Substrate Alkyl halide being attacked R-X
Nucleophile (Nu⁻) Species with lone pair that attacks electrophilic carbon OH⁻, NH₃, CN⁻, H₂O, SH⁻
Leaving Group (X⁻) Departs with electron pair Cl⁻, Br⁻, I⁻ (good)
OH⁻, OR⁻, NH₂⁻ (poor)

🧪 Important Sₙ Reactions

1. With NaOH (aq)

R-X + NaOH → R-OH + NaX (Forms alcohol)

2. With KCN

R-X + KCN → R-CN + KX (Forms nitrile)

3. With NH₃

R-X + NH₃ → R-NH₂ + HX (Forms primary amine)

4. With AgNO₃

R-X + AgNO₃ → R-NO₃ + AgX↓ (Test for halogens)

Silver Halide Tests:
• AgCl: White ppt, soluble in NH₃(aq)
• AgBr: Cream ppt, partially soluble
• AgI: Yellow ppt, insoluble in NH₃(aq)

β-Elimination Reactions

➖ Definition & Mechanism

  • Definition: Removal of two groups (X and β-H) from adjacent carbons to form C=C
  • Also called β-elimination (requires β-hydrogen)
  • β-H is slightly acidic due to halogen’s electron-withdrawing effect
CH₃-CH₂-X + NaOH(alc) → CH₂=CH₂ + NaX + H₂O

⚖️ Sₙ vs Elimination

Nucleophilic Substitution Elimination • X replaced by Nu⁻ • X and β-H removed • Forms R-Nu • Forms alkene • Aqueous conditions favor • Alcoholic conditions favor • Strong nucleophile needed • Strong base needed
Key Points:
• Nucleophile = electron-rich species
• Substitution = one group replaces another
• Elimination = two groups removed, double bond formed

Exercises – Complete Solutions

Multiple Choice Questions:

1. In primary alkyl halides, the halogen atom is attached to a carbon which is further attached to how many carbon atoms?

a. Two
b. Three
c. One
d. Four

Answer: (c) One

Explanation: Primary carbon is attached to only one other carbon atom (or zero in methyl). Primary alkyl halides have structure R-CH₂-X where the carbon with X is attached to only one alkyl group.

2. Alkyl halides are considered to be very reactive compounds towards nucleophiles, because:

a. they have an electrophilic carbon
b. they have an electrophilic carbon and a good leaving group
c. they have an electrophilic carbon and a bad leaving group
d. they have a nucleophilic carbon and a good leaving group

Answer: (b) they have an electrophilic carbon and a good leaving group

Explanation: The carbon in C-X bond is electrophilic (δ⁺) due to halogen’s electronegativity. Halogens (Cl⁻, Br⁻, I⁻) are good leaving groups as they are stable anions.

3. Which one of the following is not a nucleophile?

a. H₂O
b. H₂S
c. BF₃
d. NH₃

Answer: (c) BF₃

Explanation: BF₃ is an electrophile (electron-deficient) with empty p-orbital. H₂O, H₂S, and NH₃ have lone pairs → nucleophiles.

4. Double bond is formed as a result of:

a. Substitution reactions
b. Elimination reactions
c. Addition reactions
d. Rearrangement reactions

Answer: (b) Elimination reactions

Explanation: Elimination removes two atoms/groups from adjacent carbons, forming C=C. Addition adds to double bond, substitution replaces groups.

5. Which of the following alkyl halides cannot be formed by direct reaction of alkanes with halogen?

a. RBr
b. RCl
c. RF
d. RI

Answer: (c) RF

Explanation: Fluorine is too reactive – causes explosive reactions with alkanes. Cl₂ and Br₂ give controlled free radical substitution with UV light.

Short Answer Questions:

i. What are primary, secondary and tertiary alkyl halides?

Answer:

Primary (1°) Halogen attached to carbon that is attached to only one other carbon (R-CH₂-X) Example: CH₃-CH₂-Cl Secondary (2°) Halogen attached to carbon that is attached to two other carbons (R₂CH-X) Example: (CH₃)₂CH-Cl Tertiary (3°) Halogen attached to carbon that is attached to three other carbons (R₃C-X) Example: (CH₃)₃C-Cl

ii. What are Nucleophilic substitution reactions or Sₙ reaction?

Answer: Reactions where nucleophile (electron-rich species) replaces leaving group (halogen) from alkyl halide.

R-X + Nu⁻ → R-Nu + X⁻

Components:
• Substrate: R-X (alkyl halide)
• Nucleophile: Nu⁻ (OH⁻, CN⁻, NH₃, etc.)
• Leaving group: X⁻ (Cl⁻, Br⁻, I⁻)

iii. Tertiary alkyl halides show Sₙ1 reactions mostly, why?

Answer: Tertiary carbocations are most stable due to +I effect of three alkyl groups.

Sₙ1 Mechanism:
1. R₃C-X → R₃C⁺ + X⁻ (slow, rate-determining)
2. R₃C⁺ + Nu⁻ → R₃C-Nu (fast)

Tertiary carbocations are stabilized by hyperconjugation and inductive effect → form easily → favor Sₙ1.

iv. What are elimination reactions?

Answer: Reactions where two atoms/groups are removed from adjacent carbons to form double bond.

CH₃-CH₂-X + OH⁻ → CH₂=CH₂ + X⁻ + H₂O

Types: β-elimination (removes X and β-H)
Conditions: Strong base, heat, alcoholic medium
Requirement: β-hydrogen must be present

v. Which factor decides the reactivity of alkyl halides?

Answer: Two main factors:

1. Bond Polarity: Cδ⁺-Xδ⁻ polarity (F > Cl > Br > I)
2. Bond Energy: C-X bond strength (C-F strongest, C-I weakest)

Overall Reactivity: R-I > R-Br > R-Cl > R-F
Bond energy factor dominates over polarity for nucleophilic substitution.

Conceptual Questions:

3. Discuss the reactivity of alkyl halides.

Answer:

Alkyl halides are highly reactive due to:

A) Factors:
1. Polar C-X bond: Halogen electronegative → Cδ⁺ electrophilic center
2. Good leaving group: Halogens form stable anions (X⁻)
3. Bond strength: C-I weakest → most reactive

B) Reactivity Orders:
• By halogen: R-I > R-Br > R-Cl > R-F
• By carbon type: 3° > 2° > 1° (for Sₙ1)
• For Sₙ2: CH₃-X > 1° > 2° > 3°

C) Reaction Types:
1. Nucleophilic substitution (Sₙ1, Sₙ2)
2. Elimination (E1, E2)
3. With metals (Grignard formation)

4. Give two methods for the preparation of alkyl halides.

Answer:

1. From Alcohols (Best Method):

a) R-OH + HCl (ZnCl₂) → R-Cl + H₂O

b) R-OH + SOCl₂ → R-Cl + SO₂ + HCl (pure product)

c) R-OH + PCl₃ → 3R-Cl + H₃PO₃

d) R-OH + PCl₅ → R-Cl + POCl₃ + HCl

2. From Alkanes (Free Radical Substitution):

CH₄ + Cl₂ → CH₃Cl + HCl (UV light)

Limitation: Gives mixture of mono-, di-, tri-substituted products

3. From Alkenes (Alternative):
• Hydrohalogenation: CH₂=CH₂ + HBr → CH₃-CH₂-Br
• Halogenation: CH₂=CH₂ + Br₂ → Br-CH₂-CH₂-Br

5. What are β-elimination reactions? Explain them with detail.

Answer:

β-Elimination: Removal of two atoms/groups from adjacent carbons (α and β positions) to form C=C.

CH₃-CH₂-Br + OH⁻ → CH₂=CH₂ + Br⁻ + H₂O

Mechanism (E2 – Bimolecular Elimination):
1. Base (OH⁻) abstracts β-H proton
2. C-H bond breaks, electron pair forms π bond
3. C-X bond breaks, X⁻ leaves
4. All steps concerted (simultaneous)

Requirements:
• β-hydrogen must be present
• Strong base (OH⁻, OR⁻)
• Heat, alcoholic conditions favor elimination

Saytzeff Rule: More substituted alkene (more stable) is major product.

6. How will you convert ethyl chloride to:
(i) ethyl cyanide (ii) ethanol

Answer:

(i) Ethyl chloride → Ethyl cyanide (Propionitrile):

CH₃-CH₂-Cl + KCN → CH₃-CH₂-CN + KCl

Conditions: Heat with alcoholic KCN

(ii) Ethyl chloride → Ethanol:

CH₃-CH₂-Cl + NaOH(aq) → CH₃-CH₂-OH + NaCl

Conditions: Heat with aqueous NaOH (nucleophilic substitution)

8. Design a synthetic route for the preparation of bromoethane. What starting materials would you use?

Answer: Three possible routes:

Route 1: From Ethanol
Best method
→
Route 2: From Ethene
Addition reaction
→
Route 3: From Ethane
Free radical

1. From Ethanol (Best):

CH₃-CH₂-OH + HBr → CH₃-CH₂-Br + H₂O

OR CH₃-CH₂-OH + PBr₃ → 3CH₃-CH₂-Br + H₃PO₃

2. From Ethene:

CH₂=CH₂ + HBr → CH₃-CH₂-Br

3. From Ethane:

CH₃-CH₃ + Br₂ → CH₃-CH₂-Br + HBr (UV light)

Note: Route 3 gives mixture of products; Route 1 is best for pure bromoethane.

11. Which is an isomer of 2-chloropropane? Propane or 1-chloropropane or both, give reason.

Answer: 1-Chloropropane is an isomer of 2-chloropropane.

2-Chloropropane: CH₃-CHCl-CH₃
1-Chloropropane: CH₃-CH₂-CH₂-Cl

Explanation:

• Both have same molecular formula: C₃H₇Cl

• Different structures: Position isomers (chlorine at different positions)

• Propane (C₃H₈) has different formula → not an isomer

Types of Isomerism: Position isomerism (same carbon skeleton, different functional group position)

10. How can you convert?
(i) CH₃CH₂OH → chloroethane → a nitrile
(ii) CH₃CH₂OH → CH₃CH₂Cl → CH₃CH₂OH
(iii) CH₃CH₂OH → CH₃CH₂Br → CH₃CH₂NO₃

Answer:

(i) Ethanol → Chloroethane → Nitrile:

Step 1: CH₃CH₂-OH + HCl(ZnCl₂) → CH₃CH₂-Cl + H₂O

Step 2: CH₃CH₂-Cl + KCN → CH₃CH₂-CN + KCl

(ii) Ethanol → Chloroethane → Ethanol (Reverse):

Step 1: CH₃CH₂-OH + HCl(ZnCl₂) → CH₃CH₂-Cl + H₂O

Step 2: CH₃CH₂-Cl + NaOH(aq) → CH₃CH₂-OH + NaCl

(iii) Ethanol → Bromoethane → Ethyl nitrate:

Step 1: CH₃CH₂-OH + HBr → CH₃CH₂-Br + H₂O

Step 2: CH₃CH₂-Br + AgNO₃ → CH₃CH₂-NO₃ + AgBr↓

12. Give chemical reactions to produce the following starting with a halogenoalkane:
(i) A nitrile (ii) An amide (iii) A vicinal dihalide (iv) An amine (v) A nitrate (vi) An alkene

Answer:

ProductReactionConditions
(i) Nitrile R-X + KCN → R-CN + KX Alcoholic KCN, heat
(ii) Amide R-CN + H₂O → R-CONH₂ (via hydrolysis) Acid/alkaline hydrolysis
(iii) Vicinal dihalide CH₂=CH₂ + Br₂ → Br-CH₂-CH₂-Br CCl₄, room temp
(iv) Amine R-X + NH₃ → R-NH₂ + HX Excess NH₃, heat
(v) Nitrate R-X + AgNO₃ → R-NO₃ + AgX↓ Ethanol, heat
(vi) Alkene R-CH₂-CH₂-X + OH⁻ → R-CH=CH₂ + X⁻ + H₂O Alcoholic KOH, heat

Project Suggestion:

Create poster illustrating common reaction mechanisms involving halogenoalkanes. Include key reactions like nucleophilic substitution, elimination and free radical reactions.

Project Outline for Poster:

Section 1: Introduction • Definition of halogenoalkanes
• General formula R-X
• Classification (1°, 2°, 3°)
• Structure & polarity Section 2: Preparation Methods • From alcohols (with mechanisms)
• From alkanes (free radical)
• From alkenes (addition)
• Comparison of methods Section 3: Nucleophilic Substitution • Sₙ1 mechanism (2-step)
• Sₙ2 mechanism (1-step)
• Factors affecting Sₙ1 vs Sₙ2
• Examples with curved arrows Section 4: Elimination Reactions • E1 mechanism (carbocation)
• E2 mechanism (concerted)
• Saytzeff vs Hofmann
• Competition with substitution Section 5: Applications • Synthetic uses
• Industrial applications
• Environmental impact
• Safety considerations
Visual Elements to Include:
• Color-coded reaction mechanisms
• Molecular structure diagrams
• Comparison tables
• Flow charts of synthetic routes
• Real-life applications images