HALOALKANES: Preparation, Properties & Uses
INTRODUCTION
Haloalkanes are not only the subject of theory. They are widely used in daily life, in medicine, in industry.Let us see some halogen containing organic compounds available in the market.It will sum up more interesting about haloalkanes and also we will be familiar to these compounds.
Preparation of Haloalkanes
Haloalkanes are organic compounds in which one or more hydrogen atoms of alkanes are replaced by halogen atoms such as chlorine (Cl), bromine (Br), iodine (I), or fluorine (F).
Methods of Preparation of Haloalkanes
1. From Alcohols
Alcohols react with halogen acids or phosphorus halides to form haloalkanes.
(a) Using Hydrogen Halides (HX)
Reaction:
Example:
The reactivity order of hydrogen halides is: HI > HBr > HCl
(b) Using Phosphorus Halides
(c) Using Thionyl Chloride (SOCl₂)
This method is preferred because gaseous by-products escape easily, giving pure alkyl chloride.
2. From Alkanes (Free Radical Halogenation)
Alkanes react with chlorine or bromine in the presence of sunlight or heat.
Reaction:
(in presence of sunlight)
This reaction follows a free radical mechanism.
3. From Alkenes
Alkenes react with hydrogen halides to form haloalkanes.
Reaction:
Addition follows Markovnikov’s Rule in unsymmetrical alkenes.
Peroxide Effect
In presence of peroxide, HBr adds according to anti-Markovnikov rule.
4. Halogen Exchange Reactions
(a) Finkelstein Reaction
Alkyl chlorides or bromides react with sodium iodide in acetone to form alkyl iodides.
(b) Swarts Reaction
Alkyl chlorides or bromides are heated with metallic fluorides to form alkyl fluorides.
Important Points to Remember
- Haloalkanes contain a carbon-halogen bond.
- Thionyl chloride method gives pure alkyl chlorides.
- Free radical halogenation requires sunlight or heat.
- HBr shows peroxide effect; HCl and HI do not.
- Finkelstein reaction is used for preparing alkyl iodides.
Let us see preparation and properties in a short view.
Chemical Properties of Haloalkanes
Haloalkanes are highly reactive compounds due to the presence of the polar carbon-halogen (C–X) bond. The halogen atom can be easily replaced by other atoms or groups. Most reactions of haloalkanes involve nucleophilic substitution and elimination reactions.
1. Nucleophilic Substitution Reactions
In these reactions, the halogen atom is replaced by a nucleophile.
R–X + Nu⁻ → R–Nu + X⁻
(a) Reaction with Aqueous KOH
Haloalkanes form alcohols when heated with aqueous potassium hydroxide.
(b) Reaction with Cyanide
Alkyl cyanides (nitriles) are formed.
(c) Reaction with Ammonia
Primary amines are formed.
(d) Reaction with Silver Nitrite
Nitroalkanes are produced.
2. Elimination Reaction (Dehydrohalogenation)
Haloalkanes react with alcoholic KOH to form alkenes by elimination of HX.
This reaction is called β-elimination.
3. Wurtz Reaction
Haloalkanes react with sodium metal in dry ether to form higher alkanes.
4. Formation of Grignard Reagent
Haloalkanes react with magnesium in dry ether to form Grignard reagents.
Grignard reagents are important in organic synthesis.
SN2 Reaction Mechanism
SN2 stands for Substitution Nucleophilic Bimolecular.
Characteristics of SN2 Reaction
- Occurs in a single step.
- Rate depends on both haloalkane and nucleophile.
- Backside attack by nucleophile.
- Causes inversion of configuration.
General Reaction
Mechanism
The nucleophile attacks the carbon atom from the side opposite to the halogen atom. At the same time, the halogen leaves.
The reaction proceeds through a single transition state without formation of an intermediate.
Rate = k [R–X][Nu⁻]
SN1 Reaction Mechanism
SN1 stands for Substitution Nucleophilic Unimolecular.
Characteristics of SN1 Reaction
- Occurs in two steps.
- Formation of carbocation intermediate.
- Rate depends only on haloalkane concentration.
- Generally occurs in tertiary haloalkanes.
General Reaction
Mechanism
Step 1: Formation of Carbocation (Slow Step)
Step 2: Attack of Nucleophile (Fast Step)
Rate = k [R–X]
Difference Between SN1 and SN2 Reactions
| SN1 | SN2 |
|---|---|
| Two-step mechanism | Single-step mechanism |
| Forms carbocation intermediate | No intermediate formed |
| Rate depends on one reactant | Rate depends on two reactants |
| Occurs mainly in tertiary haloalkanes | Occurs mainly in primary haloalkanes |
| Racemization may occur | Inversion of configuration occurs |
Frequently Asked Questions (FAQs) on Haloalkanes
1. What are haloalkanes?
2. Which halogens are present in haloalkanes?
3. Why are haloalkanes reactive?
4. What is nucleophilic substitution reaction?
5. What does SN2 stand for?
6. What is the main feature of SN2 reaction?
7. What does SN1 stand for?
8. Which type of haloalkanes prefer SN1 mechanism?
9. Which type of haloalkanes prefer SN2 mechanism?
10. What is dehydrohalogenation?
11. Which reagent is used for elimination reaction?
12. What happens when haloalkanes react with aqueous KOH?
13. What is Wurtz reaction?
14. What are Grignard reagents?
15. Why is dry ether used in Grignard reaction?
16. Which haloalkane reacts fastest in SN1 reaction?
17. Which haloalkane reacts fastest in SN2 reaction?
18. What is the rate equation of SN1 reaction?
19. What is the rate equation of SN2 reaction?
20. Why are haloalkanes important?
Haloalkanes MCQ Quiz
Haloalkanes Interactive MCQ Quiz
Chemical Properties of Haloalkanes
Haloalkanes are highly reactive compounds due to the presence of the polar carbon-halogen (C–X) bond. The halogen atom can be easily replaced by other atoms or groups. Most reactions of haloalkanes involve nucleophilic substitution and elimination reactions.
1. Nucleophilic Substitution Reactions
In these reactions, the halogen atom is replaced by a nucleophile.
R–X + Nu⁻ → R–Nu + X⁻
(a) Reaction with Aqueous KOH
Haloalkanes form alcohols when heated with aqueous potassium hydroxide.
(b) Reaction with Cyanide
Alkyl cyanides (nitriles) are formed.
(c) Reaction with Ammonia
Primary amines are formed.
(d) Reaction with Silver Nitrite
Nitroalkanes are produced.
2. Elimination Reaction (Dehydrohalogenation)
Haloalkanes react with alcoholic KOH to form alkenes by elimination of HX.
This reaction is called β-elimination.
3. Wurtz Reaction
Haloalkanes react with sodium metal in dry ether to form higher alkanes.
4. Formation of Grignard Reagent
Haloalkanes react with magnesium in dry ether to form Grignard reagents.
Grignard reagents are important in organic synthesis.
SN2 Reaction Mechanism
SN2 stands for Substitution Nucleophilic Bimolecular.
Characteristics of SN2 Reaction
- Occurs in a single step.
- Rate depends on both haloalkane and nucleophile.
- Backside attack by nucleophile.
- Causes inversion of configuration.
General Reaction
Mechanism
The nucleophile attacks the carbon atom from the side opposite to the halogen atom. At the same time, the halogen leaves.
OH⁻ + CH₃Br → [Transition State] → CH₃OH + Br⁻
The reaction proceeds through a single transition state without formation of an intermediate.
Rate = k [R–X][Nu⁻]
SN1 Reaction Mechanism
SN1 stands for Substitution Nucleophilic Unimolecular.
Characteristics of SN1 Reaction
- Occurs in two steps.
- Formation of carbocation intermediate.
- Rate depends only on haloalkane concentration.
- Generally occurs in tertiary haloalkanes.
General Reaction
Mechanism
Step 1: Formation of Carbocation (Slow Step)
Step 2: Attack of Nucleophile (Fast Step)
Rate = k [R–X]
Difference Between SN1 and SN2 Reactions
| SN1 | SN2 |
|---|---|
| Two-step mechanism | Single-step mechanism |
| Forms carbocation intermediate | No intermediate formed |
| Rate depends on one reactant | Rate depends on two reactants |
| Occurs mainly in tertiary haloalkanes | Occurs mainly in primary haloalkanes |
| Racemization may occur | Inversion of configuration occurs |