Stepwise Guide to Preparing Haloalkanes and Haloarenes for JEE/NEET
The Methods of Preparation - Haloalkanes and Haloarenes topic is vital for JEE Main Chemistry, as these compounds serve as key building blocks in organic synthesis. Haloalkanes (alkyl halides) and haloarenes (aryl halides) differ in structure, properties, and their preparation routes. Understanding these differences, mastering mechanisms, and clearly recalling reagents and conditions are crucial for quick problem solving in the exam. This page provides a stepwise overview, with mechanisms, comparison charts, and short notes optimised for JEE-level clarity and revision.
What are Haloalkanes and Haloarenes?
Haloalkanes are aliphatic organic compounds with one or more halogen atoms (F, Cl, Br, I) replacing hydrogen atoms in alkanes. Haloarenes or aryl halides result from one or more hydrogens of an aromatic ring (like benzene) being replaced by halogens. Both groups are significant due to their presence in drugs, synthetic polymers, and laboratory reagents.
General Methods of Preparation of Haloalkanes
The methods of preparation of haloalkanes primarily include reactions starting from alcohols, alkanes, alkenes, or by halogen exchange. Each method uses specific reagents and follows distinct mechanisms, allowing formation of different alkyl halides under controlled conditions.
- From Alcohols: Alcohols react with halogen acids (HX), phosphorus halides (PX3, PX5), or thionyl chloride (SOCl2) to give haloalkanes. Example: R–OH + HBr → R–Br + H2O.
- Halogenation of Alkanes: Free radical substitution with Cl2 or Br2 (in presence of light/heat) forms haloalkanes. Example: CH4 + Cl2 → CH3Cl + HCl.
- Addition to Alkenes: Alkenes undergo addition with halogens or hydrogen halides to form dihaloalkanes or haloalkanes, respectively. Markovnikov’s rule governs regioselectivity unless peroxide (anti-Markovnikov) is used.
- Halogen Exchange (Finkelstein/Swarts): Alkyl bromides/chlorides are converted to iodides by sodium iodide in acetone (Finkelstein), or to fluorides by metal fluorides like AgF (Swarts reaction).
- From Carboxylic Acids (Hunsdiecker Reaction): Silver salts of carboxylic acids react with halogen to give alkyl halides.
Each route differs in specificity, yield, and use-case. Alcohols give the most controlled products, while halogenation of alkanes is less selective. For exam questions, remember key reagents and the mechanism keywords: nucleophilic substitution, free radical halogenation, and electrophilic addition.
General Methods of Preparation of Haloarenes
Preparation of haloarenes uses different strategies as compared to haloalkanes, due to the aromatic ring’s stability and unique reactivity. The main methods are:
- Direct Halogenation of Benzene: Benzene reacts with Cl2/Br2 in presence of a Lewis acid catalyst (FeCl3/AlCl3) to yield chlorobenzene or bromobenzene via electrophilic aromatic substitution.
- Sandmeyer Reaction: Aniline (C6H5NH2) is diazotised to form benzene diazonium chloride, which then reacts with CuX (X = Cl, Br, CN) to give aryl halides. Example: C6H5N2+Cl– + CuCl → C6H5Cl + N2 + CuCl2.
- Gattermann Reaction: Benzene diazonium chloride reacts with halogen acids (HCl/HBr) and copper powder to form chlorobenzene or bromobenzene.
- From Phenols: Phenols react with PCl5/SOCl2 but not as commonly used due to side reactions.
Electrophilic substitution is central in aromatic chemistry, but certain methods (like nucleophilic substitution) are rare due to the stability of the aromatic ring and resonance effects. For JEE, focus on Sandmeyer and Gattermann reaction conditions as they are commonly examined, especially for multi-step mechanism questions.
Comparative Table: Methods of Preparation - Haloalkanes vs Haloarenes
| Feature | Haloalkanes | Haloarenes |
|---|---|---|
| Usual Starting Compound | Alkanes, alcohols, alkenes | Aromatic amines (aniline), benzene, phenols |
| Main Types of Reaction | Nucleophilic/freeradical substitution, addition | Electrophilic substitution, diazotisation |
| Key Reagents | HX, PCl5, SOCl2, Cl2, Br2 | FeCl3, CuX, HCl, Sandmeyer, Gattermann reagents |
| Selectivity | Variable, often mixtures by halogenation | More specific, ortho/para products possible |
| Exam Keynote | Remember SN1/SN2 for halide formation | Know Sandmeyer & electrophilic substitution steps |
Always distinguish the preparation of aryl halides and alkyl halides on the basis of reactivity and mechanism—critical for higher-scoring problems and short notes.
Detailed Mechanisms: Key Examples
Preparation of Haloalkanes from Alcohols (SN1 mechanism): Tertiary alcohols react with HX in presence of ZnCl2. The OH group is protonated, water departs (forming a carbocation), and X– adds. Primary and secondary alcohols may follow SN2 or SN1 depending on stability.
Sandmeyer Reaction (for Haloarenes): Aniline diazotises with NaNO2/HCl at 0–5°C to form benzene diazonium chloride, which reacts with CuCl or CuBr to form chlorobenzene or bromobenzene, releasing N2 gas. The transition-metal-catalysed electron transfer enables the nucleophile (X–) to substitute the diazonium group.
Applications and Uses of Haloalkanes and Haloarenes
- Key intermediates in synthesising pharmaceuticals and agrochemicals.
- Used in manufacture of plastics, solvents, refrigerants, and pesticides.
- Serve as precursors for Grignard reagents and other organometallic compounds in advanced organic synthesis.
- Chlorobenzene and others act as solvents or reactants in dyes and polymers.
- Understanding their preparation aids quick recognition of suitable synthetic routes in exam organic synthesis questions.
Last-Minute Revision Notes: Methods of Preparation - Haloalkanes and Haloarenes
- Haloalkanes: Best prepared from alcohols; key reagents: SOCl2, PCl5, HX. Halogenation of alkanes is less selective.
- Haloarenes: Prepared by electrophilic substitution (direct halogenation with FeCl3), or from diazonium salts (Sandmeyer/Gattermann).
- Markovnikov/anti-Markovnikov selectivity crucial in addition to alkenes.
- Finkelstein and Swarts are for halide exchange (alkyl halides only, not aryl).
- Key mechanisms: SN1, SN2 for alkyl halides; electrophilic aromatic substitution and diazonium chemistry for aryl.
- In Viva/exam: stress on correct condition, reagent, and stepwise logic.
For a detailed dive into nucleophilic substitution, review Nucleophilic Substitution Reactions. If you want more on aromatic chemistry, see Preparation of Haloarene. For revision on the core topic and broader context, refer to Organic Compounds Containing Halogens and Hydrocarbons on Vedantu. Remember, clarity of mechanisms and conditions is your key advantage on JEE Main day!
FAQs on Methods of Preparation of Haloalkanes and Haloarenes Explained
1. What is the preparation method of haloalkanes and haloarenes?
Haloalkanes are prepared by halogenation of alkanes, reaction of alcohols with halogenating agents, hydrohalogenation of alkenes, and halogen exchange. Haloarenes are mainly formed through direct halogenation, the Sandmeyer reaction, and the Gattermann reaction involving aromatic compounds.
- Haloalkanes: Halogenation of alkanes, conversion of alcohols, hydrohalogenation, halogen exchange (Finkelstein reaction).
- Haloarenes: Direct halogenation of benzene, Sandmeyer and Gattermann reactions from aromatic amines.
2. How many types of preparation of haloarenes are there?
The main types of haloarene preparation include direct halogenation of aromatic hydrocarbons, Sandmeyer reaction (from diazonium salts), and Gattermann reaction. Each method uses distinct reagents and mechanisms.
- Direct halogenation
- Sandmeyer reaction (diazonium salt + CuX/HX)
- Gattermann reaction (diazonium salt + Cu/HCl + halogen)
3. What are the four methods of preparation of alkyl halides?
The four main methods to prepare alkyl halides (haloalkanes) are:
- Halogenation of alkanes (free radical mechanism)
- Halogenation of alkenes/alkynes (addition reactions)
- Reaction of alcohols with halogenating agents (like PCl5, SOCl2, PX3)
- Halogen exchange reactions (Finkelstein reaction, Swarts reaction)
4. What is the Sandmeyer reaction in haloarene preparation?
The Sandmeyer reaction is a method to prepare haloarenes by treating aromatic diazonium salts with cuprous halides, allowing replacement of a diazonium group with a halogen.
- Aromatic amine → diazonium salt (by NaNO2/HCl)
- Treat with CuX/HX (X = Cl, Br, CN)
- Forms haloarene (e.g., chlorobenzene, bromobenzene)
5. How are haloarenes synthesized from benzene?
Haloarenes are synthesized from benzene mainly by electrophilic aromatic substitution (direct halogenation) with halogens in presence of a Lewis acid catalyst, or via diazonium salt pathways (Sandmeyer or Gattermann reactions).
- Direct halogenation: Benzene + Cl2/Br2 + FeCl3/AlCl3
- Sandmeyer reaction: Benzene → Aniline → Diazonium salt → Haloarene using CuX/HX
6. Why is direct halogenation not always suitable for preparing haloarenes?
Direct halogenation of aromatic rings may lead to multi-substitution, poor selectivity, or require harsh conditions, making it less suitable for some haloarenes. Alternative methods like Sandmeyer or Gattermann reactions offer better control and yield, especially for introducing chlorides or bromides at specific positions.
7. Can alcohols be used to prepare both haloalkanes and haloarenes?
Alcohols are commonly used to prepare haloalkanes by reaction with halogenating agents. However, direct preparation of haloarenes from aromatic alcohols is not typical; haloarenes are synthesized mainly by halogenation or diazonium salt methods, not from alcohols.
8. What is the difference between nucleophilic and electrophilic substitution in these preparations?
Preparation of haloalkanes usually involves nucleophilic substitution (substitution of -OH or -X with halides), while haloarenes are formed via electrophilic substitution (halogen replaces H on aromatic ring) or by diazonium mechanisms.
- Nucleophilic substitution in alkyl systems (SN1, SN2)
- Electrophilic substitution on benzene (aromatic system)
9. What are the general methods to prepare halogens?
Halogens (Cl2, Br2, I2) can be prepared by:
- Electrolysis of halide salts (e.g., NaCl to get Cl2)
- Decomposition of metal halides
- Reaction of potassium permanganate or manganese dioxide with halide ions
These methods are more relevant to inorganic chemistry but are important for laboratory/industrial preparation.
10. What is the Finkelstein reaction in alkyl halide preparation?
The Finkelstein reaction is a halogen exchange process for synthesizing alkyl iodides or bromides from alkyl chlorides by reacting with sodium iodide or bromide in acetone.
- R–Cl + NaI (acetone) → R–I + NaCl (ppt)
- Commonly used for preparing alkyl iodides
11. What are the physical properties of haloalkanes?
Haloalkanes are colorless liquids or solids with higher boiling points than alkanes, are generally insoluble in water, and denser than water. Their physical properties depend on the type and size of halogen and carbon chain length.
- Higher boiling points than parent hydrocarbons
- Soluble in non-polar solvents, not in water
- Heavier halides (bromo-, iodo-) are denser than water
