CBSE Class 12 Chemistry Chapter 10: Haloalkanes and Haloarenes Notes

Key Terms and Definitions

Haloalkanes (Alkyl Halides)

Derivatives of alkanes where one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I).

Haloarenes (Aryl Halides)

Derivatives of aromatic hydrocarbons where one or more hydrogen atoms directly attached to the aromatic ring are replaced by halogen atoms.

SN1 Reaction

Unimolecular Nucleophilic Substitution reaction. Occurs in two steps, involving a carbocation intermediate. Favored by tertiary halides and polar protic solvents.

SN2 Reaction

Bimolecular Nucleophilic Substitution reaction. Occurs in a single step with a transition state. Favored by primary halides and polar aprotic solvents, leading to inversion of configuration.

Racemisation

A process by which an optically active compound loses its optical activity, resulting in a racemic mixture (equal amounts of enantiomers) during SN1 reactions.

Inversion of Configuration

The reversal of the spatial arrangement of bonds around an asymmetric carbon atom during a chemical reaction, characteristic of SN2 reactions.

Wurtz Reaction

Reaction of two molecules of alkyl halides with sodium metal in dry ether to form a higher alkane with an even number of carbon atoms. (R-X + 2Na + X-R → R-R + 2NaX).

Fittig Reaction

Reaction of two molecules of aryl halides with sodium metal in dry ether to form a diphenyl compound. (Ar-X + 2Na + X-Ar → Ar-Ar + 2NaX).

Wurtz-Fittig Reaction

Reaction between an alkyl halide and an aryl halide with sodium metal in dry ether to form an alkylarene. (R-X + 2Na + X-Ar → R-Ar + 2NaX).

Nomenclature, Classification & Physical Properties

Nomenclature: Haloalkanes and Haloarenes are systematically named using IUPAC rules. The longest carbon chain containing the halogen is selected, and halogens are treated as substituents. For haloarenes, halogens are directly attached to the benzene ring, and their positions are indicated using numbers or ortho-, meta-, para- prefixes.

Classification:

1. Based on number of halogen atoms: Monohalo-, dihalo-, trihalo- compounds.
2. Based on the hybridization of carbon atom to which halogen is attached:

• Alkyl halides (Haloalkanes): Halogen attached to sp³ carbon. Can be primary (1°), secondary (2°), or tertiary (3°) depending on the number of carbon atoms directly bonded to the halogen-bearing carbon.
• Allylic halides: Halogen attached to an sp³ carbon adjacent to a C=C double bond (e.g., CH₂=CH-CH₂-X).
• Benzylic halides: Halogen attached to an sp³ carbon adjacent to an aromatic ring (e.g., C₆H₅-CH₂-X).
• Vinylic halides: Halogen attached directly to an sp² carbon of a C=C double bond (e.g., CH₂=CH-X).
• Aryl halides (Haloarenes): Halogen attached directly to an sp² carbon of an aromatic ring (e.g., C₆H₅-X).

Physical Properties:

• Boiling Points: Generally increase with increasing molecular mass (I > Br > Cl > F) and with increasing size of the alkyl group. Branched isomers have lower boiling points than straight-chain isomers due to reduced surface area and weaker van der Waals forces. Haloalkanes have higher boiling points than parent alkanes due to their polarity and higher molecular mass.
• Density: Haloalkanes are generally denser than water, with bromo- and iodo-compounds being significantly denser. Density follows the order I > Br > Cl.
• Solubility: Haloalkanes are only slightly soluble in water because they cannot form hydrogen bonds with water molecules, and the energy required to break existing hydrogen bonds in water and alkyl halide-alkyl halide interactions is greater than the energy released when new alkyl halide-water interactions are formed. They are, however, soluble in organic solvents.

Methods of Preparation

Haloalkanes:

1. From Alcohols: Reaction with hydrogen halides (HX, e.g., HCl, HBr, HI) in the presence of a catalyst (ZnCl₂ for HCl, NaBr/H₂SO₄ for HBr). Reactivity order: 3° > 2° > 1° alcohols. Also, reaction with phosphorus halides (PX₃ or PX₅) or thionyl chloride (SOCl₂, Darzen's process, preferred as byproducts are gaseous).

• R-OH + HCl (anhyd. ZnCl₂) → R-Cl + H₂O
• R-OH + PCl₅ → R-Cl + POCl₃ + HCl
• R-OH + SOCl₂ → R-Cl + SO₂(g) + HCl(g)

1. From Alkenes: Addition of hydrogen halides (HX). Follows Markovnikov's rule for unsymmetrical alkenes. Peroxide effect (Anti-Markovnikov's addition) is observed only with HBr in the presence of peroxides.

• CH₂=CH₂ + HBr → CH₃-CH₂Br
• CH₃-CH=CH₂ + HBr → CH₃-CH(Br)-CH₃ (Markovnikov)
• CH₃-CH=CH₂ + HBr (peroxide) → CH₃-CH₂-CH₂Br (Anti-Markovnikov)

1. From Alkanes: By free radical halogenation (chlorination/bromination) in the presence of UV light or heat. Non-selective and yields a mixture of products.

Haloarenes:

1. From Benzene (Electrophilic Substitution): Direct halogenation with Cl₂ or Br₂ in the presence of a Lewis acid catalyst like anhydrous FeCl₃ or AlCl₃. Fluorination is too vigorous, iodination is reversible.

• C₆H₆ + Cl₂ (FeCl₃) → C₆H₅Cl + HCl

1. From Diazonium Salts (Sandmeyer Reaction): Aryl diazonium salts (prepared from primary aromatic amines) react with Cu₂X₂/HX to yield aryl halides. This is a highly regioselective method.

• Ar-N₂⁺Cl^- + CuCl/HCl → Ar-Cl + N₂
• Gattermann Reaction: Similar to Sandmeyer but uses copper powder and HX, giving lower yields.
• Ar-N₂⁺Cl^- + Cu/HCl → Ar-Cl + N₂

Chemical Reactions of Haloalkanes and Haloarenes

Haloalkanes are highly reactive due to the polar C-X bond and undergo primarily nucleophilic substitution reactions (SN1 and SN2) and elimination reactions.

1. Nucleophilic Substitution Reactions (SN1 and SN2):

• SN1 (Unimolecular Nucleophilic Substitution): Two-step mechanism. Step 1 involves slow ionization of C-X bond to form a planar carbocation intermediate. Step 2 involves rapid attack of the nucleophile on the carbocation from either side, leading to a racemic mixture (if the carbon is chiral). Rate depends only on the concentration of alkyl halide. Order of reactivity: 3° > 2° > 1° alkyl halides. Favored by polar protic solvents (e.g., water, alcohol).
• SN2 (Bimolecular Nucleophilic Substitution): One-step mechanism involving a concerted backside attack by the nucleophile and departure of the leaving group, forming a transition state. Leads to inversion of configuration (Walden inversion). Rate depends on concentrations of both alkyl halide and nucleophile. Order of reactivity: 1° > 2° > 3° alkyl halides (due to steric hindrance). Favored by polar aprotic solvents (e.g., DMSO, acetone).
• Common Nucleophiles: HO^- (alcohol), RO^- (ether), CN^- (nitrile), RCOO^- (ester), NH₃ (amine).

1. Elimination Reactions (β-elimination / Dehydrohalogenation):

• When heated with alcoholic KOH, haloalkanes undergo elimination of a hydrogen atom from a β-carbon and a halogen atom from the α-carbon, forming an alkene. Follows Saytzeff's Rule: the preferred alkene formed is the one with the greater number of alkyl groups attached to the doubly bonded carbon atoms.
• CH₃-CH₂-CH(Br)-CH₃ (2-bromobutane) + alc. KOH → CH₃-CH=CH-CH₃ (But-2-ene, major) + CH₂=CH-CH₂-CH₃ (But-1-ene, minor)

1. Reaction with Metals:

• Wurtz Reaction: 2RX + 2Na → R-R + 2NaX (in dry ether)
• Reaction with Magnesium: R-X + Mg → R-MgX (Grignard Reagent, in dry ether). Grignard reagents are highly reactive and useful intermediates.

Haloarenes are less reactive towards nucleophilic substitution reactions than haloalkanes due to:

1. Resonance effect: The lone pair on the halogen atom is in conjugation with the benzene ring, leading to partial double bond character for the C-X bond, making it stronger and shorter.
2. Difference in hybridization of carbon atom: The carbon atom attached to the halogen in haloarenes is sp² hybridized, while in haloalkanes it's sp³. An sp² carbon is more electronegative, holding the electron pair more tightly, making the C-X bond less polar and thus harder to break.
3. Instability of phenyl carbocation: Formation of a phenyl carbocation (by self-ionization) is highly unstable and thus not feasible.

Reactions of Haloarenes:

1. Electrophilic Substitution Reactions: Halogen is a deactivating group (due to -I effect) but is ortho- and para-directing (due to +R effect). Examples include nitration (conc. HNO₃/H₂SO₄), sulfonation (conc. H₂SO₄), Friedel-Crafts alkylation/acylation (R-Cl/AlCl₃ or R-COCl/AlCl₃).
2. Reaction with Metals:

• Wurtz-Fittig Reaction: Ar-X + R-X + 2Na → Ar-R + 2NaX
• Fittig Reaction: Ar-X + Ar-X + 2Na → Ar-Ar + 2NaX
• Ullmann Reaction: Two aryl iodides react with copper powder at high temperature to form biphenyls.

SN1 vs. SN2 Mechanisms

Worked Examples

• {"example":"1. Predict the major product:\nCH₃-CH₂-CH(Br)-CH₃ + alcoholic KOH → ?\n\nSolution: This is a β-elimination reaction. According to Saytzeff's rule, the major product will be the more substituted alkene. Removing H from the adjacent CH₃ group leads to But-1-ene (less substituted), and removing H from the adjacent CH₂ group leads to But-2-ene (more substituted).\nMajor Product: But-2-ene (CH₃-CH=CH-CH₃)"}
• {"example":"2. Complete the reaction:\nCH₃-CH₂-Cl + NaI (dry acetone) → ?\n\nSolution: This is a Finkelstein reaction, a type of SN2 halide exchange reaction. Chloride is replaced by iodide.\nProduct: CH₃-CH₂-I + NaCl"}
• {"example":"3. Identify A and B:\nBenzene $\\xrightarrow{Cl_2, FeCl_3}$ A $\\xrightarrow{CH_3Cl, Anhyd. AlCl_3}$ B\n\nSolution:\nStep 1: Electrophilic substitution (chlorination) of benzene.\nA: Chlorobenzene (C₆H₅Cl)\nStep 2: Friedel-Crafts alkylation of chlorobenzene. Chlorine is an ortho-para director.\nB: 1-Chloro-4-methylbenzene (p-chlorotoluene, major product)"}

Key Points to Remember

• Reactivity order of Alkyl Halides: R-I > R-Br > R-Cl > R-F due to bond dissociation enthalpy.
• SN1 reactivity: 3° > 2° > 1°. Favored by polar protic solvents, weak nucleophiles. Leads to racemisation.
• SN2 reactivity: 1° > 2° > 3°. Favored by polar aprotic solvents, strong nucleophiles. Leads to inversion of configuration.
• Haloarenes are less reactive towards nucleophilic substitution due to resonance (C-X partial double bond), sp² hybridized carbon, and instability of phenyl carbocation.
• Darzen's method (SOCl₂) is preferred for preparing alkyl chlorides from alcohols because byproducts (SO₂ and HCl) are gases and easily escape, giving pure product.
• Wurtz, Fittig, Wurtz-Fittig reactions are coupling reactions involving sodium metal in dry ether to form C-C bonds.
• Grignard reagents (RMgX) are highly versatile and react with compounds containing active hydrogen to give alkanes.
• Saytzeff's rule predicts the major product in β-elimination (more substituted alkene).
• For electrophilic substitution in haloarenes, halogens are deactivating but ortho-para directing.

Exam Tip for Haloalkanes and Haloarenes

Pay special attention to reaction mechanisms (SN1, SN2). Understand the factors influencing their rates and stereochemical outcomes. Named reactions (Sandmeyer, Gattermann, Wurtz, Fittig, Wurtz-Fittig, Finkelstein, Swarts) are frequently tested, so learn their reactants, reagents, products, and specific conditions (e.g., 'dry ether' for Wurtz reactions, 'alcoholic KOH' for elimination). Don't confuse alcoholic KOH (elimination) with aqueous KOH (substitution). Practice distinguishing between substitution and elimination, and apply Saytzeff's rule correctly. For haloarenes, remember their lower reactivity towards nucleophilic substitution and the ortho-para directing nature of halogens.

Practice Questions with Solutions

• Q: Why are haloarenes less reactive than haloalkanes towards nucleophilic substitution reactions? A: Haloarenes are less reactive due to the partial double bond character of the C-X bond (resonance effect), sp² hybridized carbon of the C-X bond, and instability of the phenyl carbocation.
• Q: Differentiate between SN1 and SN2 reactions based on their stereochemical outcome. A: SN1 reactions lead to racemisation if the carbon is chiral, while SN2 reactions result in inversion of configuration (Walden inversion).
• Q: What is the major product when 2-bromopropane is treated with alcoholic KOH? A: According to Saytzeff's rule, the major product will be propene (CH₂=CH-CH₃) because there is only one possible β-hydrogen to eliminate.
• Q: Name the reaction used to convert aryl diazonium salts to aryl chlorides. A: Sandmeyer reaction (using Cu₂Cl₂/HCl) or Gattermann reaction (using Cu powder/HCl).

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