Haloalkanes - Class 12 Karnataka Board Chemistry

Haloalkanes and Haloarenes: Core Concepts and Exam Points

Definitions and General Structure

Haloalkanes and haloarenes are organic compounds where one or more hydrogen atoms in a hydrocarbon (alkane or arene) are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). The replacement of hydrogen with a halogen changes the physical and chemical properties of the original hydrocarbon.

• Haloalkanes: Halogen attached to an sp³ hybridized carbon (alkyl group).
• Haloarenes: Halogen attached directly to an aromatic ring (sp² hybridized carbon).
• General formula for haloalkanes: $C_nH_{2n+1}X$ where $X$ is F, Cl, Br, or I.

Hybridization and Bonding…

Haloalkanes and Haloarenes: Core Concepts and Exam Points

Definitions and General Structure

Haloalkanes and haloarenes are organic compounds where one or more hydrogen atoms in a hydrocarbon (alkane or arene) are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). The replacement of hydrogen with a halogen changes the physical and chemical properties of the original hydrocarbon.

• Haloalkanes: Halogen attached to an sp³ hybridized carbon (alkyl group).
• Haloarenes: Halogen attached directly to an aromatic ring (sp² hybridized carbon).
• General formula for haloalkanes: $C_nH_{2n+1}X$ where $X$ is F, Cl, Br, or I.

Hybridization and Bonding

The hybridization of the carbon atom bonded to the halogen determines the reactivity and properties of the compound. In haloalkanes, the carbon is sp³ hybridized, while in haloarenes, it is sp² hybridized due to the aromatic ring.

• sp³ hybridized carbon: Four sigma bonds, typical of alkanes and their derivatives.
• sp² hybridized carbon: Three sigma bonds and one pi bond, typical of aromatic systems.
• Bond polarity: The C–X bond is polar due to the higher electronegativity of halogens compared to carbon, resulting in partial positive charge on carbon and partial negative on halogen.

Classification of Haloalkanes and Haloarenes

Haloalkanes and haloarenes are classified based on the number and position of halogen atoms and the nature of the carbon to which the halogen is attached.

• By number of halogen atoms:
  • Monohalides: One halogen atom
  • Dihalides: Two halogen atoms
  • Trihalides: Three halogen atoms
  • Tetrahalides: Four halogen atoms
• By type of carbon:
  • Primary (1°): Halogen attached to a carbon bonded to one other carbon
  • Secondary (2°): Halogen attached to a carbon bonded to two other carbons
  • Tertiary (3°): Halogen attached to a carbon bonded to three other carbons
• Special types:
  • Allylic halide: Halogen attached to sp³ carbon next to a C=C double bond
  • Benzylic halide: Halogen attached to sp³ carbon next to a benzene ring
  • Vinylic halide: Halogen attached directly to a C=C double bond
  • Aryl halide: Halogen attached directly to an aromatic ring

Nomenclature (IUPAC Rules)

Naming haloalkanes and haloarenes follows IUPAC rules, focusing on the longest carbon chain, position of halogen, and alphabetical order of substituents.

• Steps for naming:
  1. Identify the longest carbon chain (parent chain).
  2. Number the chain to give the lowest possible numbers to substituents.
  3. Name and number the halogen substituents (fluoro-, chloro-, bromo-, iodo-).
  4. List substituents in alphabetical order.
  5. Use prefixes di-, tri-, tetra- for multiple identical substituents.
• Examples:
  • 1-bromopropane
  • 2-chloropropane
  • 1,2-dibromoethane
  • 1-chloro-2-methylbenzene (o-chlorotoluene)

Methods of Preparation

Haloalkanes and haloarenes can be prepared by several methods, each with specific reagents and conditions.

• From alcohols:
  • React alcohol with concentrated HCl (with ZnCl₂ as catalyst, Lucas reagent) to form alkyl chloride and water.
  • React alcohol with HBr (using NaBr and H₂SO₄) to form alkyl bromide.
  • React alcohol with PX₃ (PCl₃, PBr₃, PI₃) or PCl₅ to form alkyl halide.
  • React alcohol with SOCl₂ to form alkyl chloride, SO₂, and HCl (gaseous byproducts allow easy purification).
• From hydrocarbons:
  • Free radical halogenation: Alkane reacts with X₂ (Cl₂ or Br₂) under UV light or heat to form haloalkane.
  • Addition of HX to alkenes: Follows Markovnikov’s rule (H adds to carbon with more hydrogens, X to the other).
  • Addition of X₂ to alkenes: Forms vicinal dihalides.
• From aromatic compounds:
  • Electrophilic aromatic substitution: Benzene reacts with X₂ in presence of FeX₃ to form haloarene.
  • Sandmeyer reaction: Diazonium salt reacts with CuX/HX to form haloarene.
  • Gattermann reaction: Diazonium salt reacts with Cu powder and HX to form haloarene.

Physical Properties

Physical properties of haloalkanes and haloarenes are influenced by the type and number of halogen atoms.

• State: Lower members (methyl, ethyl halides) are gases; higher members are liquids or solids.
• Colour: Pure haloalkanes are colorless; bromides and iodides may develop color on exposure to light.
• Density: Increases with the atomic mass of the halogen; bromides and iodides are denser than water.
• Solubility: Slightly soluble in water; more soluble in organic solvents.
• Boiling point:
  • Increases with molecular mass and number of halogen atoms.
  • Decreases with branching.
  • Order for same alkyl group: RI > RBr > RCl > RF.
• Melting point: Symmetrical isomers have higher melting points due to better packing.

Chemical Properties and Reactivity

The reactivity of haloalkanes and haloarenes depends on the nature of the C–X bond and the structure of the molecule.

• Nucleophilic substitution:
  • SN1 mechanism: Favored by tertiary haloalkanes, polar protic solvents, forms racemic mixture.
  • SN2 mechanism: Favored by primary haloalkanes, strong nucleophiles, polar aprotic solvents, leads to inversion of configuration.
• Elimination reactions:
  • Dehydrohalogenation with alcoholic KOH forms alkenes.
  • Zaitsev’s rule: The more substituted alkene is the major product.
• Reactivity order:
  • For SN2: 1° > 2° > 3°
  • For SN1: 3° > 2° > 1°
  • For halide leaving group: RI > RBr > RCl > RF (due to bond strength and size)
• Haloarenes: Less reactive towards nucleophilic substitution due to resonance stabilization and partial double bond character in C–X bond.

Stereochemistry: Optical Activity and Isomerism

Haloalkanes with a chiral carbon (four different groups attached) can show optical activity and exist as enantiomers.

• Chirality: Carbon with four different groups is a chiral center (asymmetric carbon).
• Optical isomerism: Non-superimposable mirror images (enantiomers) rotate plane-polarized light in opposite directions.
• Racemic mixture: Equimolar mixture of enantiomers, optically inactive.
• Inversion and retention:
  • SN2 reactions lead to inversion of configuration.
  • SN1 reactions can lead to racemization (both retention and inversion).

Applications and Environmental Impact

Haloalkanes and haloarenes have wide applications but also pose health and environmental risks.

• Medicinal uses:
  • Chloroquine: Used for malaria.
  • Chloramphenicol: Used for typhoid fever.
  • Thyroxine: Iodine-containing hormone, deficiency leads to goiter.
  • Halothane: Used as an anesthetic.
• Industrial uses:
  • Solvents (e.g., dichloromethane, chloroform).
  • Refrigerants (e.g., Freons).
  • Pesticides (e.g., DDT).
• Environmental hazards:
  • DDT: Non-biodegradable, accumulates in fatty tissues, banned in many countries.
  • Freons: Cause ozone layer depletion.
  • Chloroform: Can form toxic phosgene gas on exposure to air and light.
  • Tetrachloromethane: Causes liver and kidney damage, potential carcinogen.

Key Exam Traps and Recall Points

Several points are frequently tested and can be sources of confusion.

• Markovnikov’s rule: In addition of HX to unsymmetrical alkenes, H attaches to carbon with more hydrogens.
• Zaitsev’s rule: In elimination, the more substituted alkene is the major product.
• SN1 vs SN2: Know the conditions favoring each, the order of reactivity, and stereochemical outcomes.
• Leaving group ability: I⁻ > Br⁻ > Cl⁻ > F⁻
• Physical property trends: Boiling point increases with mass and decreases with branching; density increases with heavier halogens.
• Environmental impact: DDT, Freons, and chloroform have significant health and ecological effects.
• Nomenclature: Always use lowest locants, alphabetical order for substituents, and correct prefixes for multiple identical groups.

Special Cases and Mechanistic Insights

Some reactions and properties have exceptions or require special conditions.

• Haloarenes: Nucleophilic substitution is difficult due to resonance and partial double bond character; requires strong nucleophiles, high temperature, or electron-withdrawing groups ortho/para to halogen.

• Preparation of alkyl iodides: Often done by Finkelstein reaction (halide exchange with NaI in acetone).

• Preparation of alkyl fluorides: Swarts reaction (halide exchange with metallic fluorides like AgF, Hg₂F₂).

• Chloroform storage: Must be kept in dark bottles to prevent formation of toxic phosgene gas.

• Optical activity: Only compounds with a chiral center and no plane of symmetry are optically active.

• Table: Reactivity Order in Nucleophilic Substitution (as stated)

  Mechanism	Reactivity Order (Alkyl Halides)
  SN2	1° > 2° > 3°
  SN1	3° > 2° > 1°

• Mnemonic for halogen order: "Iodine is best at leaving" (RI > RBr > RCl > RF).

• Key recall for environmental hazards:

  • DDT: Non-biodegradable, accumulates in fat, banned.
  • Freons: Ozone depletion.
  • Chloroform: Phosgene formation, toxic.
  • Tetrachloromethane: Carcinogenic, affects liver/kidney.

• Exam tip: Always check for the presence of electron-withdrawing groups ortho/para to halogen in haloarenes for nucleophilic substitution feasibility.

• Physical property recall: Symmetry increases melting point; higher mass increases boiling point; branching decreases boiling point.

• Naming tip: For multiple substituents, use di-, tri-, tetra- and list in alphabetical order, not by position.

• Mechanistic insight: In SN2, nucleophile attacks from the side opposite to the leaving group (backside attack), leading to inversion of configuration.

• Environmental note: Use of DDT and Freons is restricted due to their persistence and ecological impact.

• Health note: Chloroquine, chloramphenicol, and halothane are important halogen-containing drugs; thyroxine is an iodine-containing hormone.

• Practical recall: Chloroform and dichloromethane are used as solvents; chloroform must be stored in dark bottles to prevent toxic gas formation.

• Preparation recall: Alcohol + SOCl₂ gives pure alkyl chloride with gaseous byproducts (SO₂, HCl) for easy purification.

• Special case: In haloarenes, nucleophilic substitution is facilitated by electron-withdrawing groups at ortho/para positions.

• Optical activity: Only possible if the carbon attached to the halogen is chiral (four different groups).

• Exam warning: Do not confuse the reactivity order of halides in SN1 and SN2 mechanisms; always check the substrate and conditions.