Classification

On The Basis Of Number Of Halogen Atoms

Halogenated compounds are classified based on the number of halogen atoms they contain: monohalogenated (one halogen), dihalogenated (two halogens), and polyhalogenated (three or more halogens).

Compounds Containing Sp3 C—X Bond (X= F, Cl, Br, I)

These compounds feature a halogen atom bonded to an sp³-hybridized carbon atom. They are further categorized:

• Alkyl Halides (Haloalkanes): Halogen bonded to an alkyl group (R-X). Classified as primary (1°), secondary (2°), or tertiary (3°) based on the carbon atom's substitution.
• Allylic Halides: Halogen attached to an sp³-hybridized carbon adjacent to a C=C double bond.
• Benzylic Halides: Halogen bonded to an sp³-hybridized carbon attached to an aromatic ring.

Compounds Containing Sp2 C—X Bond

These compounds have a halogen atom directly bonded to an sp²-hybridized carbon atom:

• Vinylic Halides: Halogen attached to a sp²-hybridized carbon of a C=C double bond.
• Aryl Halides: Halogen directly bonded to an sp²-hybridized carbon of an aromatic ring.

Dihalogenated compounds are further classified as geminal (both halogens on the same carbon) or vicinal (halogens on adjacent carbons).

Nomenclature

Alkyl halides are named using IUPAC nomenclature as halosubstituted hydrocarbons (e.g., 2-chlorobutane). Aryl halides generally use the same name as the corresponding halogenated benzene (e.g., chlorobenzene). IUPAC uses numerical locants (1,2-; 1,3-; 1,4-) instead of ortho-, meta-, para- for dihalogenated benzenes. Di- and polyhalogenated alkanes are named based on the number and position of halogen atoms, often using prefixes like 'di-', 'tri-', etc., and locants.

Nature Of C-X Bond

The carbon-halogen (C-X) bond is polar due to the higher electronegativity of halogens compared to carbon, resulting in a partial positive charge on carbon and a partial negative charge on the halogen. Bond length increases from C-F to C-I as halogen size increases. Bond enthalpies decrease down the group, indicating a weaker bond. Dipole moments are significant but decrease from C-F to C-I due to the combined effects of polarity and bond length.

Methods Of Preparation Of Haloalkanes

Haloalkanes can be prepared by several methods:

From Alcohols

The hydroxyl group (-OH) of alcohols can be replaced by a halogen using:

• Hydrogen Halides (HX): Reacting alcohols with concentrated HX (HCl, HBr, HI). Reaction efficiency varies: 3° > 2° > 1° alcohols. ZnCl₂ is often used as a catalyst for primary and secondary alcohols with HCl.
• Phosphorus Halides: $PBr_3$ and $PI_3$ (often generated in situ from red phosphorus and $Br_2$ or $I_2$) are used to prepare alkyl bromides and iodides.
• Thionyl Chloride ($SOCl_2$): Preferred for preparing alkyl chlorides as the by-products ($SO_2$ and HCl) are gases, yielding pure alkyl chlorides.

From Hydrocarbons

• Free Radical Halogenation of Alkanes: Reaction of alkanes with halogens ($Cl_2, Br_2$) under UV light or heat produces a mixture of mono- and polyhalogenated products, making separation difficult.
• Addition to Alkenes:
• Addition of Hydrogen Halides (HX): Follows Markovnikov's rule, where H adds to the carbon with more hydrogens, and the halogen adds to the more substituted carbon.
• Addition of Halogens ($X_2$): Addition of $Br_2$ in $CCl_4$ to alkenes forms vicinal dibromides and is used to detect double bonds.

Halogen Exchange

Alkyl iodides are prepared from alkyl chlorides or bromides using sodium iodide (NaI) in dry acetone (Finkelstein reaction). Alkyl fluorides are synthesized using metallic fluorides like AgF, Hg₂F₂, or SbF₃ (Swarts reaction).

Preparation Of Haloarenes

Haloarenes are prepared primarily by:

• Electrophilic Aromatic Substitution: Reaction of arenes with halogens ($Cl_2, Br_2$) in the presence of Lewis acids (e.g., $FeCl_3$) yields aryl halides. Halogens act as ortho-, para- directors but are slightly deactivating due to inductive effects. Reactions with iodine require oxidizing agents. Fluoroarenes are not typically made this way due to fluorine's high reactivity.
• Sandmeyer Reaction: Primary aromatic amines are converted to diazonium salts ($Ar-N_2^+ X^-$) using $NaNO_2$ and $HX$. The diazonium group can then be replaced by -Cl or -Br using $CuCl$ or $CuBr$ (Sandmeyer reaction), or by -I using KI.

Physical Properties

Boiling Points: Haloalkanes are generally colourless liquids with sweet smells. Boiling points increase with increasing molecular mass and molecular size of the halogen (RI > RBr > RCl > RF). Branching in alkyl halides decreases boiling points due to reduced van der Waals forces. Boiling points of isomeric dihalobenzenes are similar, but para-isomers have higher melting points due to better crystal packing.

Density: Bromo-, iodo-, and polychlorinated hydrocarbons are denser than water. Density increases with increasing carbon count, halogen count, and atomic mass of the halogen.

Solubility: Haloalkanes are sparingly soluble in water because the energy required to break solute-solute and solvent-solvent interactions is greater than the energy released by new solute-solvent interactions. They are soluble in organic solvents due to similar intermolecular forces.

Chemical Reactions

Haloalkanes undergo three main types of reactions:

Reactions Of Haloalkanes

• Nucleophilic Substitution ($S_N$): The halogen atom is replaced by a nucleophile. This proceeds via two main mechanisms:
  • $S_N2$ (Substitution Nucleophilic Bimolecular): A one-step reaction involving a back-side attack by the nucleophile, causing inversion of configuration at the carbon atom. Reactivity order: primary > secondary > tertiary haloalkanes due to steric hindrance.
  • $S_N1$ (Substitution Nucleophilic Unimolecular): A two-step reaction involving the formation of a carbocation intermediate. Reactivity order: tertiary > secondary > primary haloalkanes due to carbocation stability. Often accompanied by racemisation if the substrate is chiral.
  Ambidentate nucleophiles (like $CN^-$ and $NO_2^-$) can attack via different atoms, leading to different products (e.g., alkyl cyanide vs. isocyanide).
• Elimination Reactions (E): Haloalkanes with a beta-hydrogen undergo dehydrohalogenation (loss of H and X) when heated with a strong base (like alcoholic KOH) to form alkenes. Zaitsev's rule states that the major product is the more substituted alkene. Elimination competes with substitution, with steric hindrance favouring elimination.
• Reaction with Metals: Haloalkanes react with active metals like magnesium in dry ether to form Grignard reagents (R-Mg-X), which are important organometallic compounds. Reaction with sodium in dry ether forms alkanes via the Wurtz reaction. Aryl halides can undergo the Fittig reaction (coupling of aryl halide with alkyl halide) or Wurtz-Fittig reaction.

Reactions Of Haloarenes

• Nucleophilic Substitution: Aryl halides are much less reactive towards nucleophilic substitution than haloalkanes due to resonance (partial double bond character of C-X bond) and $sp^2$ hybridization of the carbon atom. However, strong electron-withdrawing groups (like -NO₂) at ortho- and para- positions activate the ring towards nucleophilic substitution by stabilizing the intermediate carbanion.
• Electrophilic Substitution: Haloarenes undergo electrophilic substitution reactions (like halogenation, nitration, sulfonation, Friedel-Crafts reactions). Halogens are deactivating (due to inductive effect) but ortho-, para- directing (due to resonance stabilization of intermediates).

Polyhalogen Compounds

Polyhalogen compounds contain multiple halogen atoms. Some important examples and their uses/effects include:

• Dichloromethane ($CH_2Cl_2$): Solvent, paint remover, propellant, cleaning agent; toxic to the central nervous system.
• Trichloromethane ($CHCl_3$): Solvent, precursor for refrigerants (Freon-22); previously used as an anesthetic but is toxic to the liver and kidneys. Slowly oxidizes to poisonous phosgene in the presence of light.
• Triiodomethane ($CHI_3$ or Iodoform): Used historically as an antiseptic due to iodine liberation; has an objectionable smell.
• Tetrachloromethane ($CCl_4$): Used as a solvent and in refrigerants/propellants. It depletes the ozone layer and can cause liver damage and neurological effects.
• Freons (Chlorofluorocarbons): Stable, non-toxic compounds used as refrigerants and propellants. They diffuse into the stratosphere and deplete the ozone layer through radical chain reactions.
• DDT (p,p’-Dichlorodiphenyltrichloroethane): An effective insecticide but highly persistent, fat-soluble, and toxic to fish. Its widespread use led to resistance in insects and environmental concerns, resulting in bans in many countries.

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