Revision: Organic Chemistry >> Haloalkanes and Haloarenes Chemistry (Theory) ISC (Science) ISC Class 12 CISCE

Compounds in which the halogen atom is directly attached to the carbon atom of an aromatic ring are called aryl halides.

Compounds in which the halogen atom is bonded to a carbon atom of a carbon–carbon double bond are called vinylic halides.

The halogen derivatives of aliphatic hydrocarbons are called aliphatic halogen compounds.

The halogen derivatives of alkanes are called haloalkanes.
They are obtained by replacing one or more hydrogen atoms of alkanes by halogen atoms.

The halogen derivatives of alkynes are called haloalkynes or alkynyl halides.

The halogen derivatives of alkenes are called haloalkenes or alkenyl halides.

Diazotization is the reaction in which an aromatic primary amine reacts with nitrous acid at 273 K to form a diazonium salt.

Sandmeyer reaction is the replacement of the diazonium group by Cl or Br using cuprous salts (CuCl or CuBr).

Gattermann reaction involves the replacement of the diazonium group by Cl or Br using copper powder and corresponding acid (HCl or HBr).

Haloalkanes in which the halogen atom is attached to a secondary carbon atom are called secondary alkyl halides.

Haloalkanes in which the halogen atom is attached to a tertiary carbon atom are called tertiary alkyl halides.

Balz–Schiemann reaction is used to prepare fluoroarenes by heating diazonium tetrafluoroborate, resulting in replacement of –N₂⁺ by fluorine.

Haloalkanes containing only one halogen atom are called monohaloalkanes or alkyl halides.

Haloalkanes in which the halogen atom is attached to a primary carbon atom are called primary alkyl halides.

The halogen derivatives of arenes are called aromatic halogen compounds.

When two or more haloalkanes having the same molecular formula differ in the size of the longest carbon chain, the phenomenon is called chain isomerism.

When two or more haloalkanes have the same molecular formula but differ in the position of the halogen atom on the same carbon chain, they exhibit position isomerism.

Haloarenes are halogen derivatives of aromatic hydrocarbons in which a halogen atom is directly attached to a carbon atom of the aromatic ring.

Aryl halides are compounds formed when a hydrogen atom of an aromatic ring is replaced by a halogen atom.
General formula: Ar–X

Nuclear halogenation is the replacement of a hydrogen atom of the aromatic ring by a halogen atom in the presence of a Lewis acid catalyst (Fe, FeCl₃, AlCl₃).

1. Physical state & colour:
   Lower haloalkanes are gases, middle members are liquids and higher members are solids; all are generally colourless.
2. Odour:
   Lower haloalkanes possess a pleasant sweet smell, while higher members are almost odourless.
3. Boiling points:
   Boiling points are higher than those of corresponding alkanes due to stronger intermolecular forces and molecular polarity.
4. Effect of mass & branching:
   Boiling points increase with increase in halogen size and alkyl group size, but decrease with branching of the carbon chain.
5. Solubility:
   Haloalkanes are insoluble in water but soluble in organic solvents such as alcohols, ethers and carbon tetrachloride.
6. Density & stability:
   Density follows RI > RBr > RCl; relative stability follows RF > RCl > RBr > RI.

1. Nucleophilic substitution reactions:
   Haloalkanes undergo nucleophilic substitution because the C–X bond is polar; the halogen is replaced by a nucleophile.
2. SN₁ and SN₂ mechanisms:
   Primary haloalkanes generally undergo SN₂, tertiary haloalkanes undergo SN₁, while secondary haloalkanes may follow either mechanism.
3. Order of reactivity (same alkyl group):
   Reactivity increases with decreasing C–X bond strength:
   RI > RBr > RCl > RF.
4. Order of reactivity (same halogen):
   Reactivity increases with alkyl substitution:
   3° > 2° > 1° (for SN₁) and 1° > 2° > 3° (for SN₂).
5. Elimination reactions:
   On heating with alcoholic KOH, haloalkanes undergo dehydrohalogenation to form alkenes (β-elimination).
6. Reduction reactions:
   Haloalkanes are reduced to corresponding alkanes using nascent hydrogen, Zn–alcohol, or red phosphorus with HI.
7. Reaction with metals:
   With sodium in dry ether, haloalkanes undergo Wurtz reaction to form higher alkanes; with magnesium, they form Grignard reagents.
8. Rearrangement reactions:
   Some haloalkanes undergo rearrangement in the presence of Lewis acids like anhydrous AlCl₃, giving isomeric products.

• Physical nature:
  Bromoethane (ethyl bromide) is a colourless, sweet-smelling, heavy liquid.
• Boiling point:
  It boils at 312.3 K.
• Solubility:
  Insoluble in water but soluble in organic solvents such as alcohol and ether.
• Chemical behaviour:
  It undergoes all typical reactions of haloalkanes (substitution, elimination, etc.).
• Uses:
  Used as an ethylating agent, in preparing barbiturate drugs, as an anaesthetic and refrigerant, and as a solvent.

• Physical nature:
  Chloroform is a colourless, heavy liquid with a peculiar sickly smell and sweet burning taste.
• Density & solubility:
  Heavier than water; practically insoluble in water but soluble in most organic solvents, oils, fats, and waxes.
• Boiling & freezing points:
  Boils at 334 K and freezes at 210 K.
• Inflammability & vapour effect:
  Non-inflammable, but burns with a green-edged flame; its vapour causes temporary unconsciousness (toxic).
• Oxidation (phosgene formation):
  In air and sunlight, it oxidises to poisonous phosgene (COCl₂); hence stored in dark bottles with ethanol added.
• Important chemical reactions:
  Shows oxidation, reduction, hydrolysis, chlorination, nitration, carbylamine reaction, dehalogenation, and condensation reactions.
• Special named reactions:
  Gives carbylamine test with primary amines and Reimer–Tiemann reaction with phenol to form salicylaldehyde.
• Uses:
  Used as a solvent, laboratory reagent, preservative for specimens, and in preparation of chloropicrin and chlorobutanol (now limited as anaesthetic).

• Physical nature: Iodoform is a yellow, crystalline solid with a characteristic unpleasant (antiseptic) odour; insoluble in water but soluble in alcohol and ether.
• Stability: On heating or exposure to air and light, it decomposes to liberate iodine vapours, showing antiseptic properties.
• Reduction reaction: When reduced with red phosphorus and hydriodic acid (HI), iodoform gives methylene iodide (CH₂I₂).
• Carbylamine reaction: With primary amines and alcoholic KOH on warming, iodoform forms isonitriles (carbylamines) with a very offensive smell.
• Dehalogenation: On heating with silver powder, iodoform undergoes dehalogenation to form acetylene (C₂H₂).
• Iodoform test: It is used to detect –COCH₃ (methyl ketone) or –CH₃CHOH– group; a yellow precipitate of iodoform confirms a positive test.

• Physical state & colour:
  Haloarenes are generally colourless liquids or crystalline solids.
• Solubility:
  They are insoluble in water but soluble in organic solvents like alcohol, ether, and benzene due to their low polarity.
• Density:
  Haloarenes are heavier than water, and density increases with increase in the size of the halogen atom.
• Melting and boiling points:
  Melting and boiling points of monohaloarenes increase with increase in halogen size (F < Cl < Br < I).
• Isomeric effect:
  Among o-, m- and p-dihalobenzenes, the para-isomer has the highest melting point due to symmetrical packing.
• Reactivity:
  Haloarenes are less reactive than haloalkanes towards nucleophilic substitution reactions.
• Resonance stabilisation:
  In haloarenes, the C–X bond acquires partial double bond character due to resonance, making the bond shorter and stronger.
• Hybridisation & polarity:
  The carbon attached to halogen is sp² hybridised, making the C–X bond less polar, which further reduces reactivity.