HALOPP

Introduction to Halogenoalkanes

• The chemistry of halogenoalkanes is critical reactions.

Learning Objectivesfor understanding various organic

• Recall the synthesis of chloroalkanes.

• Know less environmentally damaging substitutes for haloalkanes.

• Understand environmental concerns about haloalkanes and the mechanism of ozone depletion.

Structure of Halogenoalkanes

• Functional Group: C-X, where X is a halogen (F, Cl, Br, I).

• Halogenoalkanes: Halogen is attached to an aliphatic skeleton/alkyl group.

• Haloarenes: Halogen is directly attached to a benzene (aromatic) ring.

• Halogenoalkanes are classified according to the environment of the halogen:

  • Primary (1°)

  • Secondary (2°)

  • Tertiary (3°)

Nomenclature of Halogeno-compounds

• Naming haloalkanes follows rules similar to those for naming alkanes.

• Halogens are written as prefixes:

  • Fluoro- (F)

  • Chloro- (Cl)

  • Bromo- (Br)

  • Iodo- (I)

• When the parent chain has both a halogen and an alkyl substituent, it is numbered from the end nearer the first substituent.

Halobenzenes

• Halobenzenes have halogen atoms directly attached to a benzene ring.

• Example: A compound is not considered a halobenzene if the chlorine atom is not directly attached to the benzene ring.

Structural Isomerism in Halogenoalkanes

• Different structural isomers arise from:

  • Different positions for the halogen.

  • Branching of the carbon chain.

• Examples include:

  • 2-chlorobutane vs. 2-chloro-2-methylpropane

  • 1-chlorobutane vs. 1-chloro-2-methylpropane

Physical Properties of Halogenoalkanes

• Boiling Point (bp): Increases with molecular size due to increased van der Waals forces.

  • Example boiling points:

    • Chloroethane: 64.5°C

    • 1-chloropropane: 78.5°C

    • 1-bromopropane: 124°C

• Boiling points also increase for straight-chain isomers; greater branching results in lower intermolecular forces.

• Solubility: Halogenoalkanes are soluble in organic solvents but insoluble in water.

Preparation of Haloalkanes

• Prepared by substituting –OH group of alcohols with halogen atoms.

• Common reagents include HCl, HBr, HI, PCl3, or PBr3.

• The ease of substitution is related to the stability of reaction intermediates:

  • 3° > 2° > 1° > CH3OH (due to carbocation stability).

Mechanical Activities in Halobenzenes

• Benzene reacts readily with Cl and Br in the presence of catalysts (FeCl3, FeBr3, AlCl3).

• Halogenation of benzene leads to haloaromatic compounds.

• Addition reactions of halogens or hydrogen halides to alkenes or alkynes can produce haloalkanes.

Nucleophilic Substitution Theory

• Halogens possess higher electronegativity than carbon, resulting in polar C-X bonds.

• A dipole is induced in the C-X bond; carbon becomes open to attack by nucleophiles:

  • Nucleophiles are electron pair donors (e.g., OH¯, CN¯, NH3, H2O).

Nucleophilic Substitution Mechanism

• The nucleophile donates its lone pair to form a new bond, displacing the halogen.

• The carbon can only hold eight electrons, necessitating bond displacement.

• Mechanism: Polar C-X bond breaks heterolytically; halogen departs as a halide ion.

Rate of Reaction in Nucleophilic Substitution

• Rate of reaction is influenced by the strength of the C-X bond:

  • C-I: 238 kJmol-1 (weakest, easiest to break)

  • C-Br: 276 kJmol-1

  • C-Cl: 338 kJmol-1

  • C-F: 484 kJmol-1 (strongest, hardest to break)

• Weak C-X bonds lead to quicker precipitates in silver nitrate tests.

Hydrolysis and Nucleophilic Substitution

• Reactions with aqueous sodium hydroxide produce alcohols:

  • Example: C2H5Br + NaOH → C2H5OH + NaBr.

• Water can also act as a nucleophile, though slower than OH¯.

Elimination vs. Substitution

• Elimination mechanisms can vary based on solvent type.

• Alcoholic sodium hydroxide leads to alkene formation via elimination:

  • Example: C3H7Br + NaOH(alc) → C3H6 + H2O + NaBr.

Uses of Halogenoalkanes

• Halogenoalkanes are significant in synthetic organic chemistry and polymer formation:

  • Examples:

    • Chloroethene → poly(chloroethene) (PVC)

    • Tetrafluoroethene → poly(tetrafluoroethene) (PTFE) for non-stick surfaces.

• Chlorofluorocarbons (CFCs) are used as refrigerants and propellants due to their low reactivity and volatility.

Environmental Impact of CFCs

• CFCs contribute to ozone layer depletion by breaking down into radicals that catalyze ozone destruction.

• Real-world implications involve UV radiation absorption by ozone and the formation of detrimental radicals.

• Chemists are synthesizing alternatives to CFCs (e.g., hydrocarbons, HCFCs) to mitigate environmental issues.