Overview
This lecture covers the properties, naming, and reactions (nucleophilic substitution and elimination) of halogenoalkanes, and their environmental impact on ozone depletion.
Definition and Naming of Halogenoalkanes
- Halogenoalkanes are alkanes in which one or more hydrogen atoms are replaced by halogen atoms.
- General formula: CₙH₂ₙ₊₁X, where X is a halogen.
- Functional group is represented as RX, where R is the carbon chain.
- Naming involves identifying the longest carbon chain and using prefixes (fluoro-, chloro-, bromo-, iodo-) listed alphabetically.
- Numbers indicate the carbon(s) where halogen(s) are attached; use di-, tri-, tetra- for multiple identical halogens.
Physical Properties of Halogenoalkanes
- Carbon–halogen bonds are polar due to electronegativity difference; polarity increases from C–I < C–Br < C–Cl < C–F.
- Halogenoalkanes are insoluble in water due to their nonpolar carbon chains but are soluble in nonpolar organic solvents.
- Boiling point increases with longer carbon chain and larger halogen atoms due to greater van der Waals forces.
- Branching lowers boiling point by reducing surface area for intermolecular interactions.
Reactivity and Factors Affecting It
- Reactivity depends on the ease of breaking the C–X bond, influenced by bond enthalpy (strength) and polarity.
- C–I is the weakest and most reactive bond; reactivity increases down the group (F < Cl < Br < I).
- Hydrolysis test with AgNO₃ shows fast reaction for iodoalkanes, then bromo- and chloroalkanes.
Nucleophilic Substitution in Halogenoalkanes
- Nucleophilic substitution replaces the halogen with a nucleophile (hydroxide, cyanide, ammonia).
- Hydroxide produces alcohol; cyanide produces nitrile; ammonia produces amine.
- Curly arrows indicate electron movement from nucleophile to carbon and from C–X bond to halogen.
- For ammonia, two molecules are needed: one acts as nucleophile, the other as base.
- Typical reagents and conditions:
- Hydroxide: aqueous KOH/NaOH, warm, often with ethanol.
- Cyanide: aqueous ethanolic KCN, warm.
- Ammonia: excess concentrated NH₃, ethanol solvent, high pressure.
Elimination Reactions in Halogenoalkanes
- Elimination forms alkene, water, and halide salt, using ethanolic KOH/NaOH under heat.
- Hydroxide ion acts as a base, removing a proton to form a double bond.
- Multiple products (alkene isomers) may form if different hydrogens are removed from the adjacent carbon.
- Elimination is favored by ethanol solvent and higher temperatures; substitution by aqueous solvent and milder temperatures.
Ozone Depletion and Halogenoalkanes
- CFCs (chlorofluorocarbons) are halogenoalkanes with Cl and F, no H; used as refrigerants and solvents.
- UV light breaks C–Cl bonds, generating chlorine free radicals that catalyze ozone breakdown.
- Ozone (O₃) absorbs UV; its depletion increases harmful UV at Earth's surface.
- Regulatory actions now restrict CFC use to protect the ozone layer.
Key Terms & Definitions
- Halogenoalkane — Alkane with one or more halogen substituents.
- Nucleophile — Atom or molecule that donates a lone pair to form a bond.
- Nucleophilic substitution — Reaction where a nucleophile replaces a leaving group (halogen).
- Elimination — Reaction forming a double bond and removing atoms/groups from adjacent carbons.
- CFC — Chlorofluorocarbon compound that depletes ozone via free-radical chain reactions.
- Leaving group — Atom or group that departs with a pair of electrons in a reaction.
Action Items / Next Steps
- Practice drawing and naming halogenoalkanes and writing equations for their reactions.
- Review mechanisms for nucleophilic substitution and elimination with correct curly arrows.
- Read about free-radical mechanisms and ozone depletion for environmental chemistry context.