Alkyl halides: Class 12 Chemistry

Key points

Alkyl halides (haloalkanes) have the general formula R-X, where X is F, Cl, Br or I attached to an sp³ carbon. They are classified by the carbon holding the halogen. In a primary halide the carbon is bonded to one other carbon (CH₃CH₂Br). In a secondary halide it is bonded to two (CH₃CHBrCH₃). In a tertiary halide it is bonded to three ((CH₃)₃CBr).

IUPAC names treat the halogen as a prefix on the longest chain: CH₃CH₂Cl is chloroethane, CH₃CHBrCH₃ is 2-bromopropane and (CH₃)₃CCl is 2-chloro-2-methylpropane. Common names use alkyl and halide, such as ethyl chloride and isopropyl bromide.

Preparation of alkyl halides:

• From alcohols with HX (with ZnCl₂ for HCl), PCl₅, PCl₃ or thionyl chloride. R-OH + SOCl₂ → R-Cl + SO₂ + HCl is very convenient because the by-products are gases.
• By addition of HX or X₂ to alkenes, as in CH₂=CH₂ + HBr → CH₃CH₂Br.
• By free radical halogenation of alkanes, which gives a mixture of products and is therefore not a good laboratory method.
• Iodides by halogen exchange (Finkelstein reaction): R-Cl + NaI in dry acetone → R-I + NaCl.

The C-X bond is polar because halogens are more electronegative than carbon. The carbon carries a partial positive charge, so it is attacked by nucleophiles. Boiling points are higher than those of alkanes with the same carbon number because of dipole forces and greater mass. They rise from fluoride to iodide and fall with branching. Alkyl halides are insoluble in water because they cannot form hydrogen bonds with it, but they dissolve in organic solvents.

Bond strength falls in the order C-F > C-Cl > C-Br > C-I. So reactivity in substitution rises from chlorides to iodides. This is why iodoalkanes react fastest.

Nucleophilic substitution (a nucleophile replaces X):

• With aqueous KOH it gives an alcohol: R-X + OH⁻ → R-OH + X⁻.
• With alcoholic KCN it gives a nitrile R-CN, which adds one carbon to the chain.
• With ammonia it gives an amine and with NaOR it gives an ether (Williamson synthesis).
• With aqueous or alcoholic AgNO₃ it gives a precipitate of AgX (white AgCl, cream AgBr, yellow AgI), which is used to identify the halogen.

The SN2 mechanism is a one-step reaction. The nucleophile attacks from the side opposite to the leaving group, and the rate depends on both the halide and the nucleophile (second order). It is fastest for primary halides and gives inversion of configuration. The SN1 mechanism has two steps. The halide first ionises slowly to a carbocation, and the nucleophile then attacks it. The rate depends only on the halide (first order). It is fastest for tertiary halides, because their carbocations are most stable, and it gives a mixture of both configurations (racemisation).

Elimination (dehydrohalogenation): heating an alkyl halide with alcoholic KOH removes HX and forms an alkene. CH₃CH₂Br + KOH(alc) → CH₂=CH₂ + KBr + H₂O. Aqueous KOH favours substitution while alcoholic KOH favours elimination. When two alkenes are possible, Saytzeff's rule says the more substituted alkene is the major product.

Grignard reagents are made by reacting an alkyl halide with magnesium in dry ether: R-X + Mg → R-MgX. The carbon bonded to Mg behaves as a carbanion and is a strong nucleophile and base. Water must be absent, since R-MgX + H₂O → R-H + Mg(OH)X. Reaction with carbon dioxide followed by acid gives a carboxylic acid, and reaction with aldehydes and ketones gives alcohols.

Uses: solvents (dichloromethane, chloroform), starting materials in synthesis, anaesthetics in the past (chloroform), refrigerants and fire extinguishers. CFCs (chlorofluorocarbons) damaged the ozone layer, because UV light breaks the C-Cl bond to release chlorine radicals that destroy ozone, so their use is now restricted. Some halogenated pesticides such as DDT persist in the environment and build up in food chains.

Study tip: Make one page with the reagent chart: aqueous KOH, alcoholic KOH, KCN, NH₃, Mg in ether, AgNO₃. Most reaction questions come from this chart. In mechanism answers show the curved arrows and the inversion or carbocation clearly, and state which halide class (primary or tertiary) suits which mechanism.

Definitions and flashcards

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Short questions with model answers

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Numericals with worked steps

Common mistakes

• Using aqueous KOH when elimination is wanted. Alcoholic KOH gives alkenes and aqueous KOH gives alcohols.
• Saying SN1 reactions are fastest for primary halides. SN1 prefers tertiary and SN2 prefers primary.
• Writing the Grignard reagent as R-Mg instead of R-MgX, or preparing it in the presence of water.
• Thinking C-F is the most reactive bond because fluorine is the most electronegative. The C-F bond is the strongest.
• Mixing up the colours of silver halide precipitates: AgCl is white, AgBr cream and AgI yellow.

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