Haloalkanes and Haloarenes

The One Thing

The chemistry of haloalkanes and haloarenes is governed primarily by the polar carbon–halogen bond, the structure of the carbon skeleton and the reaction conditions. These factors determine whether substitution or elimination occurs and explain why haloarenes are generally less reactive than haloalkanes toward nucleophilic substitution.

Reactions, Processes and Experiments

Key Terms

• Haloalkane: An aliphatic compound in which a halogen atom is attached to an sp3-hybridised carbon atom, represented generally as R-X.
• Haloarene: An aromatic compound in which a halogen atom is directly attached to an aromatic ring, represented generally as Ar-X.
• Halogen: An element of Group 17, such as fluorine, chlorine, bromine or iodine, commonly represented by X in organic formulas.
• Monohalo, dihalo and polyhalo compounds: Compounds containing one, two or more halogen atoms, respectively.
• Geminal dihalide: A compound in which two halogen atoms are attached to the same carbon atom.
• Vicinal dihalide: A compound in which two halogen atoms are attached to adjacent carbon atoms.
• Primary, secondary and tertiary haloalkane: Haloalkanes classified according to whether the carbon bonded to the halogen is attached to one, two or three other carbon atoms.
• Aryl halide: A compound in which a halogen is directly attached to an aromatic ring, such as chlorobenzene.
• Benzyl halide: A compound in which the halogen is attached to the carbon next to an aromatic ring, such as C6H5CH2Cl.
• Allylic halide: A compound in which the halogen is attached to a carbon adjacent to a carbon-carbon double bond.
• IUPAC nomenclature: The systematic naming method in which halogens are named as prefixes such as fluoro-, chloro-, bromo- and iodo-.
• Nucleophile: An electron-rich species that donates an electron pair to an electron-deficient carbon atom, such as OH-, CN-, NH3 or I-.
• Electrophilic carbon: The carbon bonded to halogen in a polar C-X bond; it carries a partial positive charge and is attacked by nucleophiles.
• Leaving group: An atom or group that leaves the molecule with the electron pair during a reaction; in haloalkanes, the halide ion is the leaving group.
• Nucleophilic substitution: A reaction in which a nucleophile replaces the halogen atom in a haloalkane or haloarene.
• SN1 reaction: A unimolecular nucleophilic substitution involving two steps and a carbocation intermediate; its rate depends only on the concentration of the haloalkane.
• SN2 reaction: A bimolecular nucleophilic substitution occurring in one step through backside attack; its rate depends on both the haloalkane and the nucleophile.
• Carbocation: A positively charged carbon species formed as an intermediate in many SN1 reactions and some elimination reactions.
• Inversion of configuration: The change in spatial arrangement around a chiral carbon caused by backside attack in an SN2 reaction.
• Elimination reaction: A reaction in which hydrogen halide is removed from adjacent carbon atoms to form an alkene.
• Dehydrohalogenation: Removal of a hydrogen atom and a halogen atom from neighbouring carbon atoms, usually using alcoholic KOH and heat.
• Saytzeff rule: In beta-elimination, the major alkene is generally the more substituted, and therefore more stable, alkene.
• Aqueous KOH substitution: A haloalkane reacts with aqueous potassium hydroxide to form an alcohol: R-X + KOH(aq) -> R-OH + KX.
• Alcoholic KOH elimination: A haloalkane reacts with alcoholic potassium hydroxide on heating to form an alkene: R-CH2-CHX-R' -> R-CH=CH-R' + KX + H2O.
• Wurtz reaction: Coupling of two haloalkane molecules with sodium in dry ether to form a higher alkane: 2R-X + 2Na -> R-R + 2NaX.
• Finkelstein reaction: Preparation of alkyl iodides by treating alkyl chlorides or bromides with sodium iodide in dry acetone: R-Cl or R-Br + NaI -> R-I + NaCl or NaBr.
• Swarts reaction: Preparation of alkyl fluorides by heating alkyl chlorides or bromides with metallic fluorides such as AgF, Hg2F2 or SbF3.
• Sandmeyer reaction: Replacement of the diazonium group in an aromatic diazonium salt by Cl, Br or CN using copper(I) salts.
• Gattermann reaction: Conversion of an aromatic diazonium salt into an aryl chloride or aryl bromide using copper powder and hydrogen chloride or hydrogen bromide.
• Balz-Schiemann reaction: Preparation of aryl fluorides by heating an aromatic diazonium tetrafluoroborate.
• Friedel-Crafts reaction: An electrophilic aromatic substitution reaction used to introduce alkyl or acyl groups into an aromatic ring.
• Resonance in haloarenes: Donation of lone-pair electrons from the halogen into the aromatic ring gives the C-X bond partial double-bond character and makes it shorter and stronger.
• Relative reactivity of halides: For many nucleophilic substitution reactions of alkyl halides, the order of leaving-group ability is I- > Br- > Cl- > F-, so R-I is generally more reactive than R-F.

Easily Confused

• Haloalkane and haloarene: In a haloalkane, the halogen is attached to an sp3-hybridised aliphatic carbon; in a haloarene, it is directly attached to an aromatic ring.
• Aryl halide and benzyl halide: In an aryl halide such as chlorobenzene, the halogen is directly bonded to the ring; in a benzyl halide such as C6H5CH2Cl, it is bonded to the adjacent carbon.
• Geminal and vicinal dihalide: Geminal halides are attached to the same carbon, whereas vicinal halides are attached to adjacent carbons.
• SN1 and SN2: SN1 occurs in two steps through a carbocation and has the rate law rate = k[R-X]; SN2 occurs in one step by backside attack and depends on both reactants.
• SN2 inversion and SN1 racemisation: SN2 causes inversion of configuration through backside attack, whereas SN1 at a chiral centre may give both retention and inversion products, often approaching racemisation.
• Aqueous KOH and alcoholic KOH: Aqueous KOH favours substitution to form an alcohol, whereas alcoholic KOH with heat favours elimination to form an alkene.
• Substitution and elimination: Substitution replaces a functional group or atom; elimination removes atoms or groups from adjacent carbons to form a multiple bond.
• Markovnikov and anti-Markovnikov addition: HBr normally follows Markovnikov’s rule, but in the presence of organic peroxide it undergoes anti-Markovnikov addition.
• Aryl halide and haloalkane reactivity: Haloarenes are less reactive toward ordinary nucleophilic substitution because resonance strengthens the C-X bond and the carbon is sp2-hybridised.
• Deactivating and directing effects of halogens: Halogens deactivate the aromatic ring through their -I effect but direct incoming groups to ortho and para positions through their +R effect.
• Saytzeff and less-substituted elimination products: The more substituted alkene is generally major, but bulky bases or special structural conditions can favour the less substituted alkene.

What Gets Asked

• Nomenclature questions: Name compounds such as CH3Cl, CH3CH2Br, (CH3)3CCl and C6H5Cl using IUPAC rules. Marks are commonly lost by incorrect numbering, failure to use fluoro-, chloro-, bromo- or iodo-, or incorrect alphabetical ordering of different substituents.
• Preparation questions: Write equations for preparation from alcohols, including R-OH + HX -> R-X + H2O and R-OH + SOCl2 -> R-Cl + SO2 + HCl. The specific reagent conditions, especially anhydrous ZnCl2 for HCl and the gaseous SO2 and HCl by-products with SOCl2, must not be omitted.
• Named-reaction questions: Identify or write the Wurtz, Finkelstein, Swarts, Sandmeyer, Gattermann and Balz-Schiemann reactions. The main marking error is confusing the reagents and the halogen introduced in each reaction.
• Mechanism questions: Compare SN1 and SN2 mechanisms, including carbocation formation, the rate law rate = k[R-X], backside attack, transition state and inversion of configuration. A frequent error is assigning a carbocation intermediate to SN2.
• Condition-and-product questions: Distinguish aqueous KOH substitution from alcoholic KOH elimination and apply the Saytzeff rule. Marks are lost by giving an alcohol under alcoholic KOH and heat or by selecting the less substituted alkene without a stated structural reason.
• Haloarene reactivity questions: Explain why chlorobenzene is resistant to ordinary SN1 and SN2 reactions and state the severe conditions for conversion with aqueous NaOH at about 623 K and high pressure. The key omissions are resonance, partial double-bond character, sp2 hybridisation and the instability of a phenyl carbocation.