Haloalkanes and Haloarenes

The One Thing

The reactions of haloalkanes and haloarenes are governed mainly by the nature of the carbon–halogen bond, the structure of the carbon framework, steric effects, resonance, and the reaction conditions. Haloalkanes commonly undergo nucleophilic substitution and elimination, whereas haloarenes are less reactive towards ordinary nucleophilic substitution because resonance strengthens the aryl carbon–halogen bond.

Reactions, Processes and Experiments

Key Terms

• Haloalkane: An aliphatic compound containing one or more halogen atoms bonded to an sp3-hybridised carbon atom; its general representation is R-X.
• Haloarene: An aromatic compound in which a halogen atom is directly bonded to an aromatic ring, such as chlorobenzene, C6H5Cl.
• Alkyl halide: Another name for a haloalkane, formed by replacing an alkane hydrogen with fluorine, chlorine, bromine, or iodine.
• Aryl halide: Another name for a haloarene in which the halogen is directly attached to an aromatic carbon atom.
• Classification of haloalkanes: Haloalkanes may be primary, secondary, or tertiary according to the number of carbon groups attached to the carbon bonded to the halogen.
• Geminal dihalide: A compound having two halogen atoms attached to the same carbon atom.
• Vicinal dihalide: A compound having two halogen atoms attached to adjacent carbon atoms.
• Nucleophile: An electron-rich species that donates an electron pair to an electron-deficient carbon atom; examples include OH-, CN-, and NH3.
• Nucleophilic substitution: A reaction in which a nucleophile replaces the halogen atom in a halo compound.
• SN1 reaction: A two-step substitution reaction involving carbocation formation; its rate depends only on the concentration of the haloalkane.
• SN2 reaction: A one-step substitution reaction in which the nucleophile attacks from the side opposite the leaving group; its rate depends on both the haloalkane and nucleophile concentrations.
• Elimination reaction: A reaction in which hydrogen halide is removed from a haloalkane to form an alkene, usually using alcoholic KOH and heat.
• Saytzeff rule: In beta-elimination, the major alkene is generally the more substituted and therefore more stable alkene.
• Wurtz reaction: Coupling of two molecules of an alkyl halide with sodium in dry ether to form a higher alkane: 2R-X + 2Na -> R-R + 2NaX.
• Finkelstein reaction: Conversion of an alkyl chloride or bromide into an alkyl iodide using sodium iodide in dry acetone.
• Swarts reaction: Preparation of alkyl fluorides by heating alkyl chlorides or bromides with metallic fluorides such as AgF, Hg2F2, or SbF3.
• Grignard reagent: An organomagnesium compound with the formula R-Mg-X, prepared by reacting an alkyl or aryl halide with magnesium in dry ether.
• Aryl diazonium salt: A compound containing the diazonium group, Ar-N2+, used as an intermediate for preparing haloarenes.
• Sandmeyer reaction: Replacement of the diazonium group in an aryl diazonium salt by chlorine, bromine, or cyanide using copper(I) salts.
• Finkelstein-type limitation: The halide-exchange reaction works mainly for primary alkyl halides and is generally unsuitable for aryl halides because aryl carbon-halogen bonds are resistant to substitution.
• Resonance in haloarenes: The lone pair on the halogen can overlap with the aromatic pi system, giving the carbon-halogen bond partial double-bond character and making it shorter and stronger.
• Inductive effect: Electron withdrawal through sigma bonds; halogens show a negative inductive effect and reduce electron density near the carbon to which they are attached.
• Leaving group ability: The ease with which a halide ion leaves; for common haloalkanes, the order is generally I- > Br- > Cl- >> F-.

Additional formulae and structural terms:

• Monohaloalkane: General formula CnH2n+1X, where X = F, Cl, Br, or I.
• Monohaloarene: General formula Ar-X, where Ar represents an aromatic group.
• Carbon–halogen bond polarity: The bond is polar because halogens are more electronegative than carbon.
• Carbon–halogen bond strength: Bond strength generally follows C-F > C-Cl > C-Br > C-I.
• SN1 reactivity: Generally tertiary > secondary > primary; allylic and benzylic halides are also highly reactive because of resonance stabilisation.
• SN2 reactivity: Generally methyl > primary > secondary >> tertiary because steric hindrance inhibits backside attack.
• Walden inversion: Inversion of configuration caused by backside attack in an SN2 reaction.
• Racemisation: Formation of a mixture of configurations at a chiral carbon in an SN1 reaction because the planar carbocation can be attacked from either side.
• Polyhalogen compounds: Compounds including chloroform, CHCl3; iodoform, CHI3; carbon tetrachloride, CCl4; freons such as CCl2F2; and DDT, a formerly used insecticide.

Easily Confused

• Haloalkane and haloarene: A haloalkane has halogen bonded to an sp3-hybridised aliphatic carbon, whereas a haloarene has halogen directly bonded to an aromatic ring.
• Alkyl halide and aryl halide: Alkyl halide is another name for haloalkane; aryl halide is another name for haloarene.
• Geminal and vicinal dihalides: Geminal dihalides have both halogens on the same carbon, whereas vicinal dihalides have halogens on adjacent carbons.
• SN1 and SN2: SN1 is a two-step, carbocation-forming reaction whose rate depends only on the haloalkane; SN2 is a one-step backside attack whose rate depends on both reactants.
• SN1 and elimination: SN1 involves substitution by a nucleophile, whereas elimination removes hydrogen halide to form an alkene.
• Aqueous KOH and alcoholic KOH: Aqueous KOH generally gives alcohols by substitution, whereas alcoholic KOH and heat favour alkene formation by elimination.
• KCN and AgCN: KCN mainly forms nitriles, R-CN, while AgCN mainly forms isocyanides, R-NC.
• Wurtz and Finkelstein reactions: Wurtz couples two alkyl halides to form a higher alkane; Finkelstein exchanges chloride or bromide for iodide.
• Finkelstein and Swarts reactions: Finkelstein prepares alkyl iodides using sodium iodide in dry acetone; Swarts prepares alkyl fluorides using metallic fluorides.
• Saytzeff rule and anti-Markovnikov addition: Saytzeff rule predicts the major alkene in beta-elimination; anti-Markovnikov addition describes the orientation of HBr addition to an alkene in the presence of peroxides.
• SN1 and SN2 stereochemistry: SN1 may produce racemisation, whereas SN2 produces inversion of configuration.
• Haloarene reactivity and haloalkane reactivity: Haloarenes are comparatively resistant to nucleophilic substitution because resonance strengthens the aryl carbon–halogen bond and backside attack at an sp2 carbon is difficult.
• Deactivating and directing effects of halogens: Halogens deactivate aromatic rings through the negative inductive effect but direct electrophilic substitution to ortho and para positions through resonance donation.
• Chlorobenzene and chloroethane carbon–halogen bonds: The carbon–halogen bond in chlorobenzene is stronger than that in chloroethane because of resonance and partial double-bond character.
• Nitrile and isocyanide: Nitriles have the arrangement R-CN, whereas isocyanides have the arrangement R-NC.
• Chloroform and phosgene: Chloroform, CHCl3, can be oxidised by air and light to poisonous phosgene, COCl2; it is therefore stored in dark bottles with a small amount of ethanol.

What Gets Asked

• Questions may require preparation of haloalkanes from alcohols using HX, PCl3, PCl5, or SOCl2. The specific equation R-OH + SOCl2 -> R-Cl + SO2 + HCl and the escape of gaseous by-products are important.
• Questions may ask for the products of reactions with aqueous KOH, alcoholic KOH, KCN, AgCN, ammonia, sodium alkoxides, or magnesium in dry ether. The common mark loss is confusing substitution with elimination or confusing nitriles with isocyanides.
• Mechanism questions may compare SN1 and SN2 reactions, including their rate laws, structural preferences, steric effects, carbocation formation, backside attack, racemisation, and Walden inversion.
• Questions may ask why haloarenes are resistant to nucleophilic substitution. The required explanation includes resonance, partial double-bond character, the stronger carbon–halogen bond in chlorobenzene, the instability of the phenyl carbocation, and difficulty of backside attack at an sp2 carbon.
• Questions may test electrophilic substitution in chlorobenzene. Halogens are deactivating through the negative inductive effect but ortho- and para-directing through resonance; the para product is often favoured because of lower steric hindrance.
• Questions may ask about physical properties, bond strength, leaving group ability, or environmental and health concerns. Relevant specifics include C-F > C-Cl > C-Br > C-I, I- > Br- > Cl- >> F-, chloroform oxidation to phosgene, ozone depletion by some chlorofluorocarbons, and persistence of certain organochlorine pesticides.