Haloalkanes and their Properties

Definition and Classification

Haloalkanes are aliphatic hydrocarbons in which one or more hydrogen atoms are replaced by halogen atoms (fluorine, chlorine, bromine, iodine). Haloarenes are aromatic hydrocarbons where hydrogen atoms in the benzene ring are replaced by halogen atoms.

Types of Haloalkanes

Based on the number of halogen atoms, haloalkanes are classified as monohaloalkanes, dihaloalkanes, and polyhaloalkanes. Dihaloalkanes can be geminal (two halogens on the same carbon) or vicinal (halogens on adjacent carbons).

Hybridisation and Subtypes

In haloalkanes, the halogen is attached to an sp³ hybridised carbon. Subtypes include primary, secondary, and tertiary haloalkanes depending on the number of alkyl groups attached to the carbon bonded to the halogen. Allylic halides have halogen attached to an allylic carbon (adjacent to a double bond), and benzylic halides have halogen attached to a carbon adjacent to an aromatic ring.

Physical Properties

Haloalkanes generally have higher boiling and melting points than their parent hydrocarbons due to polar C–X bonds and dipole–dipole interactions. Boiling points increase with the size of the halogen (RF < RCl < RBr < RI). They are insoluble in water but soluble in organic solvents and have densities greater than water.

Chemical Properties

Reactivity depends on the bond dissociation energy of the C–X bond, which decreases down the group (C–Cl > C–Br > C–I). Haloalkanes undergo nucleophilic substitution, elimination, reactions with metals (forming Grignard reagents or Wurtz coupling), and reduction.

Mechanisms of Nucleophilic Substitution

Two main mechanisms are SN1 and SN2. SN1 involves a carbocation intermediate and is unimolecular, favored by tertiary haloalkanes and polar protic solvents. SN2 is bimolecular, involves a backside attack, and is favored by primary haloalkanes and polar aprotic solvents.

Stereochemistry

Haloalkanes can exhibit stereoisomerism. Optical activity arises from chiral carbons bonded to four different groups. SN2 reactions cause inversion of configuration (Walden inversion), while SN1 reactions can lead to racemisation.

Solved Examples

Example 1: Write the IUPAC names of the following compounds: (i) 4-Bromopent-2-ene, (ii) 3-Bromo-2-methylbut-1-ene.

Solution: Identify the longest chain containing the double bond, number it to give the double bond the lowest number, locate substituents, and name accordingly. The names are as given.

Example 2: Identify all possible monochloro isomers formed on free radical chlorination of (CH₃)₂CHCH₂CH₃.

Solution: Four types of hydrogens can be replaced, leading to four isomers: (CH₃)₂CHCH₂CH₂Cl, (CH₃)₂CHCH(Cl)CH₃, (CH₃)₂C(Cl)CH₂CH₃, and CH₃CH(CH₂Cl)CH₂CH₃.

Example 3: Write the products of addition of HBr to propene with and without peroxide.

Solution: Without peroxide, Markovnikov addition gives 2-bromopropane. With peroxide, anti-Markovnikov addition gives 1-bromopropane.

Practice Set

• Level 1 (Easy): Define primary, secondary, and tertiary haloalkanes with examples.
• Level 2 (Moderate): Explain the difference between SN1 and SN2 mechanisms with suitable examples.
• Level 3 (Challenging): Predict the major product and mechanism when 2-bromopropane reacts with aqueous KOH.

Answer Key

• Level 1: Primary haloalkane: halogen attached to carbon bonded to one other carbon (e.g., chloromethane). Secondary: halogen attached to carbon bonded to two carbons (e.g., 2-chloropropane). Tertiary: halogen attached to carbon bonded to three carbons (e.g., tert-butyl chloride).
• Level 2: SN1 is unimolecular, involves carbocation intermediate, favored by tertiary haloalkanes and polar protic solvents. SN2 is bimolecular, one-step backside attack, favored by primary haloalkanes and polar aprotic solvents.
• Level 3: 2-bromopropane reacts with aqueous KOH mainly via SN1 mechanism forming 2-propanol as the major product due to carbocation intermediate formation.

Haloarenes and Polyhalogen Compounds

Definition and Substitution Patterns

Haloarenes are aromatic compounds where one or more hydrogen atoms on the benzene ring are replaced by halogen atoms. Substitution patterns include ortho (1,2-), meta (1,3-), and para (1,4-) positions.

Preparation Methods

Haloarenes are prepared by direct halogenation of benzene using halogens and catalysts (Fe, FeX₃), by the Sandmeyer reaction from diazonium salts, and by other substitution reactions.

Direct halogenation: Benzene reacts with chlorine or bromine in the presence of FeCl₃ or FeBr₃ to form chlorobenzene or bromobenzene.

From diazonium salts: Aniline is first converted into a diazonium salt, which then gives haloarenes by the Sandmeyer or related reactions.

Other methods: Some haloarenes are also obtained by halogen exchange or by coupling reactions depending on the required halogen and ring substitution pattern.

Physical Properties

Isomeric haloarenes have similar boiling points, but para isomers have higher melting points due to better crystal packing.

Chemical Properties

Haloarenes are less reactive in nucleophilic substitution because the C–X bond has partial double bond character due to resonance. As a result, the bond is shorter and stronger than in haloalkanes. Electrophilic substitution occurs mainly at ortho and para positions because the halogen atom directs incoming electrophiles to these positions, although it is overall deactivating.

Why nucleophilic substitution is difficult: In haloarenes, the lone pair on the halogen is delocalized into the benzene ring, giving the C–X bond partial double bond character. This makes cleavage of the bond difficult.

Orientation in electrophilic substitution: Halogens are deactivating due to their electron-withdrawing inductive effect, but they are ortho-para directing because of resonance donation.

Key comparison: Haloarenes are much less reactive than haloalkanes toward nucleophilic substitution because the aryl carbon is sp²-hybridised and the C–X bond is strengthened by resonance.

Important Reactions

Important reactions of haloarenes include Sandmeyer, Finkelstein, Wurtz, Friedel-Crafts alkylation, Dow’s process, Hunsdiecker, and Gattermann reactions.

Sandmeyer reaction: Aryl diazonium salts are converted into aryl chlorides, bromides, or cyanides using CuCl, CuBr, or CuCN.

Finkelstein reaction: Alkyl halides are generally exchanged with iodide ion in acetone; this reaction is commonly discussed in halogen exchange, though it is not a typical method for preparing haloarenes.

Wurtz reaction: Two alkyl halide molecules couple in the presence of sodium metal to form a higher alkane.

Friedel-Crafts alkylation: Haloarenes generally do not undergo Friedel-Crafts alkylation readily because the halogen atom deactivates the ring and complexes with the catalyst.

Dow’s process: Chlorobenzene is converted into phenol by heating with aqueous NaOH at high temperature and pressure.

Hunsdiecker reaction: Silver salts of carboxylic acids give haloalkanes with one carbon atom less on heating with halogen.

Gattermann reaction: Aryl diazonium salts are converted into aryl chlorides or bromides in the presence of copper powder and the corresponding hydrogen halide.

Polyhalogen Compounds

Compounds with multiple halogen atoms, such as dichloromethane, chloroform, carbon tetrachloride, iodoform, and DDT, have industrial and agricultural uses but may pose environmental hazards.

Dichloromethane (CH₂Cl₂): Used as a solvent and paint remover.

Chloroform (CHCl₃): Used as a solvent and was formerly used as an anaesthetic.

Carbon tetrachloride (CCl₄): Used earlier as a solvent and in fire extinguishers, but now restricted because of toxicity.

Iodoform (CHI₃): Used as an antiseptic due to its characteristic smell and disinfectant properties.

DDT: An important insecticide that is highly persistent in the environment and is now banned or restricted in many places because of bioaccumulation and pollution.

Environmental concern: Many polyhalogen compounds are stable, non-biodegradable, and harmful to living organisms, so their use must be carefully controlled.

Solved Examples

Example 4: Explain why KCN reacts with haloalkanes to form alkyl cyanides, while AgCN forms isocyanides.

Solution: KCN is ionic and provides free CN⁻ ions. In CN⁻, carbon is the more nucleophilic end, so attack occurs through carbon to give alkyl cyanides (R–CN). AgCN is covalent, so the carbon end is tied up with Ag⁺ and the nitrogen end acts as the attacking site, forming isocyanides (R–NC).

Example 5: Which compound undergoes SN2 reaction faster: cyclohexylmethyl chloride or cyclohexyl chloride?

Solution: Cyclohexylmethyl chloride reacts faster in an SN2 reaction because it is less sterically hindered than cyclohexyl chloride.

Example 6: Predict the order of reactivity of isomeric bromobutanes in SN1 and SN2 reactions.

Solution: SN1: primary < secondary < tertiary. SN2: tertiary < secondary < primary.

Practice Set

• Level 1 (Easy): Define ortho, meta, and para substitution in haloarenes with examples.
• Level 2 (Moderate): Describe the Sandmeyer reaction and its significance.
• Level 3 (Challenging): Explain why nucleophilic substitution is difficult in haloarenes compared to haloalkanes.

Answer Key

• Level 1: Ortho: substituents on adjacent carbons (1,2-), meta: substituents separated by one carbon (1,3-), para: substituents opposite each other (1,4-).
• Level 2: Sandmeyer reaction replaces diazonium group with halogens using copper salts, useful for synthesizing haloarenes.
• Level 3: C–X bond in haloarenes has partial double bond character due to resonance, making nucleophilic attack difficult; also, phenyl cation intermediate is unstable.

Quick Reference Table

• Haloalkanes: Aliphatic hydrocarbons in which one or more hydrogen atoms are replaced by halogen atoms such as F, Cl, Br or I.
• Haloarenes: Aromatic hydrocarbons in which a hydrogen atom of the benzene ring is replaced by a halogen atom.
• Classification by halogen count: Monohaloalkanes contain one halogen atom, dihaloalkanes contain two, and polyhaloalkanes contain more than two.
• Geminal and vicinal dihaloalkanes: In geminal dihaloalkanes, both halogen atoms are on the same carbon; in vicinal dihaloalkanes, they are on adjacent carbons.
• Primary, secondary and tertiary haloalkanes: These are classified by the number of carbon atoms attached to the carbon bonded to halogen.
• Allylic halides: Halogen is attached to an allylic carbon, that is, a carbon adjacent to a carbon-carbon double bond.
• Benzylic halides: Halogen is attached to a carbon adjacent to a benzene ring.
• Boiling point trend: Boiling points generally increase with halogen size; RF < RCl < RBr < RI.
• Solubility: Haloalkanes are insoluble in water but soluble in organic solvents.
• Density: Many haloalkanes are denser than water.
• C–X bond strength: Bond dissociation energy decreases down the group; C–Cl > C–Br > C–I.
• SN1 mechanism: Unimolecular substitution with carbocation intermediate, favoured by tertiary haloalkanes and polar protic solvents.
• SN2 mechanism: Bimolecular substitution by backside attack, favoured by primary haloalkanes and polar aprotic solvents.
• Walden inversion: SN2 reactions cause inversion of configuration at the chiral carbon.
• SN1 stereochemistry: SN1 reactions may cause racemisation because the carbocation intermediate can be attacked from either side.
• Haloarenes and nucleophilic substitution: Haloarenes are less reactive than haloalkanes in nucleophilic substitution because the C–X bond has partial double bond character due to resonance.
• Electrophilic substitution in haloarenes: Substitution occurs mainly at ortho and para positions.
• Sandmeyer reaction: Diazonium group is replaced by halogen using copper salts.
• Finkelstein reaction: Halogen exchange reaction used in preparing haloalkanes.
• Wurtz reaction: Coupling of alkyl halides with sodium in dry ether to form higher alkanes.
• Hunsdiecker reaction: Decarboxylative halogenation used to form alkyl halides.
• DDT: A polyhalogen compound used as an insecticide, but it is environmentally hazardous.

Common Mistakes and Misconceptions

• Confusing haloalkanes with haloarenes: haloalkanes have halogen attached to an sp³ carbon, while haloarenes have halogen directly attached to the benzene ring.
• Assuming all dihaloalkanes are the same: geminal dihaloalkanes have both halogens on the same carbon, whereas vicinal dihaloalkanes have halogens on adjacent carbons.
• Thinking boiling point order follows halogen electronegativity; in haloalkanes it generally increases with halogen size: RF < RCl < RBr < RI.
• Mixing up SN1 and SN2: SN1 proceeds through a carbocation intermediate and is favored by tertiary haloalkanes, while SN2 is a one-step backside attack favored by primary haloalkanes.
• Forgetting that SN2 causes inversion of configuration (Walden inversion), not retention.
• Assuming SN1 always gives a single stereoisomer; it can lead to racemisation because the carbocation intermediate is planar.
• Confusing the products of KCN and AgCN with haloalkanes: KCN gives alkyl cyanides, while AgCN gives isocyanides.
• Expecting haloarenes to undergo nucleophilic substitution as easily as haloalkanes; resonance gives the C–X bond partial double bond character, making them less reactive.
• Thinking para isomers of haloarenes always have different boiling points; para isomers mainly show higher melting points due to better crystal packing.
• Believing carbon tetrachloride, chloroform, and DDT are only useful compounds and forgetting their environmental or health hazards.

Glossary

• Haloalkanes: Aliphatic hydrocarbons in which one or more hydrogen atoms are replaced by halogen atoms such as fluorine, chlorine, bromine or iodine.
• Haloarenes: Aromatic hydrocarbons in which a hydrogen atom of the benzene ring is replaced by a halogen atom.
• Monohaloalkanes: Haloalkanes containing one halogen atom.
• Dihaloalkanes: Haloalkanes containing two halogen atoms; they may be geminal or vicinal.
• Geminal dihalides: Dihaloalkanes in which both halogen atoms are attached to the same carbon atom.
• Vicinal dihalides: Dihaloalkanes in which the halogen atoms are attached to adjacent carbon atoms.
• Primary haloalkane: A haloalkane in which the carbon bonded to the halogen is attached to one other carbon atom.
• Secondary haloalkane: A haloalkane in which the carbon bonded to the halogen is attached to two other carbon atoms.
• Tertiary haloalkane: A haloalkane in which the carbon bonded to the halogen is attached to three other carbon atoms.
• Allylic halide: A halide in which the halogen is attached to an allylic carbon, that is, a carbon adjacent to a double bond.
• Benzylic halide: A halide in which the halogen is attached to a carbon adjacent to a benzene ring.
• Nucleophilic substitution: A reaction in which a nucleophile replaces the halogen atom in a haloalkane.
• S_N1 mechanism: A unimolecular nucleophilic substitution proceeding through a carbocation intermediate.
• S_N2 mechanism: A bimolecular nucleophilic substitution proceeding in one step by backside attack.
• Walden inversion: Inversion of configuration at a chiral carbon during an S_N2 reaction.
• Racemisation: Formation of a mixture of enantiomers, often associated with S_N1 reactions.
• Sandmeyer reaction: Replacement of the diazonium group in an aromatic diazonium salt by halogen using copper salts.
• Finkelstein reaction: Halogen exchange reaction used for the preparation of alkyl iodides.
• Wurtz reaction: Coupling of alkyl halides with sodium metal to form higher alkanes.
• Dow’s process: Preparation of phenol from chlorobenzene under high temperature and pressure.
• Hunsdiecker reaction: Decarboxylative halogenation of silver salts of carboxylic acids.
• Gattermann reaction: Preparation of aryl halides from diazonium salts using copper powder and hydrogen halides.
• Grignard reagent: An organomagnesium halide formed by the reaction of a haloalkane with magnesium in dry ether.