Haloalkanes and Haloarenes - Chapter 6

• Chapters 6 discusses Haloalkanes and Haloarenes

Key Concepts

• Replacement of hydrogen atoms in hydrocarbons by halogen atoms forms alkyl halides (haloalkanes) and aryl halides (haloarenes)

Haloalkanes

• Contain halogen atoms bonded to sp³ hybridized carbon atoms of an alkyl group

Haloarenes

• Contain halogen atoms bonded to sp² hybridized carbon atoms of an aryl group

Classification of Halogenated Hydrocarbons

Monohalides

• Have one halogen atom
• Classified based on the carbon-halogen bond's nature

Alkyl Halides

• Formula: R-X
• Carbon is sp³ hybridized

Allylic Halides

• Formula: R-CH=CH-CH₂X
• Carbon is sp³ hybridized

Benzylic Halides

• Formula: CH₂-X
• Carbon is sp³ hybridized

Aryl Halides

• Formula: -X
• Carbon is sp² hybridized

Vinylic Halides

• Formula: (R-C=C-X)
• Carbon is sp² hybridized

Alkynyl Halides

• Formula: -C≡C-X
• Carbon is sp hybridized

Dihalides

• Have two halogen atoms

Geminal Dihalides

• Two halogen atoms on the same carbon

Vicinal Dihalides

• Halogen atoms on adjacent carbons

Polyhalides

• Trihalides & Tetrahalides have three & four halogen atoms respectively

General Formula of Alkyl Halides

• R = alkyl group

Preparation Methods for Alkyl Halides

From Alcohols

• Reacting alcohols with halogen acids (HX)
• Reactivity of halogen acids: HI > HBr > HCl
• Reactivity of alcohols: tertiary > secondary > primary

Action of Phosphorus Halides

• Alcohols reacting with PCl₅, PCl₃, PBr₃, or PI₃ to yield alkyl halides

Darzen's Process

• Reaction of alcohol with thionyl chloride (SOCl₂) in pyridine yields alkyl chloride, sulfur dioxide and hydrogen chloride

Halogenation of Alkanes

• Alkanes react with halogens (X₂) under heat or light to give alkyl halides and hydrogen halides
• Reactivity order: allylic > tertiary > secondary > primary > CH₄
• Halogen reactivity order: F₂ > Cl₂ > Br₂ > I₂

Halide Exchange Method

Finkelstein Reaction

• Alkyl chlorides react with NaI in dry acetone to produce alkyl iodides and NaCl

Swarts Reaction

• Alkyl chlorides react with inorganic fluorides to yield fluoroalkanes

Reaction of Ethers w/ Hydrogen Halides

• Ethers react with hydrogen halides to produce alkyl halides and water

Addition of Hydrogen Halides to Alkenes

• Alkenes react with HX to give alkyl halides, following Markownikoff's rule

• Markownikoff's Rule dictates that, in addition to hydrogen halides to alkenes, the hydrogen atom attaches to the carbon with more hydrogen atoms, and the halide attaches to the carbon with fewer hydrogen atoms

Anti-Markownikoff’s Rule

• In the presence of peroxides, HBr adds to alkenes contrary to Markownikoff's rule
• Also known as Peroxide or Kharasch effect
• Only observed with HBr

Physical Properties of Alkyl Halides

• Alkyl halides are polar molecules insoluble in water due to their inability to break water's hydrogen bonds
• Higher melting and boiling points than comparable hydrocarbons

Boiling Point

• For the same alkyl group, Boiling point follows the order: R-I > R-Br > R-Cl > R-F
• Isomeric alkyl halides ranking: primary > secondary > tertiary

Density

• Alkyl halide density order: RI > RBr > RCI > RF
• Density increases with a rise in carbon number, halogen number, and halogen atomic mass

Bond Strength

• C-X order: CH₃-F > CH₃-Cl > CH₃-Br > CH₃-I i.e. bond strength decreases as halogen size increases

Stability

• Order for RX compounds: R-F > R-Cl > R-Br > R-I

Chemical Properties of Alkyl Halides

• Exhibit nucleophilic substitution reactions

Nucleophilic Substitution Reactions

• Stronger nucleophiles replace weaker ones Better leaving groups facilitate reactions

SN1 and SN2 Mechanisms

Unimolecular (SN1)

• 1st order reaction
• Usually in polar protic solvents (water, alcohols, & acetic acid)
• Two-step reaction involving a carbocation intermediate
• Reaction rate speed: 3° > 2° > 1° > CH₃

Bimolecular (SN2)

• 2nd order reaction
• Usually in polar aprotic solvents (acetone, DMSO, acetonitrile, or DMF)
• One-step process via a transition state
• Reaction rate speed: CH₃ > 1° > 2° > 3°
• Faster with less steric hindrance

Reactivity based on nucleophile strength

• SN1 favors weaker nucleophiles (e.g., CH₃OH, H₂O, CH₃CH₂OH)
• SN2 needs strong nucleophiles (e.g., CH₃O⁻, CN⁻, OH⁻)

Configuration Outcome

• SN1 incurs retention and inversion
• SN2 causes "Walden inversion"

Primary Allylic & Benxylic Halides

• Exhibit higher SN1 reactivity compared to others due to the greater stabilization of carbocation intermediates by resonance

Vinyl & Aryl Halides

• Unreactive in nucleophilic substitution due to the double bond character of the C-X arrangement and the lack of carbocation stability

Optical Isomerism/Enantiomerism

Dextrorotatory Isomer

• (d-form or +ve)
• Rotates polarised light to the clockwise

Laevorotatory Isomer

• (l-form or -ve)
• Rotates polarised light anti-clockwise

Racemic Mixture

• (dl form or (±)-mixture)
• Equimolar mix of d & l resulting in optical inactivity

Racemisation

• The process where enantiomers are converted into a racemic mixture

Chirality

• Molecule linked to 4 different groups as chiral centres/stereocentres

Achirality

• Molecule linked to at least two identical groups, as achiral/symmetric centres

Symmetry

• Plane of: Achiral (not molecules)
• Lack of a plane of: Chiral

Isomers

• Optical isomers, namely d- and l-isomers are mirror images that are non-superimposable or asymmetric

Diastereomers

• Optical isomers which aren't mirror images Physical attributes & levels of specific rotation diverge

Meso Compounds

• Have symmetrical plane and even chiral carbon numbers
• Internal offset gives these compounds their optical inactivity

Elimination Reactions

Dehydrohalogenation

• Alkyl halides lose a hydrogen halide to form an alkene in the presence of a base and heat

Reaction with Metals

• Alkyl halides react with metals such as Mg to form Grignard reagents

Wurtz Reaction

• Alkyl halides react with sodium in dry ether to form higher alkanes

Reduction of Alkyl Halides

• Alkyl halides can be reduced to alkanes using Zn/HCl

Haloarenes

• General formula: ArX (Ar = aryl group)

Preparation Methods for Haloarenes

From Benzenediazonium Salt

Through Sandmeyer, Gattermann, and Balz-Schiemann reactions

By Direct Halogenation of Benzene

• Requires a Lewis acid catalyst (FeCl₃, FeBr₃ etc.)

Physical Properties of Haloarenes

• Colorless, stable liquids with a pleasant odour
• Insoluble in water but miscible with organic solvents
• Higher boiling points than alkyl halides, increasing from fluoro to iodo compounds

Chemical Properties of Haloarenes

• Undergo nucleophilic substitution and electrophilic substitution reactions

Nucleophilic Substitution Reactions

• Enhanced by electron-withdrawing groups (like -NO₂) at ortho- and para- positions

Electrophilic Substitution Reactions

• Halogen atom is deactivating but ortho- and para-directing

Friedel-Crafts Alkylation

Aryl halides undergo this reaction in the presence of anhydrous aluminium chloride and an alkyl halide to yield alkylated arenes

Friedel-Crafts Acylation rx

• Aryl halides can undergo acylation on treatment with acyl halides, anhydrous aluminium chloride catalyst

Wurtz-Fittig Reaction

Combines an aryl halide and a secondary alkyl halide to form substituted aromatic compounds

Fittig Reaction

Heats aryl halides with sodium and dry ether to attach two aryl groups and create biaryl compounds

Uses of Important Compounds

Chloroform (CHCl₃)

• Used in freon refrigerant (R-22) creation
• Solvent for a variety of organic materials, & in anatomical prep it is used for
• Once used as an anaesthetic until liver damage emerged as a concern

Iodoform (CHI₃)

• Formerly used as a wound dressing antiseptic, later replaced, and acts as a methylating agent in organic chemistry

Carbon Tetrachloride (CCl₄)

• Serves as basic materials of aerosols & feed stock to CFC, and will sometimes results in cancer

Freons

• Disruption of Ozone layer & acts as greenhouse gasses

DDT

• An organic pollutant is greatly prone to bioaccumulation and absorbed into the soil