Chemistry Chapter 6 Notes of Haloalkanes & Haloarenes CBSE Class 12

Haloalkanes and haloarenes are organic compounds formed by replacing hydrogen atoms in hydrocarbons with halogen atoms. Haloalkanes have halogen attached to an sp³ carbon, while haloarenes have halogen attached to an sp² carbon of an aromatic ring. These compounds are used widely as solvents and starting materials for synthesis. Some occur naturally, like chloramphenicol and thyroxine, while others are synthetic and clinically useful, such as chloroquine and halothane. However, halogenated compounds can persist in the environment and some, like DDT and Freons, have environmental hazards. This note covers their classification, preparation methods, physical and chemical properties, including key reaction mechanisms like SN1 and SN2 substitutions and elimination reactions, and discusses some important polyhalogen compounds.

Classification

Organohalogen compounds can be classified based on the number of halogen atoms or the hybridisation of the carbon atom bonded to the halogen.

• On the Basis of Number of Halogen Atoms :
• Mono, di, or polyhalogen compounds : Contain one, two, or more halogen atoms, respectively.
• Compounds Containing sp³ C—X Bond (X= F, Cl, Br, I) :
• Halogen atom bonded to an sp³-hybridised carbon atom.
• Alkyl halides or haloalkanes (R—X) : Halogen bonded to an alkyl group (R), CnH2n+1X.
• Classified as primary (1°) , secondary (2°) , or tertiary (3°) based on whether the halogen is attached to a primary, secondary, or tertiary carbon atom.
• Allylic halides : Halogen bonded to an sp³-hybridised carbon atom adjacent to a carbon-carbon double bond (C=C). This carbon is an allylic carbon.
• Benzylic halides : Halogen bonded to an sp³-hybridised carbon atom attached to an aromatic ring.
• Compounds Containing sp² C—X Bond :
• Halogen atom bonded to an sp²-hybridised carbon atom.
• Vinylic halides : Halogen bonded to an sp²-hybridised carbon atom of a carbon-carbon double bond (C=C).
• Aryl halides : Halogen directly bonded to the sp²-hybridised carbon atom of an aromatic ring.
• Dihalogen compounds : Can be classified based on the relative position of the two halogen atoms.
• Geminal halides or gem-dihalides : Both halogen atoms are on the same carbon atom. Common name: alkylidene halides. IUPAC name: dihaloalkanes.
• Vicinal halides or vic-dihalides : Halogen atoms are on adjacent carbon atoms. Common name: alkylene dihalides. IUPAC name: dihaloalkanes.

Nomenclature

• Alkyl halides :
• Common names : Derived by naming the alkyl group followed by the name of the halide.
• IUPAC system : Named as halosubstituted hydrocarbons.
• Mono halogen substituted derivatives of benzene : Common and IUPAC names are the same.
• Dihalogen derivatives of benzene :
• Common system : Prefixes o-, m-, p- are used.
• IUPAC system : Numerals 1,2; 1,3 and 1,4 are used.
• Examples of common and IUPAC names are provided in Table 6.1.

Nature of C-X Bond

• Halogen atoms are more electronegative than carbon, causing the carbon-halogen bond (C—X) in alkyl halides to be polarised .
• The carbon atom carries a partial positive charge , and the halogen atom carries a partial negative charge .
• Bond length increases from C—F to C—I as the size of the halogen atom increases down the group.
• Bond enthalpy decreases from C—F to C—I, indicating the bond strength decreases.
• Dipole moments are also listed, showing polarity.

Methods of Preparation of Haloalkanes

From Alcohols:

• Replacement of the hydroxyl group (–OH) by halogen using concentrated halogen acids, phosphorus halides, or thionyl chloride.
• Thionyl chloride (SOCl₂) is preferred as SO₂ and HCl gases escape, yielding pure alkyl halides.
• Primary and secondary alcohols with HCl require a catalyst (ZnCl₂).
• Tertiary alcohols react simply by shaking with concentrated HCl at room temperature.
• Alkyl bromide (R—Br) is prepared using constant boiling HBr (48%).
• Alkyl iodide (R—I) is obtained by heating with NaI or KI in 95% orthophosphoric acid.
• Reactivity order of alcohols: 3° > 2° > 1°.
• Phosphorus tribromide (PBr₃) and triiodide (PI₃) are usually generated in situ .
• Aryl halides cannot be prepared from phenols by these methods due to the partial double bond character of the C-O bond in phenols.

From Hydrocarbons:

• (I) From alkanes by free radical halogenation :
• Chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, difficult to separate. Yield of any single compound is low.
• (II) From alkenes :
• (i) Addition of hydrogen halides : Alkene + HX → Alkyl halide. Markovnikov’s rule applies for unsymmetrical alkenes.
• (ii) Addition of halogens : Alkene + Halogen (e.g., Br₂ in CCl₄) → Vic-dihalide. Used for detecting double bonds (decolorisation of reddish brown bromine). Vic-dibromides are colourless.

Halogen Exchange:

• Finkelstein reaction : Alkyl chloride/bromide + NaI in dry acetone → Alkyl iodide (R—I). NaCl or NaBr precipitates, driving the reaction forward.
• Swarts reaction : Alkyl chloride/bromide + Metallic fluoride (AgF, Hg₂F₂, CoF₂, SbF₃) → Alkyl fluoride (R—F). Best accomplished by heating. Also used to manufacture Freon 12 from tetrachloromethane.

Preparation of Haloarenes

From hydrocarbons by electrophilic substitution:

• Arenes + Halogen (Cl₂ or Br₂) in presence of Lewis acid catalyst (iron or FeX₃) → Aryl halide.
• Ortho and para isomers are easily separated due to melting point difference.
• Iodination is reversible, requires an oxidising agent (HNO₃, HIO₄) to oxidise HI formed.
• Fluoro compounds not prepared by this method due to high reactivity of fluorine.

From amines by Sandmeyer’s reaction:

• Primary aromatic amine + Sodium nitrite + Cold aqueous mineral acid → Diazonium salt.
• Diazonium salt solution + Cuprous chloride (CuCl) or Cuprous bromide (CuBr) → Replacement of diazonium group by –Cl or –Br.
• Replacement by iodine is done by shaking the diazonium salt with potassium iodide (KI), no cuprous halide needed.

Physical Properties

• Colour and Smell : Alkyl halides are colourless when pure. Bromides and iodides develop colour when exposed to light. Many volatile halogen compounds have a sweet smell.
• State : Methyl chloride, methyl bromide, ethyl chloride, and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids.

Melting and Boiling points:

• Generally higher than parent hydrocarbons due to greater polarity (dipole-dipole) and higher molecular mass (van der Waals forces).
• Forces of attraction are stronger with bigger molecules and more electrons.
• For the same alkyl group, boiling points decrease in order: RI > RBr > RCl > RF. Magnitude of van der Waal forces increases with size/mass of halogen.
• Boiling points of isomeric haloalkanes decrease with increasing branching.
• Isomeric dihalobenzenes have very similar boiling points.
• Para-isomers of dihalobenzenes have higher melting points than ortho- and meta-isomers due to better fit in crystal lattice (symmetry).

Solubility:

• Very slightly soluble in water. Energy needed to break water's hydrogen bonds and overcome haloalkane attractions is not compensated by weaker attractions between haloalkanes and water.
• Tend to dissolve in organic solvents because intermolecular attractions are comparable.

Density:

• Bromo, iodo, and polychloro derivatives are heavier than water.
• Density increases with increased number of carbon atoms, halogen atoms, and atomic mass of halogen.

Chemical Reactions of Haloalkanes

Nucleophilic substitution reactions:

• Reaction where a nucleophile (electron-rich species) replaces a leaving group (halogen atom departs as halide ion) on the electron-deficient carbon bonded to halogen.
• Haloalkanes are the substrate. Halogen is bonded to sp³ hybridised carbon.
• One of the most useful classes of organic reactions for alkyl halides.
• Products formed with common nucleophiles are listed in Table 6.4.
• Ambident nucleophiles : Possess two nucleophilic centres. Examples:
• Cyanide group (CN⁻) [C≡N⁻ ↔ :C=N⁻] can link through carbon (alkyl cyanides RCN) or nitrogen (isocyanides RNC). KCN is ionic, gives CN⁻, attack is mainly through C (C—C bond more stable than C—N) → alkyl cyanides. AgCN is covalent, N is free to donate pair → isocyanides.
• Nitrite ion (NO₂⁻) [O⁻—N=O ↔ O=N—O⁻] can link through oxygen (alkyl nitrites R—O—N=O) or nitrogen (nitroalkanes R—NO₂). KNO₂ gives R—O—N=O; AgNO₂ gives R—NO₂.

Mechanism: SN1 and SN2:

• Nucleophilic substitution can proceed via two mechanisms.

Stereochemical aspects:

• Optical activity : Compounds rotating plane-polarised light are optically active.
• Dextrorotatory (+ or d): Rotates light clockwise.
• Laevorotatory (– or l): Rotates light anticlockwise.
• Optical isomers: (+) and (–) isomers. Phenomenon: optical isomerism.
• Chirality : Object is chiral if it is non-superimposable on its mirror image (like hands). Chiral molecules are optically active. Asymmetry of the molecule and non-superimposability of mirror images cause optical activity.
• Asymmetric carbon/Stereocentre : Carbon atom bonded to four different groups. A molecule with a single asymmetric carbon is typically chiral.
• Enantiomers : Stereoisomers related as non-superimposable mirror images. Possess identical physical properties (mp, bp, etc.) except for rotation of plane-polarised light (equal magnitude, opposite direction).
• Racemic mixture or racemic modification : Mixture containing equal proportions of two enantiomers. Zero optical rotation (optically inactive). Represented by dl or (±).
• Racemisation : Process of converting an enantiomer into a racemic mixture. SN1 reactions of optically active alkyl halides result in racemisation.
• Retention of configuration : Preservation of the spatial arrangement of bonds at an asymmetric centre during a reaction. Occurs when no bond to the stereocentre is broken.
• Inversion of configuration : Reversal of the spatial arrangement at an asymmetric centre. Occurs in SN2 reactions at an asymmetric carbon.

Elimination reactions (β-elimination):

• Haloalkane with β-hydrogen heated with alcoholic solution of potassium hydroxide (a base).
• Elimination of hydrogen from β-carbon and halogen from α-carbon. (α-carbon bonded to halogen, β-carbon adjacent to α-carbon).
• Forms an alkene as the product.
• Zaitsev’s rule : When multiple alkenes can form, the preferred product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbons.

Elimination vs Substitution:

• Competing reactions for alkyl halides with β-hydrogens when reacted with a base/nucleophile.
• The favoured route depends on:
• Nature of alkyl halide (1°, 2°, 3°).
• Strength and size of base/nucleophile. Bulkier nucleophile prefers to act as a base (elimination).
• Reaction conditions (e.g., solvent, temperature).
• Primary halides prefer SN2. Secondary halides can undergo SN2 or elimination depending on base/nucleophile strength. Tertiary halides prefer SN1 or elimination depending on carbocation stability or more substituted alkene formation.

Reaction with metals:

• Organic halides react with certain metals to form organo-metallic compounds (containing carbon-metal bonds).
• Grignard Reagents (RMgX) : Alkyl magnesium halides. Discovered by Victor Grignard (Nobel Prize 1912).
• Preparation: Haloalkanes + Magnesium metal in dry ether.
• Nature of bond: Covalent but highly polar C—Mg bond (carbon partially negative), essentially ionic Mg—X bond.
• Reactivity: Highly reactive. React with any source of proton (water, alcohols, amines, terminal alkynes) to give hydrocarbons. Preparation requires anhydrous conditions to prevent reaction with moisture. Can be used to convert halides to hydrocarbons.
• Wurtz reaction : Alkyl halides + Sodium metal in dry ether → Hydrocarbon with double the number of carbon atoms.

Chemical Reactions of Haloarenes

Reactivity towards Nucleophilic Substitution:

• Extremely less reactive than haloalkanes towards nucleophilic substitution reactions. Reasons:
• (i) Resonance effect : Electron pairs on halogen are in conjugation with the ring's π-electrons. C—Cl bond acquires partial double bond character due to resonance. Difficult to break compared to single bond in haloalkanes.
• (ii) Difference in hybridisation : Carbon attached to halogen is sp² hybridised in haloarenes vs sp³ in haloalkanes. sp² carbon is more electronegative (greater s-character), holds C—X electron pair more tightly. C—Cl bond shorter (169 pm) in haloarenes than in haloalkanes (177 pm). Shorter bond is stronger and harder to break.
• (iii) Instability of phenyl cation : Phenyl cation formed by self-ionisation is not stabilised by resonance. SN1 mechanism is ruled out.
• (iv) Repulsion : Electron-rich nucleophile is less likely to approach electron-rich arenes.
• Exception (Replacement by hydroxyl group) :
• Chlorobenzene can react with aqueous NaOH at high temperature (623K) and pressure (300 atm) to form phenol.
• Presence of electron-withdrawing groups (like –NO₂) at ortho- and para-positions increases reactivity towards nucleophilic substitution.
• –NO₂ effect : Electron-withdrawing –NO₂ group stabilises the carbanion intermediate formed during nucleophilic attack through resonance. The negative charge is delocalised onto the –NO₂ group when at ortho or para positions. No effect observed with –NO₂ at meta-position because the negative charge does not appear on the carbon bearing the –NO₂ group in resonating structures.

Electrophilic substitution reactions:

• Haloarenes undergo usual electrophilic reactions of the benzene ring (halogenation, nitration, sulphonation, Friedel-Crafts).
• Halogen is slightly deactivating but ortho-, para-directing .
• Reason for directing effect : Resonance increases electron density at ortho- and para-positions.
• Reason for deactivation : Halogen's –I effect withdraws electrons from the ring, making it less reactive than benzene. The inductive effect is stronger than the resonance effect for deactivation. Resonance effect opposes the inductive effect at ortho/para positions, making deactivation less pronounced there. Reactivity (rate) is controlled by the stronger inductive effect (deactivation), but orientation is controlled by the resonance effect (directing to positions with higher electron density).

Reaction with metals:

• Wurtz-Fittig reaction : Mixture of alkyl halide and aryl halide + Sodium in dry ether → Alkylarene.
• Fittig reaction : Aryl halides + Sodium in dry ether → Two aryl groups joined together (e.g., biphenyl from bromobenzene).

Polyhalogen Compounds

Carbon compounds containing more than one halogen atom. Many are useful, but some pose environmental/health risks.

Dichloromethane (Methylene chloride, CH₂Cl₂):

• Uses: Solvent (paint remover, aerosols propellant, process solvent for drugs), metal cleaning.
• Health effects: Harms central nervous system. Lower levels cause impaired hearing/vision. Higher levels cause dizziness, nausea, tingling/numbness. Skin contact causes burning/redness. Eye contact can burn cornea.
• Density: 1.336 g/mL.

Trichloromethane (Chloroform, CHCl₃):

• Uses: Solvent for fats, alkaloids, iodine. Major use: Production of refrigerant R-22 (Freon 22).
• Historical use: General anaesthetic (now replaced).
• Health effects: Depresses central nervous system. Breathing high levels causes dizziness, fatigue, headache. Chronic exposure can damage liver (metabolised to phosgene) and kidneys. Skin contact can cause sores.
• Hazard: Slowly oxidised by air/light to poisonous phosgene (carbonyl chloride). Stored in closed dark bottles completely filled.
• Density: 1.489 g/mL.

Triiodomethane (Iodoform, CHI₃):

• Historical use: Antiseptic, properties due to liberation of free iodine.
• Current status: Replaced by other iodine formulations due to objectionable smell.

Tetrachloromethane (Carbon tetrachloride, CCl₄):

• Uses: Manufacture of refrigerants and propellants, feedstock for chlorofluorocarbons, pharmaceutical manufacturing, general solvent.
• Historical use (until mid-1960s): Cleaning fluid (degreasing, spot remover), fire extinguisher.
• Health effects: Evidence of liver cancer risk. Common effects: dizziness, light headedness, nausea, vomiting (can cause permanent nerve damage). Severe cases can lead to stupor, coma, unconsciousness, death. Can cause irregular heart beat or stop heart. Eye irritant.
• Environmental effect: Rises to atmosphere and depletes the ozone layer . Ozone depletion increases UV exposure, leading to skin cancer, eye diseases, immune system disruption.
• Density: 1.595 g/mL.

Freons (Chlorofluorocarbons of methane and ethane):

• Properties: Extremely stable, unreactive, non-toxic, non-corrosive, easily liquefiable gases.
• Freon 12 (CCl₂F₂): Common industrial freon, manufactured from CCl₄ by Swarts reaction.
• Uses: Aerosol propellants, refrigeration, air conditioning.
• Environmental effect: Diffuse into stratosphere unchanged. Initiate radical chain reactions that upset natural ozone balance.

p,p’-Dichlorodiphenyltrichloroethane (DDT):

• First chlorinated organic insecticide. Effectiveness discovered by Paul Muller (Nobel Prize 1948).
• Uses (historically): Effective against malaria-spreading mosquitoes and typhus-carrying lice.
• Problems with extensive use: Insect resistance developed, high toxicity towards fish.
• Bioaccumulation: Chemical stability and fat solubility lead to deposition and storage in fatty tissues, building up over time with steady ingestion.
• Current status: Banned in United States in 1973, still used in some parts of the world. Causes environmental hazards.

Summary & Key Points

• Haloalkanes and haloarenes are classified by the number of halogen atoms and the hybridisation of the attached carbon (sp³ vs sp²).
• The C—X bond is polar.
• Preparation methods differ for haloalkanes (alcohols, alkanes, alkenes, halogen exchange) and haloarenes (electrophilic substitution, Sandmeyer's).
• Organohalogen compounds have higher boiling points than hydrocarbons due to stronger intermolecular forces. They are slightly soluble in water.
• Reactions of haloalkanes include nucleophilic substitution (SN1/SN2), elimination (β-elimination, Zaitsev's rule), and reaction with metals (Grignard reagents, Wurtz reaction).
• Stereochemistry is crucial for understanding SN1/SN2: SN2 involves inversion of configuration, while SN1 involves racemisation . Chirality, enantiomers, and racemic mixtures are key concepts.
• Haloarenes are less reactive in nucleophilic substitution due to resonance and hybridisation effects, but reactivity increases with electron-withdrawing groups at ortho/para positions.
• Haloarenes undergo electrophilic substitution, with halogens being deactivating but ortho-, para-directing.
• Polyhalogen compounds like dichloromethane, chloroform, carbon tetrachloride, freons, and DDT have industrial uses but some are persistent and environmentally harmful (e.g., ozone depletion by CCl₄, freons; bioaccumulation by DDT).

Frequently Asked Questions

1. Why are alkyl halides, though polar, only slightly soluble in water?
• Energy is required to break existing attractions: between haloalkane molecules and hydrogen bonds between water molecules.
• New attractions formed between haloalkane and water molecules are weaker than the original hydrogen bonds in water.
• As a result, the energy released is insufficient to overcome the energy required for dissolution, leading to low solubility.
2. Why must Grignard reagents be prepared under anhydrous conditions?
• Grignard reagents (RMgX) are highly reactive.
• They react readily with any source of proton, including water.
• This reaction with protons converts the Grignard reagent back into a hydrocarbon (RH), destroying the desired reagent.
3. Why are aryl halides extremely less reactive towards nucleophilic substitution reactions compared to alkyl halides?
• Due to resonance, the C—X bond in haloarenes acquires partial double bond character, making it stronger and harder to break than the single C—X bond in haloalkanes.
• The carbon bonded to halogen in haloarenes is sp² hybridised, which is more electronegative and holds the electron pair more tightly, resulting in a shorter, stronger bond compared to the sp³ carbon in haloalkanes.
• The phenyl cation that would be formed in an SN1-like mechanism is unstable and not stabilised by resonance.