Chapter: Haloalkanes and Haloarenes – Detailed Notes for NEET/JEE Mains

1. Introduction and Classification

• Haloalkanes (Alkyl halides): Hydrocarbons in which one or more hydrogen atoms of an alkane are replaced by halogen atoms (F, Cl, Br, I).
  • General formula: R−X (where R is an alkyl group and X is a halogen).
• Haloarenes (Aryl halides): Hydrocarbons in which one or more hydrogen atoms of an aromatic ring are directly attached to halogen atoms.
  • General formula: Ar−X (where Ar is an aryl group and X is a halogen).

Classification based on the number of halogen atoms:

• Monohalo compounds: Contain one halogen atom (e.g., CH3​Cl, C6​H5​Br).
• Dihalo compounds: Contain two halogen atoms (e.g., CH2​Cl2​, C6​H4​Cl2​).
  • Geminal dihalides (gem-dihalides): Both halogen atoms are attached to the same carbon atom (e.g., 1,1-dichloroethane).
  • Vicinal dihalides (vic-dihalides): Halogen atoms are attached to adjacent carbon atoms (e.g., 1,2-dichloroethane).
• Tri-, Tetra-, Polyhalo compounds: Contain three, four, or more halogen atoms respectively.

Classification based on the hybridization of carbon atom to which halogen is attached:

1. Compounds containing sp3 C-X bond:
   • Alkyl halides (Haloalkanes): Halogen attached to an alkyl group.
     • Primary (1∘): Halogen attached to a primary carbon atom (e.g., CH3​CH2​Br).
     • Secondary (2∘): Halogen attached to a secondary carbon atom (e.g., CH3​CH(Cl)CH3​).
     • Tertiary (3∘): Halogen attached to a tertiary carbon atom (e.g., (CH3​)3​CBr).
   • Allylic halides: Halogen attached to an sp3 hybridized carbon atom next to a carbon-carbon double bond (e.g., CH2​=CH−CH2​Cl).
   • Benzylic halides: Halogen attached to an sp3 hybridized carbon atom next to an aromatic ring (e.g., C6​H5​CH2​Cl).
2. Compounds containing sp2 C-X bond:
   • Vinylic halides: Halogen attached directly to an sp2 hybridized carbon atom of a carbon-carbon double bond (e.g., CH2​=CH−Cl).
   • Aryl halides (Haloarenes): Halogen attached directly to an sp2 hybridized carbon atom of an aromatic ring (e.g., C6​H5​Cl).
3. Compounds containing sp C-X bond:
   • Halogen attached directly to an sp hybridized carbon atom of a carbon-carbon triple bond (e.g., CH≡C−Cl). (Less common)

2. Nomenclature

• Common names: Derived from the alkyl group followed by the halide name (e.g., CH3​Cl is methyl chloride).
• IUPAC names: Halogen is treated as a substituent, and the compound is named as a haloalkane (e.g., CH3​Cl is chloromethane, CH3​CH2​Br is bromoethane). For haloarenes, halogens are prefixes to the benzene ring. Positions are indicated by numbers (e.g., 1-chlorobenzene or chlorobenzene). For di- or polyhaloarenes, prefixes o-, m-, p- are used or numbers (e.g., 1,2-dichlorobenzene or o-dichlorobenzene).

3. Nature of C-X Bond

• The carbon-halogen bond is a polar covalent bond because halogens are more electronegative than carbon.
• The carbon atom carries a partial positive charge (δ+), and the halogen atom carries a partial negative charge (δ−).
• Bond Length: C-F < C-Cl < C-Br < C-I (increases down the group as halogen size increases).
• Bond Enthalpy: C-F > C-Cl > C-Br > C-I (decreases down the group as bond length increases).
• Dipole Moment: Generally, C-Cl > C-F > C-Br > C-I (due to a balance between electronegativity difference and bond length). Chloromethane has a higher dipole moment than fluoromethane.

4. Methods of Preparation of Haloalkanes

1. From Alcohols:
   • Reaction with Hydrogen halides (HX): R-OH+HX→R-X+H2​O
     • Reactivity of HX: HI > HBr > HCl.
     • Reactivity of alcohol: 3∘>2∘>1∘.
     • Lucas Reagent (Conc. HCl + Anhydrous ZnCl2​): Used for alcohols.
       • 3∘ alcohols react immediately (turbidity appears).
       • 2∘ alcohols react in 5-10 minutes.
       • 1∘ alcohols react only on heating.
   • Reaction with Phosphorus halides (PX3​, PX5​):
     • 3R-OH+PX3​→3R-X+H3​PO3​ (where X=Cl,Br,I)
     • R-OH+PCl5​→R-Cl+POCl3​+HCl
   • Reaction with Thionyl chloride (SOCl2​): (Darzen’s process – best method for chlorides)
     • R-OH+SOCl2​Pyridine​R-Cl+SO2​↑+HCl↑
     • Both byproducts (SO2​ and HCl) are gases and escape, leaving behind pure haloalkane.
2. From Hydrocarbons:
   • Free Radical Halogenation (for alkanes):
     • CH4​+Cl2​hv/heat​CH3​Cl+HCl (Mixture of products; not suitable for preparation)
     • Yields a mixture of mono-, di-, tri-halogenated products.
   • Electrophilic Addition (for alkenes/alkynes):
     • Addition of HX: Follows Markovnikov’s rule (H adds to the carbon with more hydrogens, X to the carbon with fewer hydrogens).
       • CH3​CH=CH2​+HBr→CH3​CH(Br)CH3​ (major product)
       • Anti-Markovnikov addition (Peroxide effect/Kharasch effect): Only with HBr in the presence of peroxides.
         • CH3​CH=CH2​+HBrPeroxide​CH3​CH2​CH2​Br
     • Addition of Halogens (X2​): Forms vicinal dihalides.
       • CH2​=CH2​+Br2​CCl4​​BrCH2​−CH2​Br (test for unsaturation)
3. Halogen Exchange Reactions:
   • Finkelstein Reaction: For preparing iodoalkanes.
     • R-Cl/R-Br+NaIAcetone, reflux​R-I+NaCl/NaBr
     • NaCl/NaBr are insoluble in acetone and precipitate, shifting the equilibrium to the right.
   • Swarts Reaction: For preparing fluoroalkanes.
     • R-Cl/R-Br+metallic fluorideheat​R-F+metal chloride/bromide
     • Common metallic fluorides: AgF, Hg2​F2​, CoF2​, SbF3​.
       • CH3​Br+AgF→CH3​F+AgBr

5. Methods of Preparation of Haloarenes

1. From Hydrocarbons (Electrophilic Substitution):
   • Direct halogenation of benzene in the presence of a Lewis acid catalyst (FeCl3​,FeBr3​,AlCl3​).
     • C6​H6​+Cl2​FeCl3​​C6​H5​Cl+HCl
     • Iodination is reversible and requires an oxidizing agent (HNO3​,HIO4​) to remove HI.
     • Fluorination is too vigorous to be controlled.
   • If an activating group (e.g., −CH3​) is present, ortho- and para-substituted products are formed.
     • Toluene Cl2​/FeCl3​​ o-chlorotoluene + p-chlorotoluene.
2. From Diazo Salts (Sandmeyer Reaction): (Best method for preparing chloro- and bromoarenes)
   • Aniline is converted to benzene diazonium chloride.
   • C6​H5​NH2​NaNO2​/HCl,0−5∘C​C6​H5​N2+​Cl− (Benzene diazonium chloride)
   • C6​H5​N2+​Cl−Cu2​Cl2​/HCl​C6​H5​Cl+N2​
   • C6​H5​N2+​Cl−Cu2​Br2​/HBr​C6​H5​Br+N2​
   • Gattermann Reaction: Similar to Sandmeyer, but uses Cu powder/HX. Lower yield than Sandmeyer.
     • C6​H5​N2+​Cl−Cu powder/HCl​C6​H5​Cl+N2​+CuCl
   • For iodobenzene:
     • C6​H5​N2+​Cl−+KI→C6​H5​I+N2​+KCl (No copper catalyst needed)
   • For fluorobenzene (Balz-Schiemann Reaction):
     • C6​H5​N2+​Cl−HBF4​​C6​H5​N2+​BF4−​heat​C6​H5​F+BF3​+N2​

6. Physical Properties

1. Boiling Points:
   • Haloalkanes: Increase with increasing molecular mass (R-I > R-Br > R-Cl > R-F for same alkyl group).
   • For isomeric haloalkanes, boiling point decreases with increasing branching (as branching leads to a more spherical shape, reducing surface area and weaker van der Waals forces).
   • Boiling points of haloalkanes are higher than corresponding alkanes due to greater polarity and higher molecular mass (stronger dipole-dipole interactions and van der Waals forces).
   • Haloarenes are generally higher boiling than haloalkanes of comparable carbon count due to their larger size and more effective packing. For dihalobenzenes, para-isomers have higher melting points than ortho– and meta-isomers due to symmetry, allowing better crystal packing.
2. Density:
   • Haloalkanes: Density increases with increasing molecular mass (R-I > R-Br > R-Cl > R-F).
   • Density increases with the number of halogen atoms.
   • Bromides and iodides are generally denser than water; chlorides and fluorides are often lighter.
   • Haloarenes are generally denser than haloalkanes and water.
3. Solubility:
   • Both haloalkanes and haloarenes are very slightly soluble in water. Although they are polar, they cannot form strong hydrogen bonds with water molecules (or cannot compensate for the H-bonds already broken in water).
   • They are readily soluble in organic solvents (e.g., ether, alcohol, benzene) due to similar intermolecular forces.

7. Chemical Reactions of Haloalkanes

Haloalkanes primarily undergo nucleophilic substitution and elimination reactions.

A. Nucleophilic Substitution Reactions (SN​ Reactions):

• Involves the replacement of the halogen atom (a good leaving group) by a stronger nucleophile.
• Nucleophile (Nu$^{-}$): Electron-rich species that attacks an electron-deficient center. Examples: OH−, CN−, RO−, NH3​, H2​O.
• Leaving Group (X$^{-}$): The halide ion (Cl−, Br−, I−). Good leaving groups are weak bases. I−>Br−>Cl−>F− (reactivity order of haloalkanes: R-I > R-Br > R-Cl > R-F).

Two main mechanisms:

1. SN​1 (Unimolecular Nucleophilic Substitution):
   • Two-step process:
     • Step 1 (Slow & Rate-determining): Formation of a carbocation (heterolytic cleavage of C-X bond).
       • R-XSlow​R+(carbocation)+X−
     • Step 2 (Fast): Nucleophile attacks the planar carbocation from either side.
       • R++Nu−Fast​R-Nu
   • Rate Law: Rate =k[R-X] (depends only on the concentration of haloalkane).
   • Reactivity Order: 3∘>2∘>1∘>CH3​X (due to stability of carbocation: 3∘>2∘>1∘). Allylic and benzylic halides also undergo SN​1 readily due to resonance stabilization of their carbocations.
   • Solvent: Favored by polar protic solvents (e.g., water, alcohol, acetic acid) as they stabilize the carbocation and the leaving group through solvation.
   • Stereochemistry: Leads to racemization if the reactant is chiral. The planar carbocation allows attack from both sides, yielding a nearly 50:50 mixture of enantiomers (some inversion also occurs due to ion pair formation).
2. SN​2 (Bimolecular Nucleophilic Substitution):
   • One-step (concerted) process: Nucleophile attacks the carbon atom from the back side (opposite to the leaving group) while the C-X bond is simultaneously breaking and the C-Nu bond is forming.
     • Transition State: A five-membered transition state where both the nucleophile and leaving group are partially bonded to the carbon atom.
   • Rate Law: Rate =k[R-X][Nu−] (depends on concentrations of both haloalkane and nucleophile).
   • Reactivity Order: CH3​X>1∘>2∘>3∘ (due to steric hindrance). Methyl halides have the least steric hindrance. Tertiary halides have too much steric hindrance for backside attack.
   • Solvent: Favored by polar aprotic solvents (e.g., acetone, DMF, DMSO, acetonitrile) as they solvate the cation part of the nucleophile but leave the nucleophile itself free and highly reactive.
   • Stereochemistry: Always leads to inversion of configuration (Walden inversion). The product has the opposite configuration to the reactant if the carbon is chiral.

Summary of Nucleophilic Substitution with common nucleophiles:

Nucleophile (Nu$^{-}$)	Product (R-Nu)	Example Reaction
OH− (aq. KOH/NaOH)	Alcohol (R-OH)	R-X+KOH(aq)→R-OH+KX
CN− (KCN/NaCN)	Alkyl Cyanide (R-CN)	R-X+KCN→R-CN+KX (C-N bond is more stable)
CN− (AgCN)	Alkyl Isocyanide (R-NC)	R-X+AgCN→R-NC+AgX (AgCN is covalent, N attacks)
NO2−​ (KNO$_2$)	Alkyl Nitrite (R-ONO)	R-X+KNO2​→R-ONO+KX (KNO$_2$ is ionic, O attacks)
NO2−​ (AgNO$_2$)	Nitroalkane (R-NO$_2$)	R-X+AgNO2​→R-NO2​+AgX (AgNO$_2$ is covalent, N attacks)
RO− (NaOR’)	Ether (R-OR’) (Williamson)	R-X+R’ONa→R-OR’+NaX
R′COO− (silver salt)	Ester (R-OOCR’)	R-X+R’COOAg→R-OOCR’+AgX
NH3​ (excess)	Amine (RNH2​)	R-X+NH3​→RNH2​+HX (can go to 2∘,3∘)

B. Elimination Reactions (β-Elimination / Dehydrohalogenation):

• Involves the removal of a hydrogen atom from a β-carbon (adjacent to the carbon bearing the halogen) and the halogen atom from the α-carbon, forming an alkene.
• Reagent: Alcoholic KOH (ethanol/KOH).
• Saytzeff’s (Zaitsev’s) Rule: In dehydrohalogenation reactions, the preferred product is the alkene that has the greater number of alkyl groups attached to the doubly bonded carbon atoms (more substituted alkene). This is usually the more stable alkene.
  • Example: CH3​CH2​CH(Br)CH3​alc. KOH​CH3​CH=CHCH3​ (But-2-ene, major)+CH3​CH2​CH=CH2​ (But-1-ene, minor)
• Competition between SN​ and E reactions:
  • Stronger and bulkier bases favor elimination.
  • Higher temperatures favor elimination.
  • 3∘ haloalkanes favor elimination with strong bases.
  • 1∘ haloalkanes favor SN​2 with strong nucleophiles, but elimination with very strong, bulky bases.

C. Reaction with Metals:

1. Wurtz Reaction:
   • Forms higher alkanes by coupling of two alkyl halides in the presence of sodium metal in dry ether.
   • 2R-X+2NaDry Ether​R-R+2NaX
   • Best for preparing symmetrical alkanes (e.g., butane from ethyl bromide). Not suitable for unsymmetrical alkanes due to a mixture of products.
2. Formation of Grignard Reagents:
   • Alkyl halides react with magnesium metal in dry ether to form alkylmagnesium halides (Grignard reagents).
   • R-X+MgDry Ether​RMgX
   • Grignard reagents are highly reactive and powerful nucleophiles. They are susceptible to moisture and should be prepared in anhydrous conditions.
   • Reaction with water: RMgX+H2​O→R-H+Mg(OH)X (used to convert alkyl halide to alkane).

8. Chemical Reactions of Haloarenes

Haloarenes are generally less reactive towards nucleophilic substitution reactions compared to haloalkanes due to:

1. Resonance effect: The C-X bond acquires partial double bond character due to resonance, making it stronger and shorter, and harder to break.
   • Canonical structures show the halogen’s lone pair delocalized into the ring, giving partial double bond character to C-X bond.
2. Difference in hybridization of carbon atom: The carbon atom attached to the halogen in haloarenes is sp2 hybridized, which is more electronegative than sp3 hybridized carbon in haloalkanes. This makes the C-X bond stronger and shorter. Also, sp2 carbon forms a more stable cation, which is less likely to form in a substitution reaction.
3. Instability of phenyl cation: If haloarene were to undergo SN​1 (like haloalkanes), it would form a highly unstable phenyl cation, which is not resonance stabilized.
4. Repulsion between nucleophile and electron-rich benzene ring: The benzene ring is electron-rich and would repel an incoming nucleophile.

A. Nucleophilic Aromatic Substitution:

• Occurs only under drastic conditions (high temperature and pressure) or if electron-withdrawing groups (EWG) are present at ortho– or para-positions to the halogen.
  • Example (Dow’s Process): Chlorobenzene NaOH, 623K, 300 atm​Sodium phenoxideH+​Phenol
  • Presence of electron-withdrawing groups (e.g., −NO2​) at ortho and para positions activates the ring towards nucleophilic substitution by stabilizing the intermediate carbanion formed during the reaction.
    • o- or p-nitrochlorobenzene reacts more easily than chlorobenzene.
    • 2,4,6-trinitrochlorobenzene reacts very readily with water on heating.

B. Electrophilic Substitution Reactions:

• Halogen atom is a deactivating group but ortho-para directing (due to resonance effects which increase electron density at ortho and para positions, despite overall deactivation).
• Halogenation:
  • C6​H5​Cl+Cl2​FeCl3​​1,4-dichlorobenzene (major)+1,2-dichlorobenzene (minor)
• Nitration:
  • C6​H5​Cl+conc. HNO3​conc. H2​SO4​​p-chloronitrobenzene (major)+o-chloronitrobenzene (minor)
• Sulphonation:
  • C6​H5​Cl+conc. H2​SO4​heat​p-chlorobenzenesulphonic acid (major)+o-chlorobenzenesulphonic acid (minor)
• Friedel-Crafts Alkylation/Acylation:
  • C6​H5​Cl+CH3​ClAnhy. AlCl3​​p-chlorotoluene (major)+o-chlorotoluene (minor)
  • Haloarenes are less reactive than benzene towards Friedel-Crafts due to deactivating nature of halogens.

C. Reaction with Metals:

1. Wurtz-Fittig Reaction:
   • Reaction between an alkyl halide, an aryl halide, and sodium metal in dry ether to form an alkylbenzene.
   • R-X+Ar-X+2NaDry Ether​R-Ar+2NaX
2. Fittig Reaction:
   • Coupling of two aryl halide molecules in the presence of sodium metal in dry ether to form biphenyl (diaryl).
   • 2Ar-X+2NaDry Ether​Ar-Ar+2NaX

9. Polyhalogen Compounds

Compounds containing more than one halogen atom.

1. Dichloromethane (CH2​Cl2​ / Methylene chloride):
   • Uses: Solvent for paint remover, propellent in aerosols, process solvent for drug manufacturing, metal cleaning agent.
   • Harmful effects: Mildly narcotic, causes dizziness, nausea, damage to CNS.
2. Trichloromethane (CHCl3​ / Chloroform):
   • Uses: Solvent for fats, alkaloids, iodine, etc. Production of Freon refrigerant R-22.
   • Harmful effects: Depresses CNS, causes dizziness, fatigue, headache. Prolonged exposure damages liver and kidneys. Oxidizes to highly poisonous phosgene gas in presence of light and air (COCl2​). Stored in dark brown bottles, filled completely, with 1% ethanol (to convert phosgene to harmless diethyl carbonate).
3. Triiodomethane (CHI3​ / Iodoform):
   • Uses: Antiseptic (due to iodine released, not due to CHI3​ itself). Its objectionable smell has led to its replacement by other iodine-containing antiseptics.
   • Preparation: Haloform reaction (e.g., ethanol or acetone with I2​/NaOH).
4. Tetrachloromethane (CCl4​ / Carbon Tetrachloride):
   • Uses: Solvent in synthesis, dry cleaning, fire extinguishers (Pyrene), refrigerant production.
   • Harmful effects: Liver cancer (carcinogenic), damages nerve cells, kidney and liver damage, dizziness, coma. Depletes ozone layer.
5. Freons (Chlorofluorocarbons, CFCs):
   • Methane and ethane derivatives containing both chlorine and fluorine atoms. Extremely stable, non-reactive, non-toxic, non-corrosive.
   • Uses: Refrigerants, aerosol propellants, air conditioning systems, blowing agents for foams.
   • Environmental impact: Serious damage to stratospheric ozone layer. (Example: Freon-12, CCl2​F2​).
6. DDT (Dichlorodiphenyltrichloroethane):
   • Structure: (ClC6​H4​)2​CHCCl3​
   • Uses: Powerful insecticide. First prepared in 1874, discovered as an insecticide by Paul Muller (1939).
   • Environmental impact: Highly persistent in the environment. Not biodegradable. Accumulates in food chains (biomagnification). Causes toxicity to fish, birds, and other animals. Banned in many countries.