Quick Revision Notes & Formula Sheet

Class 12 Chemistry: Haloalkanes and Haloarenes (MP Board)

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1. Introduction & Classification

Haloalkanes (Alkyl halides) are halogen derivatives of alkanes, and Haloarenes (Aryl halides) are halogen derivatives of arenes (aromatic hydrocarbons).

• Classification based on Halogen Atoms:
  • Mono-, Di-, Polyhalogen compounds: Depending on whether they contain one, two, or more halogen atoms.
• Classification based on sp^3 C-X bond (Alkyl halides):
  • Primary ($1^\circ$): Halogen is bonded to a $1^\circ$ carbon atom (e.g., $CH_3CH_2Cl$).
  • Secondary ($2^\circ$): Halogen is bonded to a $2^\circ$ carbon atom (e.g., $(CH_3)_2CHCl$).
  • Tertiary ($3^\circ$): Halogen is bonded to a $3^\circ$ carbon atom (e.g., $(CH_3)_3CCl$).
• Allylic Halides: $X$ is bonded to an $sp^3$ carbon next to a carbon-carbon double bond ($C=C$).
• Benzylic Halides: $X$ is bonded to an $sp^3$ carbon attached to an aromatic ring.
• Vinylic Halides: $X$ is bonded to an $sp^2$ carbon of a $C=C$ bond.
• Aryl Halides: $X$ is bonded directly to an $sp^2$ carbon of an aromatic ring.

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2. Nomenclature (IUPAC Rules)

• Alkyl halides are named as haloalkanes (Prefix 'halo' + Root word + Suffix 'ane').
• Example: $CH_3-CH(Cl)-CH_3 \rightarrow$ 2-Chloropropane.
• Aryl halides are named as haloarenes (e.g., Chlorobenzene).

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3. Methods of Preparation

A. From Alcohols (Best methods)

• Using Lucas Reagent ($ZnCl_2 + conc. HCl$): R-OH + HCl --(ZnCl2)--> R-Cl + H_2O
• Using Phosphorus Halides: 3 R-OH + PX_3 -> 3 R-X + H_3PO_3 (where $X = Cl, Br$) R-OH + PCl_5 -> R-Cl + POCl_3 + HCl
• Using Thionyl Chloride (Preferred as byproducts are gases): R-OH + SOCl_2 -> R-Cl + SO_2 (g) + HCl (g)

B. From Hydrocarbons (Alkanes & Alkenes)

• Free Radical Halogenation (Alkanes): R-H + Cl_2 --(UV Light/Heat)--> R-Cl + HCl
• Electrophilic Addition (Alkenes):
  • Addition of Hydrogen halides ($HX$): Follows Markovnikov's Rule (Negative part goes to carbon with fewer hydrogens).
  • Peroxide Effect (Anti-Markovnikov addition): Only applicable for HBr in the presence of organic peroxide.

C. Halogen Exchange Reactions (Name Reactions)

• Finkelstein Reaction (Preparation of Alkyl Iodides): R-Cl + NaI --(Dry Acetone)--> R-I + NaCl
• Swarts Reaction (Preparation of Alkyl Fluorides): R-Br + AgF -> R-F + AgBr (Other metallic fluorides used: $Hg_2F_2$, $CoF_2$, $SbF_3$).

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4. Physical Properties

• Melting & Boiling Points:
  • Boiling points increase with an increase in molecular mass (order: $R-I > R-Br > R-Cl > R-F$).
  • For isomeric haloalkanes, boiling point decreases with branching due to decrease in surface area ($Straight\ chain > Branched$).
• Density: Polyhalogen derivatives and bromo/iodo derivatives are heavier than water.
• Solubility: Though polar, haloalkanes are only slightly soluble in water because they cannot form H-bonds with water molecules.

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5. Chemical Reactions of Haloalkanes

A. Nucleophilic Substitution Reactions ($S_N1$ vs $S_N2$)

Feature	$S_N2$ (Substitution Nucleophilic Bimolecular)	$S_N1$ (Substitution Nucleophilic Unimolecular)
Order of Kinetics	Bimolecular (Rate = $k[RX][Nu^-]$)	Unimolecular (Rate = $k[RX]$)
Steps	Single-step (concerted mechanism)	Two-step (via Carbocation intermediate)
Stereochemistry	Inversion of configuration (Walden inversion)	Racemization (50% retention, 50% inversion)
Reactivity Order	$1^\circ > 2^\circ > 3^\circ$ (Steric hindrance)	$3^\circ > 2^\circ > 1^\circ$ (Carbocation stability)
Solvent	Aprotic polar solvent (e.g., Acetone)	Polar protic solvent (e.g., $H_2O, ROH$)

B. Elimination Reactions ($\beta$-Elimination / Dehydrohalogenation)

• When haloalkane is heated with alcoholic $KOH$, it undergoes elimination of $HX$ to form alkenes.
• Saytsev's Rule: In elimination reactions, the preferred alkene is the one that is most alkylated (contains more alkyl groups attached to the doubly bonded carbons). CH_3-CH_2-CH(Br)-CH_3 + alc. KOH -> CH_3-CH=CH-CH_3 (Major) + CH_3-CH_2-CH=CH_2 (Minor)

C. Reaction with Metals

• Wurtz Reaction: Alkyl halides react with sodium in dry ether to give higher alkanes. 2 R-X + 2 Na --(Dry Ether)--> R-R + 2 NaX
• Grignard Reagent Formation: R-X + Mg --(Dry Ether)--> R-MgX (Organometallic compound)

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6. Chemical Reactions of Haloarenes (Aryl Halides)

Haloarenes are less reactive towards nucleophilic substitution due to resonance stabilization, $sp^2$ hybridized C-X bond, and repulsion of electron-rich rings.

A. Nucleophilic Substitution (under drastic conditions)

• Chlorobenzene + NaOH (623K, 300 atm) -> Phenol

B. Electrophilic Aromatic Substitution (o- and p-directing)

• Halogenation: $C_6H_5Cl + Cl_2 --(FeCl_3)--> o-Dichlorobenzene + p-Dichlorobenzene$
• Nitration: $C_6H_5Cl + HNO_3 --(conc. H_2SO_4)--> o- & p-Chloronitrobenzene$
• Sulphonation: $C_6H_5Cl + conc. H_2SO_4 \rightarrow 2-$ and $4-$Chlorobenzenesulfonic acid
• Friedel-Crafts Alkylation: $C_6H_5Cl + CH_3Cl --(anhyd. AlCl_3)--> 1-$ and $4-$Chlorotoluene

C. Reaction with Metals

• Wurtz-Fittig Reaction: Aryl halide + Alkyl halide + $Na$ in dry ether $\rightarrow$ Alkylbenzene.
• Fittig Reaction: Two moles of Aryl halide + $Na$ in dry ether $\rightarrow$ Diphenyl.

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7. Important Polyhalogen Compounds (Brief)

• Dichloromethane (Methylene chloride, $CH_2Cl_2$): Used as a solvent, paint remover.
• Trichloromethane (Chloroform, $CHCl_3$): Used in production of freon refrigerant R-22. (Stored in dark colored bottles filled to the rim to prevent oxidation to poisonous phosgene gas, $COCl_2$).
• Triiodomethane (Iodoform, $CHI_3$): Used as an antiseptic due to liberation of free iodine.
• Tetrachloromethane (Carbon tetrachloride, $CCl_4$): Used as fire extinguisher (Pyrene).
• Freons: Chlorofluorocarbons of methane and ethane (e.g., $CF_2Cl_2$). Responsible for ozone depletion.
• DDT (p,p'-Dichlorodiphenyltrichloroethane): Powerful insecticide.