NCERT Class XII Chemistry Unit 10: Haloalkanes and Haloarenes Study Guide

Introduction to Haloalkanes and Haloarenes

• The replacement of hydrogen atom(s) in a hydrocarbon, whether aliphatic or aromatic, by halogen atom(s) results in the formation of alkyl halides ($haloalkanes$) and aryl halides ($haloarenes$).

• Haloalkanes: Feature halogen atom(s) attached to the $sp^3$ hybridised carbon atom of an alkyl group.

• Haloarenes: Feature halogen atom(s) attached to $sp^2$ hybridised carbon atom(s) of an aryl group.

• Clinical and Industrial Utility:

  • Chloramphenicol: A chlorine-containing antibiotic produced by soil microorganisms, used for typhoid fever.

  • Thyroxine: An iodine-containing hormone; deficiency causes goiter.

  • Chloroquine: A synthetic halogen compound used to treat malaria.

  • Halothane: Used as an anaesthetic during surgery.

  • Fluorinated compounds: Potential blood substitutes in surgery.

Classification of Organohalogen Compounds

• Based on Number of Halogen Atoms: Classified as mono, di, or polyhalogen (tri-, tetra-, etc.) compounds.

• Compounds Containing $sp^3$ C-X Bond:

  • Alkyl halides (R-X): Represented by the homologous series $C_nH_{2n+1}X$. Sub-classified into primary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$).

  • Allylic halides: The halogen is bonded to an $sp^3$ carbon adjacent to a carbon-carbon double bond ($C=C$).

  • Benzylic halides: The halogen is bonded to an $sp^3$ carbon adjacent to an aromatic ring.

• Compounds Containing $sp^2$ C-X Bond:

  • Vinylic halides: Halogen is bonded to an $sp^2$ carbon of a $C=C$ bond.

  • Aryl halides: Halogen is bonded directly to the $sp^2$ carbon of an aromatic ring.

Nomenclature and Structural Isomerism

• Common Names: Derived by naming the alkyl group followed by the halide.

• IUPAC Names: Named as halosubstituted hydrocarbons.

• Dihalogen Derivatives: Common prefixes are $o-$, $m-$, and $p-$; IUPAC uses numerals $1,2$, $1,3$, and $1,4$.

• Geminal vs Vicinal:

  • Gem-dihalides: Halogen atoms on the same carbon atom (alkylidene halides).

  • Vic-dihalides: Halogen atoms on adjacent carbon atoms (alkylene dihalides).

• Examples:

  • Vinyl chloride: $\text{Chloroethene}$

  • Chloroform: $\text{Trichloromethane}$ ($CHCl_3$)

  • Carbon tetrachloride: $\text{Tetrachloromethane}$ ($CCl_4$)

Nature of the Carbon-Halogen Bond

• Polarity: Halogen atoms are more electronegative than carbon, resulting in a polarised $C-X$ bond where carbon bears a partial positive charge and the halogen bears a partial negative charge.

• Bond Parameters (Table 10.2):

  • Bond Lengths ($pm$): $CH_3-F$ ($139$), $CH_3-Cl$ ($178$), $CH_3-Br$ ($193$), $CH_3-I$ ($214$).

  • Bond Enthalpies ($kJ\,mol^{-1}$): $CH_3-F$ ($452$), $CH_3-Cl$ ($351$), $CH_3-Br$ ($293$), $CH_3-I$ ($234$).

  • Dipole Moments ($Debye$): $CH_3-F$ ($1.847$), $CH_3-Cl$ ($1.860$), $CH_3-Br$ ($1.830$), $CH_3-I$ ($1.636$).

Methods of Preparation of Haloalkanes

• From Alcohols: Reagents include concentrated halogen acids ($HX$), phosphorus halides ($PX_3, PCl_5$), or thionyl chloride ($SOCl_2$).

  • $SOCl_2$ is preferred as by-products ($SO_2$ and $HCl$) are escapable gases.

  • Order of reactivity of alcohols with $HX$: 3^\circ > 2^\circ > 1^\circ.

  • This method is not used for aryl halides because the $C-O$ bond in phenols has partial double bond character.

• From Hydrocarbons:

  • Free Radical Halogenation: Alkanes give mixtures of mono- and polyhaloalkanes which are hard to separate.

  • Electrophilic Substitution: Arenes react with $Cl_2$ or $Br_2$ in the presence of Lewis acids ($Fe$ or $FeCl_3$).

  • Sandmeyer's Reaction: Primary aromatic amines treated with $NaNO_2 + HCl$ form diazonium salts. Mixing with cuprous chloride/bromide replaces the diazonium group with $-Cl$ or $-Br$.

• From Alkenes:

  • Addition of $HX$: Follows Markovnikov’s rule.

  • Addition of Halogens: Addition of $Br_2$ in $CCl_4$ results in vic-dibromides and discharge of reddish-brown color (test for unsaturation).

• Halogen Exchange:

  • Finkelstein Reaction: $R-X + NaI \rightarrow R-I + NaX$ (in dry acetone).

  • Swarts Reaction: Heating alkyl chloride/bromide with metallic fluorides ($AgF, Hg_2F_2, CoF_2, SbF_3$) produces alkyl fluorides.

Physical Properties

• State: Methyl chloride, methyl bromide, and ethyl chloride are gases; higher members are liquids or solids.

• Boiling Points:

  • Higher than parents due to dipole-dipole and van der Waals forces.

  • Trend for a given alkyl group: RI > RBr > RCl > RF.

  • Branching decreases boiling point due to decreased surface area (e.g., 2-bromo-2-methylpropane has lower B.P. than n-butyl bromide).

• Density: Increases with increasing carbon atoms and halogen atomic mass.

• Solubility: Very slightly soluble in water; dissolves in organic solvents.

Nucleophilic Substitution Reactions

• Mechanisms:

  • $S_N2$ (Bimolecular): Single-step, second-order kinetics. Involves inversion of configuration. Reactivity order: Primary > Secondary > Tertiary (Methyl > 1^\circ > 2^\circ > 3^\circ).

  • $S_N1$ (Unimolecular): Two-step, first-order kinetics. Step 1 involves slow formation of a carbocation. Favored in polar protic solvents. Reactivity order: 3^\circ > 2^\circ > 1^\circ.

• Stereochemistry:

  • Optical Activity: Compounds rotating plane polarised light.

  • Chirality: Objects non-superimposable on their mirror images are chiral (asymmetric).

  • Enantiomers: Mirror image isomers with identical physical properties (except rotation direction).

  • Racemic Mixture: Equimolar mixture of enantiomers resulting in zero optical rotation ($racemisation$).

Elimination and Metal Reactions

• Elimination: Haloalkanes with $\beta$-hydrogen heated with alcoholic $KOH$ undergo $\beta$-elimination to form alkenes.

  • Zaitsev (Saytzeff) Rule: The preferred product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbons.

• Reaction with Metals:

  • Grignard Reagents: Prepared as $RMgX$ by reacting haloalkanes with $Mg$ in dry ether. Extremely reactive with moisture.

  • Wurtz Reaction: $2RX + 2Na \rightarrow R-R + 2NaX$ (doubles carbon chain).

  • Wurtz-Fittig Reaction: Mixture of alkyl and aryl halide reacts with $Na$ to give alkylarenes.

  • Fittig Reaction: Two aryl halides join together.

Polyhalogen Compounds

• Dichloromethane ($CH_2Cl_2$): Solvent, propellant, harms the central nervous system.

• Trichloromethane ($CHCl_3$): Chloroform; once used as an anaesthetic. Oxidises in light to poisonous Phosgene ($COCl_2$).

• Triiodomethane ($CHI_3$): Iodoform; previously used as an antiseptic due to released free iodine.

• Tetrachloromethane ($CCl_4$): Carbon tetrachloride; causes liver cancer and depletes the ozone layer.

• Freons: Chlorofluorocarbons ($CFCs$); extremely stable gases used in refrigeration; causes ozone depletion.

• DDT ($p,p'-Dichlorodiphenyltrichloroethane$): First chlorinated organic insecticide; persistent in environment; banned in USA in 1973.

Questions & Discussion

• Question: Why is sulfuric acid not used during the reaction of alcohols with $KI$?

  • Answer: $H_2SO_4$ is an oxidising agent. It converts $KI$ to $HI$ and then oxidises $HI$ to $I_2$, preventing its reaction with the alcohol.

• Question: Which alkyl halide undergoes faster $S_N2$: $CH_3Br$ or $CH_3I$?

  • Answer: $CH_3I$ reacts faster because $I^-$ is a better leaving group due to its larger size.

• Question: Why are haloarenes less reactive than haloalkanes in nucleophilic substitution?

  • Answer: 1. Resonance effect (partial double bond character). 2. Carbon is $sp^2$ hybridised (more electronegative/holds electrons tighter). 3. Instability of phenyl cation. 4. Repulsion between electron-rich nucleophile and arene ring.

• Question: What are ambident nucleophiles?

  • Answer: Nucleophiles possessing two nucleophilic centers, such as cyanide ($CN^-$) and nitrite ($NO_2^-$).