Haloalkanes and Haloarenes

1. Classification & Nature of the C-X Bond

• Classification by Carbon Hybridisation:
  • Alkyl Halides ($sp^3\text{ C-X}$): Primary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$) haloalkanes based on alpha-carbon substitution.
  • Allylic Halides ($sp^3\text{ C-X}$): Halogen attached to an $sp^3$-carbon adjacent to a carbon-carbon double bond ($\text{CH}_2=\text{CH}-\text{CH}_2\text{X}$).
  • Benzylic Halides ($sp^3\text{ C-X}$): Halogen attached to an $sp^3$-carbon adjacent to an aromatic ring ($\text{Ar}-\text{CH}_2\text{X}$).
  • Vinylic Halides ($sp^2\text{ C-X}$): Halogen bonded directly to an $sp^2$-carbon of an alkene ($\text{CH}_2=\text{CH}-\text{X}$).
  • Aryl Halides ($sp^2\text{ C-X}$): Halogen bonded directly to an aromatic ring ($\text{Ar}-\text{X}$).
• Bond Polarity and Physical Properties:
  • The $\text{C}-\text{X}$ bond is polarized ($\text{C}^{\delta+} - \text{X}^{\delta-}$) due to halogen electronegativity.
  • Bond length increases down Group 17: $\text{C-F}\ (139\text{ pm}) < \text{C-Cl}\ (178\text{ pm}) < \text{C-Br}\ (193\text{ pm}) < \text{C-I}\ (214\text{ pm})$.
  • Bond enthalpy decreases: $\text{CH}_3\text{F}\ (452) > \text{CH}_3\text{Cl}\ (351) > \text{CH}_3\text{Br}\ (293) > \text{CH}_3\text{I}\ (234\text{ kJ/mol})$.
  • Dipole moment: $\text{CH}_3\text{Cl}\ (1.860\text{ D}) > \text{CH}_3\text{F}\ (1.847\text{ D}) > \text{CH}_3\text{Br}\ (1.830\text{ D}) > \text{CH}_3\text{I}\ (1.636\text{ D})$.
  • Boiling points decrease with branching due to reduced surface contact area: $n\text{-butyl} > \text{isobutyl} > \text{tert-butyl}$.
  • Para-dihalobenzenes have higher melting points than ortho and meta isomers due to crystal lattice symmetry.

2. Methods of Preparation

• From Alcohols:
  • Darzens Process (Best Laboratory Route):

$$\text{R-OH} + \text{SOCl}_2 \xrightarrow{\text{Pyridine}} \text{R-Cl} + \text{SO}_2\uparrow + \text{HCl}\uparrow$$

• Phosphorus Halides:

$$\text{R-OH} + \text{PCl}_5 \longrightarrow \text{R-Cl} + \text{POCl}_3 + \text{HCl}$$

$$3\text{R-OH} + \text{PBr}_3 \longrightarrow 3\text{R-Br} + \text{H}_3\text{PO}_3$$

• Grooves Process (Lucas Reagent): Primary and secondary alcohols require anhydrous $\text{ZnCl}_2$ with concentrated $\text{HCl}$; tertiary alcohols react at room temperature.
• Halogen Exchange Reactions:
  • Finkelstein Reaction: Preparation of alkyl iodides using sodium iodide in dry acetone:

$$\text{R-X} + \text{NaI} \xrightarrow{\text{dry acetone}} \text{R-I} + \text{NaX}\downarrow \quad (\text{X} = \text{Cl, Br})$$

• Swarts Reaction: Synthesis of alkyl fluorides using metallic fluorides:

$$\text{R-Br} + \text{AgF} \longrightarrow \text{R-F} + \text{AgBr}\downarrow \quad (\text{or } \text{Hg}_2\text{F}_2, \text{CoF}_2, \text{SbF}_3)$$

• From Arenes and Diazonium Salts:
  • Direct electrophilic halogenation with $\text{Cl}_2/\text{Fe}$ in the dark gives ortho and para chlorotoluenes.
  • Sandmeyer Reaction: Primary aromatic amines undergo diazotisation followed by reaction with cuprous halides:

$$\text{Ar-NH}_2 \xrightarrow[273-278\text{ K}]{\text{NaNO}_2 + \text{HCl}} \text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{Cu}_2\text{Cl}_2/\text{HCl}} \text{Ar-Cl} + \text{N}_2$$

• Aryl iodides are synthesized by shaking diazonium salts with aqueous $\text{KI}$ without a copper catalyst.
• Balz-Schiemann Reaction: Diazonium fluoroborate heated to give pure aryl fluorides ($\text{Ar-N}_2^+\text{BF}_4^- \xrightarrow{\Delta} \text{Ar-F} + \text{BF}_3 + \text{N}_2$).

3. Nucleophilic Substitution: SN1 vs SN2 Pathways

• Kinetic and Mechanistic Comparison:

4. Stereochemical Principles: Chirality & Enantiomers

• Chirality and Asymmetry: A molecule that is non-superimposable on its mirror image is chiral and optically active. The asymmetric carbon atom is bonded to four chemically distinct groups.
• Enantiomers: Stereoisomers related as non-superimposable mirror images that rotate plane-polarized light in equal and opposite directions ($+$ and $-$). They share identical melting points, boiling points, and refractive indices.
• Racemic Mixture: An equimolar ($1:1$) mixture of two enantiomers exhibiting zero net optical rotation due to external compensation.
• Diastereomers & Meso Compounds:
  • Diastereomers are stereoisomers that are not mirror images of each other and possess different physical constants.
  • Meso compounds contain asymmetric carbons but possess an internal plane of symmetry, rendering them optically inactive via internal compensation.

5. Elimination Reactions & Metal Chemistry

• Dehydrohalogenation ($\beta$-Elimination): Heating alkyl halides with alcoholic $\text{KOH}$ abstracts a $\beta$-hydrogen and eliminates a halide ion.
  • Saytzeff (Zaitsev) Rule: The major product is the more highly substituted alkene with the maximum number of alkyl groups on double-bonded carbons:

$$\text{CH}_3\text{CH}_2\text{CH(Br)CH}_3 \xrightarrow[\Delta]{\text{alc. KOH}} \text{CH}_3\text{CH}=\text{CHCH}_3\ (81\%\text{ Saytzeff}) + \text{CH}_3\text{CH}_2\text{CH}=\text{CH}_2\ (19\%\text{ Hofmann})$$

• Organometallic Reagents:
  • Grignard Synthesis: Alkyl halides react with magnesium turnings in anhydrous ether:

$$\text{R-X} + \text{Mg} \xrightarrow{\text{dry ether}} \text{R-MgX} \quad (\text{Organomagnesium halide})$$

• The carbon-magnesium bond is highly polarized covalent ($\text{R}^{\delta-} - \text{Mg}^{\delta+}\text{X}$); it acts as a carbanion source and hydrolyzes to alkanes with trace water:

$$\text{R-MgX} + \text{H}_2\text{O} \longrightarrow \text{R-H} + \text{Mg(OH)X}$$

• Wurtz-Fittig and Fittig Coupling:

$$\text{Ar-X} + 2\text{Na} + \text{R-X} \xrightarrow{\text{dry ether}} \text{Ar-R} + 2\text{NaX} \quad (\text{Wurtz-Fittig})$$

$$2\text{Ar-X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{Ar-Ar} + 2\text{NaX} \quad (\text{Fittig reaction})$$

6. Haloarenes: Low Reactivity & Electrophilic Aromatic Substitution

• Resistance to Nucleophilic Substitution:
  1. Resonance Effect: Delocalization of halogen lone pairs into the aromatic ring imparts partial double-bond character to the $\text{C}-\text{Cl}$ bond ($169\text{ pm}$ vs $177\text{ pm}$ in $\text{R-Cl}$).
  2. Hybridisation: $sp^2$ aromatic carbon is more electronegative than $sp^3$ alkyl carbon, holding bonding electrons tighter.
  3. Instability of Phenyl Cation: Self-ionization to $\text{C}_6\text{H}_5^+$ is energetically disfavoured, precluding $\text{S}_\text{N}1$.
  4. Repulsion: $\pi$-electron cloud of the arene ring repels incoming nucleophiles.
• Activated Nucleophilic Displacement:
  • Heating chlorobenzene with aqueous $\text{NaOH}$ requires $623\text{ K}$ and $300\text{ atm}$ (Dow's Process).
  • Ortho/para electron-withdrawing nitro groups stabilize the anionic Meisenheimer intermediate via resonance:
    • 4-Nitrochlorobenzene reacts at $443\text{ K}$.
    • 2,4-Dinitrochlorobenzene reacts at $368\text{ K}$.
    • 2,4,6-Trinitrochlorobenzene (picryl chloride) hydrolyzes in warm water.

• Electrophilic Aromatic Substitution on Chlorobenzene:
  • Chlorine is weakly deactivating due to its strong $-I$ effect, but directs incoming electrophiles to ortho and para positions due to $+M$ resonance stabilization of the arenium intermediate.
  • Halogenation: $\text{Cl}_2 / \text{FeCl}_3$ gives 1,4-dichlorobenzene (major) and 1,2-dichlorobenzene (minor).
  • Nitration: $\text{HNO}_3 + \text{H}_2\text{SO}_4$ gives 1-chloro-4-nitrobenzene (major).
  • Friedel-Crafts Alkylation & Acylation: $\text{CH}_3\text{Cl} / \text{anhyd. AlCl}_3$ gives 1-chloro-4-methylbenzene (major).

7. Polyhalogen Derivatives & Environmental Impact

• Dichloromethane ($\text{CH}_2\text{Cl}_2$): Paint remover and pharmaceutical process solvent; causes central nervous system depression, impaired hearing, and corneal damage.
• Chloroform ($\text{CHCl}_3$): Former surgical anaesthetic and precursor to Freon R-22. Oxidizes in air and sunlight to poisonous phosgene:

$$2\text{CHCl}_3 + \text{O}_2 \xrightarrow{h\nu} 2\text{COCl}_2 + 2\text{HCl}$$

Stored in completely filled, dark amber bottles with 1% ethanol to convert traces of phosgene into harmless diethyl carbonate.

• Iodoform ($\text{CHI}_3$): Yellow crystalline solid used as an antiseptic due to liberated elemental iodine; identified by the characteristic triiodomethane test.
• Tetrachloromethane ($\text{CCl}_4$): Fire extinguisher (Pyrene) and refrigerant feedstock; exposure causes liver cancer and severe nervous system toxicity. Produces phosgene when exposed to high-temperature steam.
• Freons (CFCs): Freon-12 ($\text{CCl}_2\text{F}_2$) manufactured via Swarts reaction from $\text{CCl}_4$. Stable in the troposphere, but photolytically generates chlorine radicals in the stratosphere, destroying the protective ozone layer.
• DDT ($p,p'$-Dichlorodiphenyltrichloroethane): Synthesized from chlorobenzene and chloral with concentrated sulphuric acid. Highly persistent, fat-soluble contact insecticide that accumulates in food chains; banned due to avian toxicity and biomagnification.