Hydrocarbons: Alkanes, Alkenes, Alkynes & Arenes

1. Alkanes: Synthesis & Conformational Analysis

• General Formula: $\text{C}_n\text{H}_{2n+2}$. Characterised by $sp^3$-hybridised carbons with tetrahedral geometry and $109.5^\circ$ bond angles.
• Preparative Methods:
  • Hydrogenation (Sabatier-Senderens): $\text{R}-\text{CH}=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni/Pt/Pd}} \text{R}-\text{CH}_2-\text{CH}_3$.
  • Wurtz Coupling: $2\text{R}-\text{X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{R}-\text{R} + 2\text{NaX}$. High yields only for symmetrical alkanes with an even number of carbons; methane cannot be prepared.
  • Soda-Lime Decarboxylation: $\text{R}-\text{COONa} + \text{NaOH} \xrightarrow[\Delta]{\text{CaO}} \text{R}-\text{H} + \text{Na}_2\text{CO}_3$ (yields alkane with one less carbon).
  • Kolbe's Electrolytic Method: Aqueous sodium carboxylate electrolysed at the anode to generate alkyl radicals:

$$2\text{R-COO}^- \longrightarrow 2\text{R-COO}^\bullet + 2e^- \longrightarrow 2\text{R}^\bullet + 2\text{CO}_2 \longrightarrow \text{R-R}$$

• Conformations of Ethane: Rotation around the $\text{C}-\text{C}$ single $\sigma$-bond generates an infinite number of conformers.
  • Staggered Conformer: Dihedral angle $\theta = 60^\circ$. Maximum separation between $\text{C}-\text{H}$ electron pairs, minimum torsional strain, maximum stability.
  • Eclipsed Conformer: Dihedral angle $\theta = 0^\circ$. Minimum distance between $\text{C}-\text{H}$ bonds, maximum torsional strain, minimum stability.
  • The energy barrier separating staggered and eclipsed forms is $12.5\text{ kJ/mol}$, permitting free rotation at room temperature.

2. Alkanes: Free-Radical Halogenation & Combustion

• Halogenation Reactivity: $\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2$. Fluorination is explosive; iodination is reversible and requires oxidising agents ($\text{HNO}_3$ or $\text{HIO}_3$) to destroy $\text{HI}$.
• Relative Replacement of Hydrogens: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$).
• Free-Radical Chain Mechanism:
  • Initiation: Homolytic cleavage of halogen bond by heat or light:

    $$\text{Cl}_2 \xrightarrow{h\nu} 2\text{Cl}^\bullet$$

  • Propagation: Radical abstracts hydrogen to form alkyl radical, which reacts with another halogen molecule:

    $$\text{CH}_4 + \text{Cl}^\bullet \longrightarrow \text{CH}_3^\bullet + \text{HCl}$$

    $$\text{CH}_3^\bullet + \text{Cl}_2 \longrightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet$$

  • Termination: Coupling of remaining radicals ($\text{Cl}^\bullet + \text{Cl}^\bullet$, $\text{CH}_3^\bullet + \text{Cl}^\bullet$, $\text{CH}_3^\bullet + \text{CH}_3^\bullet \rightarrow \text{CH}_3-\text{CH}_3$).
• Controlled Catalytic Oxidation:
  • Methanol formation: $2\text{CH}_4 + \text{O}_2 \xrightarrow[523\text{ K},\ 100\text{ atm}]{\text{Cu}} 2\text{CH}_3\text{OH}$
  • Methanal formation: $\text{CH}_4 + \text{O}_2 \xrightarrow[\Delta]{\text{Mo}_2\text{O}_3} \text{HCHO} + \text{H}_2\text{O}$
  • Tertiary hydrogen oxidation: $(\text{CH}_3)_3\text{CH} \xrightarrow{[\text{O}],\ \text{KMnO}_4} (\text{CH}_3)_3\text{C}-\text{OH}$ (2-methylpropan-2-ol)

3. Alkenes: Structure, Synthesis & Geometric Isomerism

• Double Bond Structure: One $\sigma$-bond ($sp^2-sp^2$ head-on) and one $\pi$-bond ($2p-2p$ sideways overlap). Carbon-carbon bond length is $134\text{ pm}$.
• Methods of Preparation:
  • Dehydrohalogenation: $\text{R}-\text{CH}_2-\text{CH}_2\text{X} + \text{alc. KOH} \xrightarrow{\Delta} \text{R}-\text{CH}=\text{CH}_2 + \text{KX} + \text{H}_2\text{O}$. Follows Saytzeff rule (more substituted alkene is major).
  • Dehalogenation of Vicinal Dihalides: $\text{R}-\text{CHBr}-\text{CH}_2\text{Br} + \text{Zn} \longrightarrow \text{R}-\text{CH}=\text{CH}_2 + \text{ZnBr}_2$.
  • Acid-Catalyzed Dehydration of Alcohols: $\text{CH}_3-\text{CH}_2\text{OH} \xrightarrow[\Delta]{\text{conc. } \text{H}_2\text{SO}_4} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}$.
• Geometric (Cis-Trans) Isomerism: Arises from restricted rotation about the $\text{C}=\text{C}$ double bond.
  • Cis-Isomer: Similar groups on the same side; higher dipole moment, higher boiling point.
  • Trans-Isomer: Similar groups on opposite sides; lower dipole moment, higher symmetry and higher melting point.

4. Addition Reactions: Markovnikov Rule vs Peroxide Effect

• Markovnikov's Rule: In electrophilic addition of an unsymmetrical reagent ($\text{H}-\text{X}$) to an unsymmetrical alkene, the negative part adds to the double-bonded carbon bearing fewer hydrogen atoms. This proceeds via the more stable carbocation intermediate ($3^\circ > 2^\circ > 1^\circ$).
• Anti-Markovnikov Addition (Kharasch / Peroxide Effect): Observed exclusively with $\text{HBr}$ in the presence of organic peroxides. The bromine radical adds first to generate the more stable secondary free radical, yielding 1-bromopropane as the major product.

5. Alkenes: Ozonolysis & Baeyer's Test

• Ozonolysis: Alkene reacts with $\text{O}_3$ to form a cyclic ozonide, followed by reductive cleavage with $\text{Zn}/\text{H}_2\text{O}$ to carbonyl compounds. Used to determine the position of double bonds:

$$\text{R}_2\text{C}=\text{CH}-\text{R}' \xrightarrow{\text{(i) } \text{O}_3} \text{Ozonide} \xrightarrow{\text{(ii) } \text{Zn}/\text{H}_2\text{O}} \text{R}_2\text{C}=\text{O} + \text{R}'\text{CHO}$$

• Baeyer's Test (Test for Unsaturation): Decolourisation of cold, dilute, alkaline $\text{KMnO}_4$ solution with formation of vicinal glycols:

$$\text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} + [\text{O}] \xrightarrow[\text{alk. } \text{KMnO}_4]{273\text{ K}} \text{CH}_2(\text{OH})-\text{CH}_2(\text{OH})$$

• Bromine Water Test: Addition of bromine in $\text{CCl}_4$ discharges the reddish-orange colour by forming colourless vicinal 1,2-dibromoalkanes.

6. Alkynes: Acidic Character & Additions

• Structure of Triple Bond: One $\sigma$-bond and two perpendicular $\pi$-bonds with linear geometry ($180^\circ$ bond angle, $\text{C}\equiv\text{C}$ length $120\text{ pm}$).
• Acidic Nature of Terminal Alkynes: The $sp$-hybridised carbon has 50% $s$-character, exhibiting higher electronegativity than $sp^2$ or $sp^3$ carbons. Terminal acetylenic hydrogens are weakly acidic:

  $$\text{R}-\text{C}\equiv\text{C}-\text{H} + \text{NaNH}_2 \longrightarrow \text{R}-\text{C}\equiv\text{C}^-\text{Na}^+ + \text{NH}_3$$

  $$\text{Acidity: } \text{CH}\equiv\text{CH} > \text{CH}_3-\text{C}\equiv\text{CH} \gg \text{CH}_2=\text{CH}_2 > \text{CH}_3-\text{CH}_3$$

• Hydration to Carbonyls (Kucherov Reaction): Addition of water in the presence of $\text{HgSO}_4 / \text{dil. } \text{H}_2\text{SO}_4$ at 333 K yields carbonyls via enol-keto tautomerism:
  • Ethyne yields Ethanal ($\text{CH}_3\text{CHO}$).
  • Propyne and higher alkynes yield Ketones ($\text{CH}_3\text{COCH}_3$).
• Cyclic Trimerisation: Passing ethyne through a red-hot iron tube at 873 K yields benzene:

  $$3\text{CH}\equiv\text{CH} \xrightarrow[873\text{ K}]{\text{Red-hot Fe tube}} \text{C}_6\text{H}_6$$

7. Aromatic Hydrocarbons: Aromaticity & EAS Mechanism

• Hückel's Rule of Aromaticity: A cyclic planar system is aromatic if it contains $(4n+2)\ \pi$-electrons completely delocalised across the ring. All ring carbons in benzene are $sp^2$-hybridised with equal carbon-carbon bond lengths of $139\text{ pm}$.
• Electrophilic Aromatic Substitution (EAS) Steps:
  1. Generation of the Electrophile ($E^+$): Lewis acids like $\text{AlCl}_3$ assist in generating $\text{Cl}^+$, $\text{NO}_2^+$, $\text{R}^+$, or $\text{RCO}^+$.
  2. Formation of Arenium Ion ($\sigma$-Complex): Attack of $E^+$ disrupts the aromatic sextet to create a resonance-stabilised cyclohexadienyl cation.
  3. Proton Loss: Rapid removal of the proton by a base restores the $6\ \pi$-electron aromatic sextet.

• Directive Influence of Substituents:
  • Ortho/Para-Directing Activating Groups: Donate electron density by resonance ($+M / +R$), increasing electron density at ortho and para positions ($-\text{OH}, -\text{NH}_2, -\text{OCH}_3, -\text{CH}_3$).
  • Ortho/Para-Directing Deactivating Groups (Halogens): Net electron withdrawing due to strong inductive effect ($-I$), but direct ortho/para due to $+M$ resonance stabilization.
  • Meta-Directing Deactivating Groups: Withdraw electron density from the ring via $-M$ and $-I$ effects, leaving the meta position least electron-deficient ($-\text{NO}_2, -\text{CN}, -\text{CHO}, -\text{COOH}, -\text{SO}_3\text{H}$).