Comprehensive Study Notes on Hydrocarbons: Alkanes, Alkenes, Alkynes, and Arenes

Classification and General Introduction to Hydrocarbons

• Definition of Hydrocarbons:

  • Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms.
  • They function as primary sources of energy globally and form the chemical foundation for petrochemical industries.

• Everyday and Commercial Sources of Hydrocarbons:

  • Liquefied Petroleum Gas (LPG): A domestic fuel composed of light alkanes (primarily butane and propane) that burns with minimal pollution.
  • Compressed Natural Gas (CNG): Methane-rich natural gas derived from the upper strata of oil wells, compressed for use as an environmentally cleaner automotive fuel.
  • Liquefied Natural Gas (LNG): Natural gas converted into a liquid state through refrigeration for efficient bulk storage and transport.
  • Fractional Distillation Fractions of Petroleum: Commercial fuels including petrol (gasoline), diesel, and kerosene oil isolated from crude oil within the Earth's crust.
  • Coal Gas: A combustible gaseous mixture produced via the destructive distillation of coal.
  • Industrial Chemical Feedstocks: Raw materials used in the synthesis of polymers such as polythene, polypropene, and polystyrene; higher molecular weight fractions function as paint solvents, industrial lubricants, and precursors for synthetic dyes and pharmaceuticals.

• Broad Structural Classification:

  • Saturated Hydrocarbons:
  • Hydrocarbons containing exclusively single covalent carbon-carbon ($C-C$) and carbon-hydrogen ($C-H$) bonds.
  • Open-chain (acyclic) saturated hydrocarbons are designated as alkanes.
  • Closed-chain (cyclic) saturated hydrocarbons are termed cycloalkanes.
  • Unsaturated Hydrocarbons:
  • Molecules possessing one or more multiple carbon-carbon bonds.
  • Hydrocarbons containing carbon-carbon double bonds ($C=C$) are designated as alkenes.
  • Hydrocarbons containing carbon-carbon triple bonds ($C\equiv C$) are designated as alkynes.
  • Aromatic Hydrocarbons (Arenes):
  • Highly unsaturated cyclic systems characterized by continuous cyclic conjugation, planarity, and exceptional thermodynamic stability governed by Hückel's rule.

Alkanes: Structure, Nomenclature, and Isomerism

• Homologous Series and General Formula:

  • Alkanes conform to the general molecular formula $C_n H_{2n+2}$, where $n$ corresponds to the integer number of carbon atoms.
  • Historically termed paraffins (derived from Latin parum = little, affinis = affinity) due to their general chemical inertness toward strong acids, alkalis, oxidizing agents, and reducing agents under ambient conditions.

• Methane and Alkane Hybridization Geometry:

  • The simplest homologue is methane ($CH_4$), commonly identified in marsh gases and coal mines.
  • Carbon exhibits $sp^3$ hybridization in all alkanes. The four hybrid orbitals point toward the vertices of a regular tetrahedron.
  • Bond parameters in methane:
  • Geometry: Tetrahedral.
  • Bond angle ($H-C-H$): $109.5^\circ$.
  • Carbon-carbon single bond length ($C-C$): $154\,pm$.
  • Carbon-hydrogen bond length ($C-H$): $112\,pm$.
  • Bonding mechanism: Pure head-on axial overlap between $sp^3$ hybrid orbitals of carbon and $1s$ atomic orbitals of hydrogen.

• Constitutional (Structural) Isomerism:

  • Alkanes containing 1 to 3 carbon atoms ($CH_4$, $C_2H_6$, $C_3H_8$) exhibit only one unique structural arrangement.
  • Chain Isomers: Compounds possessing identical molecular formulas but varying in the skeletal branching of their carbon parent chain.
  • Isomers of Butane ($C_4H_{10}$):
  • n-Butane: Continuous unbranched chain: $CH_3-CH_2-CH_2-CH_3$, boiling point $273\,K$.
  • 2-Methylpropane (Isobutane): Branched chain: $CH_3-CH(CH_3)-CH_3$, boiling point $261\,K$.
  • Isomers of Pentane ($C_5H_{12}$):
  • n-Pentane: Unbranched chain: $CH_3-CH_2-CH_2-CH_2-CH_3$, boiling point $309.1\,K$.
  • 2-Methylbutane (Isopentane): Single methyl branch: $CH_3-CH(CH_3)-CH_2-CH_3$, boiling point $300.9\,K$.
  • 2,2-Dimethylpropane (Neopentane): Quaternary symmetric center: $C(CH_3)_4$, boiling point $282.5\,K$.
  • Scaling of Isomers with Chain Length:
  • Hexane ($C_6H_{14}$): 5 isomers.
  • Heptane ($C_7H_{16}$): 9 isomers.
  • Decane ($C_{10}H_{22}$): 75 structural isomers.

• Classification of Carbon and Hydrogen Atoms:

  • Primary Carbon ($1^\circ$): Attached to zero other carbons (as in $CH_4$) or bonded to exactly one other carbon atom (terminal carbons).
  • Secondary Carbon ($2^\circ$): Bonded directly to two adjacent carbon atoms.
  • Tertiary Carbon ($3^\circ$): Bonded directly to three adjacent carbon atoms.
  • Quaternary Carbon ($4^\circ$): Bonded directly to four adjacent carbon atoms.
  • Hydrogens attached to $1^\circ$, $2^\circ$, and $3^\circ$ carbon atoms are termed primary, secondary, and tertiary hydrogen atoms, respectively.

• Alkyl Radicals and IUPAC Nomenclature Rules:

  • Removal of a single terminal hydrogen atom from an alkane generates an alkyl substituent ($C_n H_{2n+1}$, symbol $-R$).
  • Nomenclature Rules:
  • Identification of the longest continuous carbon chain as the parent alkane.
  • Numbering the parent chain from the terminal end that awards the lowest locant set to substituents (lowest sum rule).
  • When multiple substituents are present, they are cited alphabetically irrespective of multiplying prefixes (di-, tri-, tetra-), except where complex substituent names utilize specific prefixes.
  • Multi-branched complex alkyl groups are numbered starting from the carbon attached directly to the parent chain ($C1$).

Preparation of Alkanes

• Catalytic Hydrogenation of Unsaturated Hydrocarbons:

  • Alkenes and alkynes undergo syn-addition of dihydrogen gas over finely divided transition metal catalysts ($Pt$, $Pd$, or Raney $Ni$).
  • Platinum and palladium catalyze the reaction efficiently at room temperature; nickel catalysts require elevated temperatures and pressures.   $\text{Ethene: } CH_2=CH_2 + H_2 \xrightarrow{Pt/Pd/Ni} CH_3-CH_3$$\text{Propene: } CH_3-CH=CH_2 + H_2 \xrightarrow{Pt/Pd/Ni} CH_3-CH_2-CH_3$$\text{Propyne: } CH_3-C\equiv C-H + 2H_2 \xrightarrow{Pt/Pd/Ni} CH_3-CH_2-CH_3$

• Reduction of Alkyl Halides:

  • Reduction by Nascent Hydrogen:
  • Alkyl chlorides, bromides, and iodides (fluorides are unreactive) reduce to parent alkanes upon treatment with zinc and dilute hydrochloric acid ($Zn/H^+$):     $CH_3-Cl + H_2 \xrightarrow{Zn, H^+} CH_4 + HCl$$C_2H_5-Cl + H_2 \xrightarrow{Zn, H^+} C_2H_6 + HCl$$CH_3CH_2CH_2-Cl + H_2 \xrightarrow{Zn, H^+} CH_3CH_2CH_3 + HCl$
  • Wurtz Reaction:
  • Coupling of alkyl halides with sodium metal in dry ether solution to produce symmetrical alkanes having an even number of carbon atoms:     $2CH_3-Br + 2Na \xrightarrow{\text{dry ether}} CH_3-CH_3 + 2NaBr$$2C_2H_5-Br + 2Na \xrightarrow{\text{dry ether}} CH_3-CH_2-CH_2-CH_3 + 2NaBr$
  • Limitation: When a mixture of two dissimilar alkyl halides ($R-X$ and $R'-X$) is utilized, a statistical mixture of three distinct alkanes ($R-R$, $R'-R'$, and $R-R'$) is formed, creating separating difficulties due to proximate boiling points.

• Decarboxylation of Carboxylic Acids:

  • Soda-Lime Decarboxylation:
  • Fusing dry sodium salts of carboxylic acids with soda lime (a dry mixture of $NaOH$ and $CaO$ in an approximate $3:1$ ratio) yields an alkane containing one less carbon atom than the precursor carboxylate:     $CH_3COO^-Na^+ + NaOH \xrightarrow{CaO, \Delta} CH_4 + Na_2CO_3$$CH_3CH_2CH_2COO^-Na^+ + NaOH \xrightarrow{CaO, \Delta} CH_3CH_2CH_3 + Na_2CO_3$
  • Calcium oxide ($CaO$) maintains the dryness of the reaction blend and prevents melting of glass apparatus by elevating the fusion temperature.
  • Kolbe's Electrolytic Method:
  • Electrolysis of a concentrated aqueous solution of sodium or potassium carboxylate yields symmetrical alkanes containing an even number of carbon atoms at the anode.
  • Overall Reaction:     $2CH_3COO^-Na^+ + 2H_2O \xrightarrow{\text{Electrolysis}} CH_3-CH_3 + 2CO_2 + H_2 + 2NaOH$
  • Reaction Mechanism:
    • Dissociation in water:       $2CH_3COONa \rightleftharpoons 2CH_3COO^- + 2Na^+$
    • Anodic oxidation (electron loss and radical decarboxylation):       $2CH_3COO^- \xrightarrow{-2e^-} 2[CH_3COO\cdot] \xrightarrow{\text{unstable}} 2\cdot CH_3 + 2CO_2$
    • Radical dimerisation at the anode:       $\cdot CH_3 + \cdot CH_3 \rightarrow CH_3-CH_3$
    • Cathodic reduction:       $2H_2O + 2e^- \rightarrow 2OH^- + 2H\cdot \rightarrow H_2(g)$
  • Note: Methane ($CH_4$) cannot be synthesized via Kolbe's electrolysis because the minimum hydrocarbon produced requires the dimerization of at least one radical pair ($C_1 + C_1 = C_2$).

Physical and Chemical Properties of Alkanes

• Physical States and Intermolecular Forces:

  • Alkanes are non-polar molecules exhibiting negligible dipole moments due to nearly equal electronegativities of carbon ($2.55$) and hydrogen ($2.20$).
  • Intermolecular interactions are limited to weak London dispersion (van der Waals) forces.
  • States at room temperature ($298\,K$):
  • $C_1$ to $C_4$ (methane to butane): Colourless, odourless gases.
  • $C_5$ to $C_{17}$: Colourless liquids.
  • $C_{18}$ and higher: Waxy solids.

• Solubility Trends:

  • Alkanes adhere strictly to the principle of "like dissolves like". They are insoluble and hydrophobic in water, but dissolve freely in non-polar organic solvents such as benzene, diethyl ether, and chloroform.
  • Greasy deposits comprise higher non-polar alkanes and are readily extracted via petroleum ether or dry-cleaning solvents.

• Boiling Point and Melting Point Regularities:

  • Boiling points demonstrate a steady rise of approximately $20\,K$ to $30\,K$ per added methylene ($-CH_2-$) unit owing to increasing molecular surface area and polarizability.
  • Effect of Branching on Boiling Point:
  • For structural isomers, branching diminishes molecular surface area by converting the molecule into a more compact, spherical geometry.
  • Reduced contact area lowers London dispersion forces, depressing the boiling point:
    • n-Pentane: $309.1\,K$
    • 2-Methylbutane: $300.9\,K$
    • 2,2-Dimethylpropane: $282.5\,K$

• Free-Radical Substitution (Halogenation):

  • Reactivity order of halogens: $F_2 > Cl_2 > Br_2 > I_2$.
  • Relative ease of carbon-hydrogen bond abstraction: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$).
  • Direct fluorination occurs with explosive violence, cleaving $C-C$ frameworks.
  • Iodination is sluggish and reversible; it requires the addition of an oxidizing agent like $HIO_3$ or concentrated $HNO_3$ to consume $HI$ byproduct:     $CH_4 + I_2 \rightleftharpoons CH_3I + HI$$HIO_3 + 5HI \rightarrow 3I_2 + 3H_2O$
  • Mechanism of Chlorination of Methane:
  • Initiation: Photolytic homolysis of the weak chlorine-chlorine single bond ($242\,kJ\,mol^{-1}$):       $Cl-Cl \xrightarrow{h\nu} 2\cdot Cl$
  • Propagation:       $\cdot Cl + CH_4 \rightarrow \cdot CH_3 + HCl$$\cdot CH_3 + Cl-Cl \rightarrow CH_3Cl + \cdot Cl$
    • Continued propagation steps lead to polychlorination products:       $CH_3Cl + \cdot Cl \rightarrow \cdot CH_2Cl + HCl$$\cdot CH_2Cl + Cl_2 \rightarrow CH_2Cl_2 + \cdot Cl$$CH_2Cl_2 + \cdot Cl \rightarrow \cdot CHCl_2 + HCl$$\cdot CHCl_2 + Cl_2 \rightarrow CHCl_3 + \cdot Cl$$CHCl_3 + \cdot Cl \rightarrow \cdot CCl_3 + HCl$$\cdot CCl_3 + Cl_2 \rightarrow CCl_4 + \cdot Cl$
  • Termination: Coupling of reactive radical pairs:       $\cdot Cl + \cdot Cl \rightarrow Cl_2$$\cdot CH_3 + \cdot CH_3 \rightarrow CH_3-CH_3$$\cdot CH_3 + \cdot Cl \rightarrow CH_3Cl$
  • Significance: The coupling of two methyl free radicals accounts for the formation of trace ethane ($CH_3-CH_3$) during the chlorination of pure methane.

• Combustion Reactions:

  • Alkanes burn in an excess of oxygen exothermically to form carbon dioxide and water vapour:   $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l); \quad \Delta_c H^\circ = -890\,kJ\,mol^{-1}$$C_4H_{10}(g) + \frac{13}{2}O_2(g) \rightarrow 4CO_2(g) + 5H_2O(l); \quad \Delta_c H^\circ = -2875.84\,kJ\,mol^{-1}$
  • General Stoichiometric Combustion Equation:   $C_n H_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow n CO_2 + (n+1)H_2O$
  • Incomplete Combustion: With limited air, combustion generates toxic carbon monoxide or carbon black (soot):   $CH_4(g) + O_2(g) \xrightarrow{\text{incomplete}} C(s) + 2H_2O(l)$
  • Carbon black is consumed industrially in printing inks, tyre pigments, and filtration media.

• Controlled Catalytic Oxidation:

  • Regulated oxidation using specific catalysts yields select functional groups:   $2CH_4 + O_2 \xrightarrow{Cu / 523\,K / 100\,atm} 2CH_3OH \quad \text{(Methanol)}$$CH_4 + O_2 \xrightarrow{Mo_2O_3, \Delta} HCHO + H_2O \quad \text{(Methanal)}$$2CH_3CH_3 + 3O_2 \xrightarrow{(CH_3COO)_2Mn, \Delta} 2CH_3COOH + 2H_2O \quad \text{(Ethanoic acid)}$
  • Resistance of primary and secondary positions: Typical alkanes resist oxidation by aqueous potassium permanganate ($KMnO_4$), but tertiary carbons oxidize selectively to tertiary alcohols:   $(CH_3)_3CH \xrightarrow{KMnO_4, [O]} (CH_3)_3COH \quad \text{(2-Methylpropan-2-ol)}$

• Isomerisation:

  • Refluxing unbranched alkanes with anhydrous aluminium chloride ($AlCl_3$) and hydrogen chloride ($HCl$) induces rearrangement to branched isomers:   $CH_3(CH_2)_4CH_3 \xrightarrow{\text{Anhydrous } AlCl_3 / HCl} CH_3-CH(CH_3)-(CH_2)_2-CH_3 + CH_3-CH_2-CH(CH_3)-CH_2-CH_3$

• Aromatization (Reforming):

  • Straight-chain alkanes containing 6 to 8 carbon atoms, when heated to $773\,K$ under $10-20\,atm$ pressure over transition metal oxides ($Cr_2O_3$, $V_2O_5$, or $Mo_2O_3$) supported on alumina ($Al_2O_3$), undergo simultaneous dehydrogenation and ring cyclization:   $n\text{-Hexane } (C_6H_{14}) \xrightarrow{Cr_2O_3 / V_2O_5 / Mo_2O_3, 773\,K, 10-20\,atm} \text{Benzene } (C_6H_6) + 4H_2$$n\text{-Heptane } (C_7H_{16}) \xrightarrow{Cr_2O_3 / V_2O_5 / Mo_2O_3, 773\,K, 10-20\,atm} \text{Toluene } (C_6H_5CH_3) + 4H_2$

• Steam Reforming:

  • Industrial production of synthesis gas (syngas) and dihydrogen gas from methane over nickel catalysts at elevated temperatures:   $CH_4 + H_2O(g) \xrightarrow{Ni, 1273\,K} CO + 3H_2$

• Pyrolysis (Cracking):

  • Thermal breakdown of high molecular weight alkanes into lower alkanes, alkenes, and hydrogen under oxygen-free conditions via free-radical scission:   $C_6H_{14} \xrightarrow{773\,K} C_6H_{12} + H_2 \quad \text{or} \quad C_4H_8 + C_2H_6 \quad \text{or} \quad C_3H_6 + C_2H_4 + CH_4$$C_{12}H_{26} \xrightarrow{Pt/Pd/Ni, 973\,K} C_7H_{16} + C_5H_{10} + \text{other products}$

Conformations of Alkanes

• Free Rotation Around Carbon-Carbon Sigma Bonds:

  • Electron density within a $\\sigma$ molecular orbital is symmetrically distributed around the internuclear cylindrical axis of the $C-C$ single bond.
  • This symmetry permits unhindered spatial rotation around the axis, producing an infinite continuum of transient geometries known as conformations, conformational isomers, or rotamers.
  • True free rotation is restricted by a small internal barrier ($1-20\,kJ\,mol^{-1}$) known as torsional strain, stemming from repulsive electrostatic interactions between adjacent bonding electron pairs.

• Conformational Extremes of Ethane ($C_2H_6$):

  • Staggered Conformation:
  • Hydrogens attached to adjacent carbon atoms are oriented at maximum dihedral angles ($\theta = 60^\circ$).
  • Repulsion between electron clouds is minimized, rendering this conformation the thermodynamic energy minimum.
  • Eclipsed Conformation:
  • Hydrogens directly align with one another across the $C-C$ bond axis ($\theta = 0^\circ$).
  • Maximizes orbital pair repulsion and torsional strain, creating a relative potential energy maximum.
  • Skew Conformations: Any non-staggered, non-eclipsed spatial arrangement intermediate between the two extremes.

• Graphic Projection Modes:

  • Sawhorse Projections:
  • The molecule is viewed at an oblique angle down the central $C-C$ bond axis.
  • The bond is drawn as an elongated slanting diagonal line; the lower left terminus designates the front carbon atom, while the upper right designates the rear carbon atom.
  • Newman Projections:
  • The molecule is viewed directly end-on down the $C-C$ bond axis.
  • The front carbon is represented as a central point with three radiating bonds placed at $120^\circ$ angles.
  • The rear carbon is projected as an overlapping circle, with three radiating bonds projecting from the perimeter.

• Thermodynamic Stability and Rotational Energy Barrier:
  • The torsional energy barrier separating staggered and eclipsed ethane is approximately $12.5\,kJ\,mol^{-1}$.
  • At ambient temperatures ($298\,K$), the average thermal kinetic energy acquired through intermolecular collisions exceeds this barrier.
  • Consequently, rotation occurs millions of times per second, rendering individual conformational isomers non-isolable under standard laboratory conditions, though molecules spend over $99\%$ of their time in staggered configurations.

Alkenes: Structure, Nomenclature, and Isomerism

• Structure and Bonding of the Carbon-Carbon Double Bond:
  • Alkenes contain at least one $C=C$ bond and exhibit the general formula $C_n H_{2n}$.
  • Historically called olefins (Latin oleum = oil, facere = to make), because lower members like ethene form oily liquids upon reacting with halogens.
  • Each double-bonded carbon undergoes $sp^2$ hybridization, yielding a planar trigonal framework with bond angles around $120^\circ$.
  • The double bond consists of:
  • One strong $\sigma$ bond formed via coaxial head-on overlap of two $sp^2$ orbitals (bond dissociation enthalpy $\approx 397\,kJ\,mol^{-1}$).
  • One weaker $\pi$ bond formed through lateral (sideways) overlap of unhybridized parallel $2p_z$ orbitals (bond dissociation enthalpy $\approx 284\,kJ\,mol^{-1}$).
  • Total double bond enthalpy is $681\,kJ\,mol^{-1}$, compared to $348\,kJ\,mol^{-1}$ for a single $C-C$ bond in ethane.
  • Bond length: The double bond ($134\,pm$) is shorter than the single bond ($154\,pm$).
  • Chemical Implications: The diffuse, exposed $\pi$-electron cloud above and below the molecular plane is loosely held and acts as a nucleophilic electron source readily susceptible to electrophilic attack.

• Structural and Geometrical Isomerism in Alkenes:
  • Structural Isomerism:
  • Chain Isomerism: Varied branching in the main chain (e.g., but-1-ene vs. 2-methylprop-1-ene).
  • Position Isomerism: Altered position of the unsaturation site (e.g., but-1-ene vs. but-2-ene).
  • Geometrical (Cis-Trans) Isomerism:
  • Arises due to restricted rotation about the $C=C$ double bond, as twisting the bond would rupture the lateral $\pi$ overlap.
  • Cis-Isomer: Identical groups or priority chains positioned on the same side of the double bond.
  • Trans-Isomer: Identical groups positioned on opposite sides of the double bond.
  • Prerequisite for Geometrical Isomerism: Each individual carbon atom of the double bond must be attached to two distinct atoms or groups ($XYC=CXY$, $XYC=CXZ$, or $XYC=CZW$). Molecules containing geminal identical groups (such as $H_2C=CR_2$ or $R_2C=CR'_2$) cannot form cis-trans isomers.
  • Physical Properties of Geometrical Isomers:
  • Dipole Moment ($\mu$): Cis-isomers are generally more polar than trans-isomers because individual bond dipoles add constructively. In symmetrical trans-alkenes, opposing bond moments cancel out:
    • Cis-but-2-ene: $\mu = 0.33\,D$
    • Trans-but-2-ene: $\mu = 0.0\,D$
  • Boiling Point: The higher polarity of cis-isomers leads to stronger dipole-dipole attractions, resulting in slightly elevated boiling points.
  • Melting Point: Trans-isomers are more symmetrical and pack more efficiently into crystal lattices, giving them higher melting points than their cis counterparts.

Preparation of Alkenes

• Partial Reduction of Alkynes:

  • Controlled Catalytic Hydrogenation (Lindlar's Catalyst):
  • Hydrogenation over palladised charcoal ($Pd/C$) deactivated (poisoned) with quinoline or sulfur compounds produces stereoselective cis-alkenes:     $R-C\equiv C-R + H_2 \xrightarrow{Pd/C, \text{ quinoline}} \begin{pmatrix} R & & R \\ & C=C & \\ H & & H \end{pmatrix} \quad \text{(Cis-alkene)}$
  • Birch-Type Chemical Reduction:
  • Reduction of non-terminal alkynes using sodium in liquid ammonia ($Na / \text{liq. } NH_3$) produces stereoselective trans-alkenes:     $R-C\equiv C-R \xrightarrow{Na / \text{liq. } NH_3} \begin{pmatrix} R & & H \\ & C=C & \\ H & & R \end{pmatrix} \quad \text{(Trans-alkene)}$

• Dehydrohalogenation of Alkyl Halides:

  • Heating alkyl halides ($R-X$) with alcoholic potassium hydroxide ($KOH$ in ethanol) results in 1,2-elimination (termed $\\beta$-elimination) of halogen and hydrogen:   $CH_3-CH_2-CH_2-X \xrightarrow{\text{alc. } KOH, \Delta} CH_3-CH=CH_2 + KX + H_2O$
  • Ease of halogen departure: Iodide > Bromide > Chloride ($I^- > Br^- > Cl^-$).
  • Reactivity of alkyl substrate: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$).

• Dehalogenation of Vicinal Dihalides:

  • Vicinal dihalides (compounds with halogen atoms on adjacent carbons) lose $ZnX_2$ when treated with zinc dust in boiling alcohol:   $CH_2Br-CH_2Br + Zn \xrightarrow{\Delta} CH_2=CH_2 + ZnBr_2$$CH_3-CHBr-CH_2Br + Zn \xrightarrow{\Delta} CH_3-CH=CH_2 + ZnBr_2$

• Acidic Dehydration of Alcohols:

  • Heating alcohols with concentrated sulphuric acid ($H_2SO_4$) at $433-443\,K$ removes a molecule of water via a $\beta$-elimination pathway:   $CH_3-CH_2-OH \xrightarrow{\text{conc. } H_2SO_4, 443\,K} CH_2=CH_2 + H_2O$

Physical and Chemical Properties of Alkenes

• Physical Properties of Alkenes:

  • Physical states: Ethene, propene, and butene isomers ($C_2-C_4$) are gases; $C_5$ to $C_{17}$ are liquids; higher members are solids.
  • Ethene is a colourless gas with a faint sweet odour; all other alkenes are odourless and colourless.
  • Insoluble in water; soluble in organic solvents (petroleum ether, benzene).
  • Boiling points rise by $20-30\,K$ per added methylene unit.

• Electrophilic Addition of Hydrogen and Halogens:

  • Addition of Dihydrogen: Yields saturated alkanes over $Pt$, $Pd$, or $Ni$.
  • Addition of Halogens: Chlorine and bromine react in inert non-polar media ($CCl_4$) to produce vicinal dihalides via a cyclic halonium ion intermediate.
  • Bromine Water / $Br_2$ in $CCl_4$ Test: The rapid discharge of the characteristic reddish-orange bromine colour serves as a standard diagnostic analytical test for unsaturation:   $CH_2=CH_2 + Br_2 \xrightarrow{CCl_4} CH_2Br-CH_2Br \quad \text{(1,2-Dibromoethane, colourless)}$

• Addition of Hydrogen Halides and Regioselectivity:

  • Relative reactivity of hydrogen halides: $HI > HBr > HCl$.
  • Symmetrical Alkenes: Addition across symmetrical alkenes produces a single monohaloalkane:   $CH_2=CH_2 + HBr \rightarrow CH_3-CH_2Br$$CH_3-CH=CH-CH_3 + HBr \rightarrow CH_3-CH_2-CHBr-CH_3$
  • Markovnikov's Rule (1869):
  • When an unsymmetrical reagent ($H-X$) adds to an unsymmetrical alkene, the negative part of the addendum (halide) bonds preferentially to the double-bonded carbon bearing fewer hydrogen atoms.
  • Mechanism: Electrophilic addition begins with the attack of a proton ($H^+$) to generate the more thermodynamically stable carbocation ($3^\circ > 2^\circ > 1^\circ$ via hyperconjugation and inductive effects). The nucleophilic halide then attacks this carbocation:     $CH_3-CH=CH_2 + HBr \rightarrow CH_3-CH(Br)-CH_3 \quad \text{(2-Bromopropane, major)}$
  • Anti-Markovnikov Addition (Peroxide / Kharash Effect):
  • Discovered by M.S. Kharash and F.R. Mayo (1933) at the University of Chicago.
  • In the presence of organic peroxides (e.g., benzoyl peroxide, $(C_6H_5CO)_2O_2$), the addition of $HBr$ exclusively proceeds contrary to Markovnikov's rule, generating 1-bromopropane:     $CH_3-CH=CH_2 + HBr \xrightarrow{(C_6H_5CO)_2O_2} CH_3-CH_2-CH_2Br \quad \text{(1-Bromopropane, major)}$
  • Mechanism:
    • Homolytic cleavage of peroxide initiates benzoyl radicals, yielding phenyl radicals:       $(C_6H_5COO)_2 \rightarrow 2C_6H_5COO\cdot \rightarrow 2\cdot C_6H_5 + 2CO_2$
    • Radical generation: Phenyl radicals abstract hydrogen from $HBr$ to yield a bromine free radical:       $\cdot C_6H_5 + H-Br \rightarrow C_6H_6 + \cdot Br$
    • Radical addition: $\cdot Br$ adds to propene forming a more stable secondary carbon radical:       $CH_3-CH=CH_2 + \cdot Br \rightarrow CH_3-\dot{C}H-CH_2Br$
    • Chain propagation: Hydrogen abstraction yields the anti-Markovnikov product and regenerates the bromine radical:       $CH_3-\dot{C}H-CH_2Br + HBr \rightarrow CH_3-CH_2-CH_2Br + \cdot Br$
  • Thermodynamic Exclusion of $HCl$ and $HI$:
    • $H-Cl$ bond dissociation energy ($430.5\,kJ\,mol^{-1}$) is too high to be cleaved homolytically by the peroxy radical.
    • $H-I$ bond is weaker ($296.8\,kJ\,mol^{-1}$) and easily cleaved, but iodine free radicals preferentially recombine into iodine molecules ($2\cdot I \rightarrow I_2$) rather than adding across the double bond.

• Addition of Sulphuric Acid and Hydration:

  • Cold, concentrated sulphuric acid adds via an electrophilic Markovnikov pathway to form alkyl hydrogen sulphates:   $CH_3-CH=CH_2 + H_2SO_4 \rightarrow CH_3-CH(OSO_3H)-CH_3$
  • Direct acid-catalyzed hydration in the presence of dilute mineral acids yields alcohols via carbocation intermediates:   $CH_3-CH=CH_2 + H_2O \xrightarrow{H^+} CH_3-CH(OH)-CH_3$

• Oxidation Reactions:

  • Hydroxylation (Baeyer's Test):
  • Reaction with cold, dilute, alkaline potassium permanganate ($KMnO_4$, Baeyer's reagent) yields vicinal glycols via syn-dihydroxylation. Decolorisation of the purple permanganate solution serves as a positive test for unsaturation:     $CH_2=CH_2 + H_2O + [O] \xrightarrow{\text{cold, dil. } KMnO_4} CH_2OH-CH_2OH \quad \text{(Ethane-1,2-diol)}$$CH_3-CH=CH_2 + H_2O + [O] \xrightarrow{\text{cold, dil. } KMnO_4} CH_3-CH(OH)-CH_2OH \quad \text{(Propane-1,2-diol)}$
  • Vigorous Acidic Oxidation:
  • Hot acidic potassium permanganate ($KMnO_4 / H^+$) or dichromate ($K_2Cr_2O_7 / H^+$) oxidatively cleaves the double bond into ketones and/or carboxylic acids:     $(CH_3)_2C=CH_2 + 2[O] \xrightarrow{KMnO_4 / H^+} (CH_3)_2C=O + CO_2 + H_2O$$CH_3-CH=CH-CH_3 + 4[O] \xrightarrow{KMnO_4 / H^+} 2CH_3COOH$

• Ozonolysis:

  • Addition of ozone ($O_3$) across the alkene forms a cyclic molozonide that rearranges to an ozonide.
  • Reductive cleavage of the ozonide with zinc dust and water ($Zn/H_2O$) prevents over-oxidation of aldehydes, generating carbonyl compounds:   $\text{Propene } (CH_3CH=CH_2) \xrightarrow{O_3} \text{Ozonide} \xrightarrow{Zn / H_2O} CH_3CHO + HCHO + ZnO$$\text{2-Methylpropene } ((CH_3)_2C=CH_2) \xrightarrow{O_3} \text{Ozonide} \xrightarrow{Zn / H_2O} (CH_3)_2C=O + HCHO + ZnO$
  • Ozonolysis is used to establish the exact structural location of double bonds in unknown hydrocarbons.

• Polymerisation Reactions:

  • High-temperature, high-pressure catalytic addition of alkene monomers yields synthetic polymers:   $n(CH_2=CH_2) \xrightarrow{\text{High } T, P, \text{ Cat.}} -(-CH_2-CH_2-)_n- \quad \text{(Polythene)}$$n(CH_3-CH=CH_2) \xrightarrow{\text{High } T, P, \text{ Cat.}} -(-CH(CH_3)-CH_2-)_n- \quad \text{(Polypropene)}$

Alkynes: Structure, Nomenclature, and Isomerism

• General Formula and Homology:
  • Alkynes contain at least one $C\equiv C$ triple bond and follow the general formula $C_n H_{2n-2}$.
  • The simplest member is ethyne ($C_2H_2$, commonly named acetylene), used industrially in oxyacetylene torches ($>3000^\circ C$) for metal welding and cutting.

• Structure and Electronic Character of the Triple Bond:
  • Each triply bonded carbon is $sp$ hybridized, producing a linear geometry with a $180^\circ$ bond angle.
  • Overlap Framework:
  • One strong $\sigma$ bond formed via coaxial overlap of two $sp$ hybrid orbitals.
  • Two mutually perpendicular $\pi$ bonds formed by sideways overlap of unhybridized $2p_y$ and $2p_z$ orbitals.
  • The resulting $\pi$-electron cloud forms a continuous, cylindrically symmetrical sheath enclosing the internuclear $C-C$ axis.
  • Physical parameters:
  • $C\equiv C$ bond enthalpy: $823\,kJ\,mol^{-1}$ (greater than $C=C$ at $681\,kJ\,mol^{-1}$ and $C-C$ at $348\,kJ\,mol^{-1}$).
  • $C\equiv C$ bond length: $120\,pm$ (significantly shorter than alkene $134\,pm$ and alkane $154\,pm$).

Preparation and Properties of Alkynes

• Industrial Synthesis from Calcium Carbide:

  • Thermal decomposition of limestone ($CaCO_3$) produces quicklime ($CaO$), which is reduced with coke in an electric arc furnace to yield calcium carbide ($CaC_2$):   $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$$CaO + 3C \rightarrow CaC_2 + CO$
  • Hydrolysis of calcium carbide produces pure ethyne gas:   $CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2$

• Dehydrohalogenation of Vicinal Dihalides:

  • Two-step elimination of hydrogen halides from vicinal dihaloalkanes:   $CH_2Br-CH_2Br \xrightarrow{\text{alc. } KOH, -HBr} CH_2=CHBr \xrightarrow{NaNH_2, -HBr} CH\equiv CH$
  • The second elimination step requires a stronger base, sodamide ($NaNH_2$ in liquid ammonia), because the vinylic halide intermediate resists elimination due to partial double-bond character from resonance.

• Acidity of Terminal Alkynes:

  • Terminal alkynes ($R-C\equiv C-H$) possess weakly acidic hydrogens.
  • Orbital Explanation: The $sp$ hybridized carbon has $50\%$ s-character (compared to $33.3\%$ in $sp^2$ and $25\%$ in $sp^3$). Electrons in $s$ orbitals are held closer to the positive nucleus, making the $sp$ carbon more electronegative. This polarizes the $C-H$ bond, facilitating proton removal by strong bases.
  • Reactions with Strong Bases:   $HC\equiv CH + Na \rightarrow HC\equiv C^-Na^+ + \frac{1}{2}H_2 \quad \text{(Monosodium ethynide)}$$HC\equiv C^-Na^+ + Na \rightarrow Na^+C^-\equiv C^-Na^+ + \frac{1}{2}H_2 \quad \text{(Disodium ethynide)}$$CH_3-C\equiv CH + NaNH_2 \rightarrow CH_3-C\equiv C^-Na^+ + NH_3$
  • Non-terminal alkynes (such as but-2-yne) lack terminal acetylenic protons and do not react with sodium metal or sodamide.
  • Relative Acidic Order:   $HC\equiv CH > CH_2=CH_2 > CH_3-CH_3$$HC\equiv CH > CH_3-C\equiv CH \gg CH_3-C\equiv C-CH_3$

• Addition Reactions of Alkynes:

  • Halogen Addition: Bromine in $CCl_4$ adds in two successive steps to produce vicinal tetrahaloalkanes; this reaction discharges the red bromine colour, serving as a test for unsaturation:   $HC\equiv CH + Br_2 \xrightarrow{CCl_4} CHBr=CHBr \xrightarrow{Br_2} CHBr_2-CHBr_2$
  • Hydrogen Halide Addition: Reacts via Markovnikov electrophilic addition in two stages, ultimately forming geminal dihalides (where both halogens attach to the same carbon):   $CH_3-C\equiv CH + HBr \rightarrow CH_3-C(Br)=CH_2 \xrightarrow{HBr} CH_3-C(Br)_2-CH_3 \quad \text{(2,2-Dibromopropane)}$
  • Mercuric-Catalyzed Hydration:
  • Alkynes add a single water molecule when warmed to $333\,K$ in dilute sulphuric acid with a mercuric sulphate catalyst ($HgSO_4 / H_2SO_4$).
  • The unstable enol intermediate rapidly tautomerizes to a stable carbonyl:     $HC\equiv CH + H_2O \xrightarrow{HgSO_4 / H_2SO_4, 333\,K} [CH_2=CH-OH] \rightleftharpoons CH_3-CHO \quad \text{(Ethanal)}$$CH_3-C\equiv CH + H_2O \xrightarrow{HgSO_4 / H_2SO_4, 333\,K} [CH_3-C(OH)=CH_2] \rightleftharpoons CH_3-CO-CH_3 \quad \text{(Propanone)}$

• Polymerisation Modes:

  • Linear Polymerisation: Acetylene polymerizes under specific conditions to form polyacetylene ($-(CH=CH-CH=CH)_n-$), a conjugated polyene that conducts electricity when doped, making it useful in thin-film battery electrodes.
  • Cyclic Trimerisation: Passing ethyne gas through red-hot iron tubes at $873\,K$ results in cyclic trimerization to benzene:   $3C_2H_2 \xrightarrow{\text{Red hot } Fe, 873\,K} C_6H_6$

Aromatic Hydrocarbons: Structure, Resonance, and Aromaticity

• Discovery and Nomenclature of Arenes:
  • Benzene was first isolated by Michael Faraday in 1825.
  • Aromatic hydrocarbons were originally termed "aromatic" (Greek aroma = pleasant-smelling) due to their distinct fragrances, though modern aromatic classification relies strictly on electronic structure.
  • Benzenoids possess one or more benzene rings (e.g., benzene, toluene, naphthalene, biphenyl, anthracene); non-benzenoids are aromatic systems lacking a benzene nucleus (e.g., azulene, tropylium ion).

• The Kekulé Model and Historical Anomalies:

  • In 1865, Friedrich August Kekulé proposed a dynamic equilibrium of cyclohexatriene structures with alternating single and double bonds.
  • Anomalies of the Classical Kekulé Hypothesis:
  • Kekulé's structure predicted two distinct 1,2-dibromobenzene isomers (one with the bromines linked across a double bond, and one across a single bond); experimentally, only one 1,2-disubstituted product is ever isolated.
  • Kekulé postulated an oscillating alternation of double bonds to explain this equivalency, but this model failed to account for benzene's unusual thermodynamic stability and its marked resistance to addition reactions.

• Resonance and Molecular Orbital Theory of Benzene:

  • According to Valence Bond Theory, benzene is a resonance hybrid of two equivalent contributing Kekulé structures along with minor Dewar structures.
  • Orbital Features:
  • All six carbon atoms are $sp^2$ hybridized, forming a planar hexagonal ring with interior bond angles of $120^\circ$.
  • Each carbon forms two $C-C\,\sigma$ bonds with neighbouring carbons and one $C-H\,\sigma$ bond with a hydrogen atom.
  • Six unhybridized $2p_z$ atomic orbitals lie perpendicular to the ring plane and overlap sideways equally with orbitals on adjacent carbons, forming continuous delocalized $\pi$ electron clouds above and below the ring.
  • X-ray Diffraction Data:
  • Demonstrates that all six carbon-carbon bond lengths in benzene are identical at $139\,pm$.
  • This value lies between the standard aliphatic single bond ($154\,pm$) and localized double bond ($134\,pm$), confirming the absence of isolated double bonds and explaining benzene's resistance to addition reactions.

• Hückel's Rule of Aromaticity:

  • For a compound to be classified as aromatic, it must satisfy four criteria:
  • Cyclic structure: Must consist of an unbroken ring.
  • Planarity: Atoms in the ring must lie in the same geometric plane to permit parallel orbital overlap.
  • Complete Delocalization: A continuous cycle of overlapping $p$-orbitals around the entire ring.
  • Hückel's $(4n + 2)\pi$ Electron Rule: The conjugated system must contain $(4n + 2)$ delocalized $\pi$ electrons, where $n$ is a non-negative integer ($n = 0, 1, 2, 3, \dots$).
  • Examples of Systems Fulfilling Hückel's Rule:
  • $n=0$ ($2\pi$ electrons): Cyclopropenyl cation.
  • $n=1$ ($6\pi$ electrons): Benzene, cyclopentadienyl anion, cycloheptatrienyl cation (tropylium ion), pyridine, pyrrole.
  • $n=2$ ($10\pi$ electrons): Naphthalene, bicyclic aromatic rings.
  • $n=3$ ($14\pi$ electrons): Anthracene, phenanthrene.

Preparation and Electrophilic Substitution Reactions of Arenes

• Methods of Preparation of Benzene:

  • Cyclic Trimerisation of Ethyne: Passing acetylene through red-hot iron tubes at $873\,K$.
  • Decarboxylation of Benzoic Acid Salts:     $C_6H_5COO^-Na^+ + NaOH \xrightarrow{CaO, \Delta} C_6H_6 + Na_2CO_3$
  • Reduction of Phenol:     $C_6H_5OH + Zn \xrightarrow{\Delta} C_6H_6 + ZnO$

• Mechanism of Electrophilic Aromatic Substitution ($S_E Ar$):

  • Electrophilic aromatic substitution proceeds via a common three-step pathway:
  • Step 1: Generation of the Electrophile ($E^+$):
    • Lewis acids like anhydrous $AlCl_3$ or $FeCl_3$ polarize reagent bonds to generate strong electrophiles ($Cl^+$, $R^+$, $R-C^+=O$).
  • Step 2: Formation of the Arenium Ion (Carbocation Intermediate / $\sigma$-complex):
    • The electrophile attacks the conjugated ring, using two $\pi$ electrons to form a new $C-E$ bond.
    • This produces a resonance-stabilized arenium ion ($\sigma$-complex) in which delocalization is interrupted at the tetrahedral $sp^3$ carbon, temporarily breaking aromaticity.
  • Step 3: Deprotonation to Restore Aromaticity:
    • A base (such as $[AlCl_4]^-$ or $[HSO_4]^-$) abstracts the proton from the $sp^3$ carbon, returning the electron pair to the ring to regenerate aromatic stability.

• Characteristic Electrophilic Substitution Reactions:

  • Nitration:
  • Reagents: Concentrated nitric acid ($HNO_3$) and concentrated sulphuric acid ($H_2SO_4$) (nitrating mixture) heated below $333\,K$:     $C_6H_6 + HNO_3 \xrightarrow{\text{conc. } H_2SO_4, \le 333\,K} C_6H_5NO_2 + H_2O$
  • Electrophile: Nitronium ion ($NO_2^+$), generated via an acid-base interaction where $H_2SO_4$ acts as a Brønsted acid and $HNO_3$ behaves as a base:     $HNO_3 + 2H_2SO_4 \rightleftharpoons NO_2^+ + H_3O^+ + 2HSO_4^-$
  • Halogenation:
  • Benzene reacts with chlorine or bromine in the presence of a Lewis acid ($FeCl_3$, $FeBr_3$, or anhydrous $AlCl_3$) in the dark:     $C_6H_6 + Cl_2 \xrightarrow{\text{Anhydrous } AlCl_3} C_6H_5Cl + HCl$
  • With excess chlorine and catalyst, all six ring hydrogens are replaced, forming hexachlorobenzene ($C_6Cl_6$).
  • Sulphonation:
  • Heating benzene with fuming sulphuric acid (oleum, $H_2S_2O_7$) introduces a sulphonic acid group via electrophilic sulfur trioxide ($SO_3$):     $C_6H_6 + SO_3 \xrightarrow{H_2SO_4, \Delta} C_6H_5SO_3H \quad \text{(Benzenesulphonic acid)}$
  • Friedel-Crafts Alkylation:
  • Reaction with an alkyl halide in the presence of anhydrous aluminium chloride:     $C_6H_6 + CH_3Cl \xrightarrow{\text{Anhydrous } AlCl_3} C_6H_5CH_3 + HCl \quad \text{(Toluene)}$
  • Carbocation Rearrangement: Alkylation of benzene with 1-chloropropane produces isopropylbenzene (cumene) rather than n-propylbenzene, because the initial primary carbocation rearranges to a more stable secondary carbocation via a 1,2-hydride shift:     $CH_3-CH_2-CH_2^+ \xrightarrow{1,2\text{-hydride shift}} CH_3-\overset{+}{C}H-CH_3$
  • Friedel-Crafts Acylation:
  • Treatment of benzene with an acyl halide or acid anhydride in the presence of anhydrous $AlCl_3$ yields acylbenzenes via acylium ions ($R-C^+=O$):     $C_6H_6 + CH_3COCl \xrightarrow{\text{Anhydrous } AlCl_3} C_6H_5COCH_3 + HCl \quad \text{(Acetophenone)}$$C_6H_6 + (CH_3CO)_2O \xrightarrow{\text{Anhydrous } AlCl_3} C_6H_5COCH_3 + CH_3COOH$

• Addition and Combustion Reactions of Benzene:

  • Catalytic Hydrogenation: Vigorous reaction with hydrogen over nickel at high temperature and pressure yields cyclohexane:   $C_6H_6 + 3H_2 \xrightarrow{Ni, \Delta, P} C_6H_{12} \quad \text{(Cyclohexane)}$
  • Photochemical Halogenation: Benzene adds three molecules of chlorine under ultraviolet light to form benzene hexachloride (BHC, gammaxane, lindane, $C_6H_6Cl_6$):   $C_6H_6 + 3Cl_2 \xrightarrow{h\nu} C_6H_6Cl_6$

• Combustion: Burns with a characteristically smoky, sooty flame due to high carbon content: $C_6H_6 + \frac{15}{2}O_2 \rightarrow 6CO_2 + 3H_2O$

• General Combustion Equation for Any Hydrocarbon: $C_x H_y + \left(x + \frac{y}{4}\right)O_2 \rightarrow x CO_2 + \frac{y}{2}H_2O$