Comprehensive Study Notes on Hydrocarbons: Alkanes, Alkenes, Alkynes, and Aromatic Compounds

Introduction and Importance of Hydrocarbons

• Definition: Hydrocarbons are compounds composed exclusively of carbon and hydrogen.

• Daily Life Role and Energy Sources:

  • LPG (Liquefied Petroleum Gas): Used as a domestic fuel; known for generating the least pollution.

  • CNG (Compressed Natural Gas): Formed by compressing natural gas; used as a cleaner fuel for automobiles.

  • LNG (Liquefied Natural Gas): Obtained through the liquefaction of natural gas; currently a significant fuel source.

  • Petroleum Products: Petrol, diesel, and kerosene oil are obtained via fractional distillation of petroleum found in the earth's crust.

  • Coal Gas: Produced through the destructive distillation of coal.

  • Natural Gas: Found in upper geographical strata during oil well drilling.

• Industrial Applications:

  • Polymers: Used in the manufacture of polythene, polypropene, and polystyrene.

  • Solvents: Higher hydrocarbons serve as solvents for paints.

  • Chemical Synthesis: Starting materials for the manufacture of many dyes and drugs.

Classification of Hydrocarbons

• Saturated Hydrocarbons: Contain only carbon-carbon ($C-C$) and carbon-hydrogen ($C-H$) single bonds.

  • Alkanes: Open-chain saturated hydrocarbons.

  • Cycloalkanes: Saturated hydrocarbons in which carbon atoms form a ring or closed chain.

• Unsaturated Hydrocarbons: Contain carbon-carbon multiple bonds ($C=C$ double bonds, $C\equiv C$ triple bonds, or both).

• Aromatic Hydrocarbons: A special category of cyclic compounds featuring unique stability and characteristics (benzenoids and non-benzenoids).

Alkanes

• General Characteristics: Earlier known as paraffins (Latin: parum = little, affinis = affinity) because they are relatively inert under normal conditions, reacting minimally with acids, bases, or other reagents.

• General Formula: $C_nH_{2n+2}$.

• Structure of Methane ($CH_4$):

  • Tetrahedral structure according to VSEPR theory.

  • The carbon atom is at the center, and four hydrogen atoms occupy the corners of a regular tetrahedron.

  • Bond Angle: All $H-C-H$ bond angles are $109.5^{\circ}$.

  • Bond Lengths: $C-C$ is $154\,pm$; $C-H$ is $112\,pm$.

  • Bonding: Carbon uses $sp^3$ hybrid orbitals to overlap head-on with the $1s$ orbitals of hydrogen, forming sigma ($\sigma$) bonds.

Nomenclature and Isomerism in Alkanes

• Homologous Series: Methane ($CH_4$), Ethane ($C_2H_6$), Propane ($C_3H_8$), etc. Each member differs from the next by a $-CH_2$ group.

• Structural Isomerism: Compounds with the same molecular formula but different structures.

  • Chain Isomers: Isomers that differ in the arrangement of the carbon chain (e.g., straight vs. branched).

  • Examples:

    • Butane ($C_4H_{10}$): Has 2 isomers ($n-butane$ and isobutane/2-methylpropane).

    • Pentane ($C_5H_{12}$): Has 3 isomers ($n-pentane$, isopentane/2-methylbutane, and neopentane/2,2-dimethylpropane).

    • Hexane ($C_6H_{14}$): Has 5 isomers.

    • Heptane ($C_7H_{16}$): Has 9 isomers.

    • Decane ($C_{10}H_{22}$): Possible isomers reach 75.

• Classification of Carbons:

  • Primary ($1^{\circ}$): Attached to no other carbon or one other carbon atom.

  • Secondary ($2^{\circ}$): Attached to two carbon atoms.

  • Tertiary ($3^{\circ}$): Attached to three carbon atoms.

  • Quaternary ($4^{\circ}$): Attached to four carbon atoms.

• Alkyl Groups: Derived from alkanes by removing one hydrogen atom. General formula: $C_nH_{2n+1}$. Examples include methyl ($-CH_3$), ethyl ($-C_2H_5$), and propyl ($-C_3H_7$).

Preparation of Alkanes

1. From Unsaturated Hydrocarbons (Hydrogenation):

   • Dihydrogen gas adds to alkenes or alkynes in the presence of catalyst (Pt, Pd, or Ni).

   • Pt and Pd work at room temperature; Ni requires higher temperature and pressure.

   • Reaction: $CH_2=CH_2 + H_2 \rightarrow CH_3-CH_3$.

2. From Alkyl Halides:

   • Reduction: Alkyl halides (except fluorides) reduced with Zn and dilute HCl. Example: $CH_3-Cl + H_2 \xrightarrow{Zn, H^+} CH_4 + HCl$.

   • Wurtz Reaction: Alkyl halides treated with sodium metal in dry ether to produce higher alkanes with an even number of carbon atoms. Example: $2CH_3Br + 2Na \xrightarrow{dry\,ether} C_2H_6 + 2NaBr$.

3. From Carboxylic Acids:

   • Decarboxylation: Heating sodium salts of carboxylic acids with soda lime ($NaOH$ and $CaO$ in a 3:1 ratio). The product has one less carbon than the parent acid. Example: $CH_3COONa + NaOH \xrightarrow{CaO, \Delta} CH_4 + Na_2CO_3$.

   • Kolbe's Electrolytic Method: Electrolysis of aqueous sodium or potassium salts of carboxylic acids produces alkanes at the anode. Methane cannot be prepared via this method.

Properties of Alkanes

Physical Properties

• Polarity: Almost non-polar due to covalent bonds and minimal electronegativity difference between C and H.

• Forces: Weak van der Waals forces.

• Physical State (at 298 K):

  • $C_1$ to $C_4$: Gases.

  • $C_5$ to $C_{17}$: Liquids.

  • $C_{18}$ and above: Solids.

• Solubility: Insoluble in water (hydrophobic); soluble in non-polar solvents like petroleum ether or benzene ("like dissolves like").

• Boiling Point: Increases steadily with molecular mass due to increasing van der Waals forces. Branching decreases surface area, making the molecule more spherical, thus decreasing the boiling point (e.g., $n-pentane$ b.p. $309.1\,K$ vs. $2,2-dimethylpropane$ b.p. $282.5\,K$).

Chemical Properties

1. Substitution Reactions (Halogenation):

   • Occurs at high temperatures ($573-773\,K$) or under UV light/diffused sunlight.

   • Rate of hydrogen replacement: 3^{\circ} > 2^{\circ} > 1^{\circ}.

   • Halogen reactivity: F_2 > Cl_2 > Br_2 > I_2. Fluorination is violent; Iodination is reversible and requires oxidizing agents like $HIO_3$ or $HNO_3$.

   • Mechanism: Free radical chain mechanism (Steps: Initiation, Propagation, Termination).

2. Combustion:

   • Complete oxidation in air/oxygen: $C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O$.

   • Produces large amounts of heat ($\Delta_c H$ for methane = $-890\,kJ\,mol^{-1}$).

   • Incomplete combustion produces carbon black (used in inks, pigments, and filters).

3. Controlled Oxidation:

   • Methane with $Cu$ at $523\,K$/$100\,atm$ yields methanol ($CH_3OH$).

   • Methane with $Mo_2O_3$ yields methanal ($HCHO$).

   • Alkanes with tertiary H atoms can be oxidized to alcohols by $KMnO_4$.

4. Isomerization: Heating $n-alkanes$ with anhydrous $AlCl_3$ and $HCl$ gas converts them to branched isomers.

5. Aromatization (Reforming): $n-alkanes$ with 6+ carbons heated to $773\,K$ at $10-20\,atm$ with oxides of V, Mo, or Cr over alumina produce benzene or its derivatives.

6. Reaction with Steam: Methane reacts with steam at $1273\,K$ over Ni to produce carbon monoxide and $H_2$ (industrial preparation of dihydrogen).

7. Pyrolysis (Cracking): Higher alkanes decompose into lower alkanes and alkenes upon heating. Dodecane ($C_{12}H_{26}$) at $973\,K$ over Pt/Pd/Ni yields heptane and pentene.

Conformations of Ethane

• Definition: Infinite spatial arrangements arising from rotation around the $C-C$ sigma bond. Also called conformers or rotamers.

• Hindrance: Rotation is not completely free; hindered by a small energy barrier ($1-20\,kJ\,mol^{-1}$) called torsional strain.

• Extreme Types:

  • Eclipsed: Hydrogen atoms of both carbons are as close as possible. Maximum repulsion (torsional strain), minimum stability.

  • Staggered: Hydrogen atoms are as far apart as possible. Minimum repulsion, maximum stability.

  • Skew: Any intermediate conformation.

• Representations:

  • Sawhorse Projections: Central bond drawn as a long tilted line.

  • Newman Projections: Molecule viewed head-on. Front carbon is a point; rear carbon is a circle.

• Stability: Staggered form is more stable. The energy difference in ethane is only $12.5\,kJ\,mol^{-1}$, making rotation practically free at room temperature.

Alkenes

• General Characteristics: Unsaturated hydrocarbons with at least one double bond. Also called olefins (oil-forming).

• General Formula: $C_nH_{2n}$.

• Structure of Double Bond:

  • Consists of one strong sigma ($\sigma$) bond ($sp^2-sp^2$ overlap, $397\,kJ\,mol^{-1}$) and one weak pi ($\pi$) bond (lateral orbital overlap, $284\,kJ\,mol^{-1}$).

  • Bond Length: $C=C$ is $134\,pm$ (shorter than alkane $154\,pm$).

  • The $\pi$ cloud makes alkenes susceptible to electrophilic reagents.

Isomerism in Alkenes

• Structural Isomerism: Includes chain and position isomerism.

• Geometrical Isomerism: Arises due to restricted rotation around the double bond.

  • Cis Isomer: Identical groups on the same side of the double bond. Generally has a higher dipole moment (e.g., $cis-but-2-ene$ = $0.33\,D$).

  • Trans Isomer: Identical groups on opposite sides. Generally has zero or near-zero dipole moment and higher melting point.

Preparation of Alkenes

1. From Alkynes: Partial reduction with dihydrogen.

   • Lindlar's Catalyst (Palladized charcoal deactivated with sulfur/quinoline) yields cis-alkenes.

   • Sodium in Liquid Ammonia yields trans-alkenes.

2. From Alkyl Halides: Dehydrohalogenation ($\beta$-elimination) using alcoholic potash ($KOH$). Rate: I > Br > Cl; 3^{\circ} > 2^{\circ} > 1^{\circ}.

3. From Vicinal Dihalides: Dehalogenation using zinc metal.

4. From Alcohols: Acidic dehydration using concentrated $H_2SO_4$.

Properties of Alkenes

• Addition Reactions:

  • Addition of Halogens: Forms vicinal dihalides. Bromine in $CCl_4$ (reddish orange) decolorizes on addition to double bonds (test for unsaturation).

  • Addition of Hydrogen Halides ($HX$): Follows Markovnikov Rule (the negative part of the addendum attaches to the carbon with fewer hydrogens).

  • Peroxide Effect (Kharash Effect): In the presence of organic peroxides, $HBr$ (only) adds to unsymmetrical alkenes via Anti-Markovnikov addition.

  • Addition of Water: Reacts with water in acid to form alcohols (Markovnikov).

• Oxidation:

  • Baeyer's Reagent (cold, dilute $KMnO_4$): Produces vicinal glycols; decolorization is a test for unsaturation.

  • Acidic $KMnO_4$: Oxidizes alkenes to ketones or acids based on structure.

• Ozonolysis: Addition of ozone followed by cleavage with $Zn/H_2O$ gives aldehydes/ketones. Used to locate the double bond.

• Polymerisation: Ethene forms polythene; propene forms polypropene.

Alkynes

• General Characteristics: Contain at least one triple bond. General Formula: $C_nH_{2n-2}$.

• Structure of Triple Bond:

  • One sigma bond ($sp-sp$ overlap) and two pi bonds (lateral overlap of unhybridized $p$ orbitals).

  • Bond Length: $C\equiv C$ is $120\,pm$.

  • Bond Angle: $180^{\circ}$ (linear geometry).

Preparation of Alkynes

1. From Calcium Carbide: $CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2$.

2. From Vicinal Dihalides: Double dehydrohalogenation using alcoholic $KOH$ followed by sodamide ($NaNH_2$).

Properties of Alkynes

• Acidic Nature: Terminal hydrogens in ethyne and propyne are acidic due to high $s$-character (50%) of $sp$ hybrid orbitals. They react with sodium or sodamide to form acetylides.

• Addition Reactions: Add two molecules of $H_2, X_2,$ or $HX$ (Markovnikov). Water adds in the presence of $HgSO_4$ and $H_2SO_4$ to form carbonyl compounds.

• Polymerisation:

  • Linear: Polyacetylene/Polyethyne (conducts electricity).

  • Cyclic: Three molecules of ethyne passed through a red hot iron tube at $873\,K$ form benzene.

Aromatic Hydrocarbons (Arenes)

• Classification: Benzenoids (contain benzene ring) and Non-benzenoids.

• Structure of Benzene ($C_6H_6$):

  • Proposed by August Kekul\u00e9 (1865) as a cyclic hexagon with alternating single and double bonds.

  • Modern view: Resonance hybrid; all $C-C$ bond lengths are equal at $139\,pm$.

• Aromaticity (H\u00fcckel Rule): To be aromatic, a compound must be planar, have complete delocalization of $\pi$ electrons, and possess $(4n+2) \pi$ electrons.

Chemical Properties of Benzene

• Electrophilic Substitution Reactions:

  • Nitration: Conc. $HNO_3$ + Conc. $H_2SO_4$ (nitrating mixture) introduces $-NO_2$.

  • Halogenation: Halogen + Lewis acid ($FeCl_3, AlCl_3$).

  • Sulphonation: Fuming sulfuric acid (oleum).

  • Friedel-Crafts Alkylation: Alkyl halide + anhydrous $AlCl_3$.

  • Friedel-Crafts Acylation: Acyl halide + anhydrous $AlCl_3$.

• Mechanism: Involves three steps: (1) Generation of electrophile ($E^+$), (2) Formation of arenium ion/sigma complex (resonance stabilized), (3) Removal of proton to restore aromaticity.

Directive Influence of Substituents

• Ortho and Para Directing Groups: Activate the ring (except halogens) and direct incoming groups to $o-$ and $p-$ positions (e.g., $-OH, -NH_2, -CH_3$).

• Meta Directing Groups: Deactivate the ring and direct incoming groups to meta position (e.g., $-NO_2, -CN, -CHO, -COOH$).

Carcinogenicity and Toxicity

• Benzene and polynuclear hydrocarbons with fused rings (formed by incomplete combustion of tobacco, coal, or petroleum) are carcinogenic.

• They undergo biochemical transformations in the body that can damage DNA and cause cancer.