Hydrocarbons

1. Classification & Alkanes: Nomenclature, Preparation

Hydrocarbons are compounds of carbon and hydrogen only, and are the major sources of energy (LPG, CNG, petrol, diesel, kerosene) and the starting materials for polymers, dyes and drugs. Based on the type of carbon–carbon bonds present, hydrocarbons are classified into:

• Saturated hydrocarbons (alkanes): contain only C–C and C–H single bonds; open-chain alkanes have general formula CₙH₂ₙ₊₂; ring forms are called cycloalkanes.
• Unsaturated hydrocarbons (alkenes and alkynes): contain at least one carbon–carbon double bond (alkenes, CₙH₂ₙ) or triple bond (alkynes, CₙH₂ₙ₋₂).
• Aromatic hydrocarbons: a special class of (usually) cyclic compounds showing unique stability (arenes).

Alkane nomenclature and isomerism: methane (CH₄), ethane (C₂H₆) and propane (C₃H₈) have only one possible structure each, but from C₄H₁₀ onward, chain isomers are possible — n-butane and isobutane (2-methylpropane) are the two chain isomers of C₄H₁₀; C₅H₁₂ has three (pentane, isopentane, neopentane); C₆H₁₄ has five; C₇H₁₆ has nine; and C₁₀H₂₂ has as many as 75 isomers. Carbon atoms are classified by how many other carbons they are bonded to: primary (1°) — bonded to one other carbon (or none, as in methane); secondary (2°) — bonded to two; tertiary (3°) — bonded to three; quaternary (4°) — bonded to four other carbons.

Preparation of alkanes:

1. From unsaturated hydrocarbons (hydrogenation): alkenes/alkynes + H₂ in the presence of finely divided Pt, Pd or Ni catalyst → alkanes. Platinum and palladium work at room temperature; nickel requires higher temperature and pressure.
2. From alkyl halides: (i) reduction with zinc and dilute HCl (except for fluorides); (ii) Wurtz reaction — alkyl halide + sodium metal in dry ether → higher alkane with an EVEN number of carbon atoms (2R–X + 2Na → R–R + 2NaX).
3. From carboxylic acids: (i) decarboxylation — sodium salt of a carboxylic acid heated with soda lime (NaOH + CaO) gives an alkane with ONE CARBON LESS than the acid; (ii) Kolbe's electrolytic method — electrolysis of an aqueous solution of a sodium/potassium salt of a carboxylic acid; at the anode, the carboxylate radical loses CO₂ to form an alkyl free radical, and two such radicals combine to give an alkane with an even number of carbons (methane cannot be prepared by this method).

2. Alkane Reactions & Conformations

Physical properties: alkanes are almost non-polar; the first four (C₁–C₄) are gases, C₅–C₁₇ are liquids, C₁₈ and above are solids at 298 K; boiling point increases steadily with molecular mass (increasing van der Waals forces), while for isomers, more branching lowers the boiling point (a more spherical shape means smaller surface area of contact and weaker intermolecular forces).

Chemical properties (alkanes are generally inert but undergo):

1. Substitution (halogenation): one or more H atoms are replaced by halogen, e.g. CH₄ + Cl₂ →(hv) CH₃Cl + HCl (and further to CH₂Cl₂, CHCl₃, CCl₄). Reactivity order of halogens: F₂ > Cl₂ > Br₂ > I₂ (fluorination is too violent to control; iodination is reversible and needs an oxidising agent like HIO₃). Ease of replacement of hydrogens: 3° > 2° > 1°. Halogenation proceeds by a free radical chain mechanism: initiation (Cl–Cl homolysis by light/heat to give Cl• radicals), propagation (Cl• abstracts H from CH₄ to give •CH₃ + HCl; •CH₃ then attacks Cl₂ to give CH₃Cl + Cl•, repeating the cycle), and termination (two radicals combine, e.g. Cl• + Cl• → Cl₂, or •CH₃ + •CH₃ → CH₃CH₃, which explains ethane as a byproduct).
2. Combustion: complete combustion in excess O₂ gives CO₂ + H₂O with large heat evolution (general equation: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O); incomplete combustion (insufficient air) gives carbon black.
3. Controlled oxidation: with a regulated O₂/catalyst supply, alkanes give alcohols, aldehydes, or carboxylic acids; alkanes with a tertiary hydrogen can be oxidised directly to tertiary alcohols by KMnO₄.
4. Isomerisation: n-alkanes heated with anhydrous AlCl₃/HCl gas rearrange to branched-chain isomers.
5. Aromatization: n-alkanes with 6 or more carbons, heated at 773 K and 10–20 atm over Cr₂O₃/V₂O₅/Mo₂O₃ catalyst, undergo dehydrogenation and cyclisation to benzene and its homologues.
6. Reaction with steam: methane + steam at 1273 K over Ni catalyst → CO + 3H₂ (industrial route to dihydrogen).
7. Pyrolysis (cracking): higher alkanes decompose into smaller alkanes/alkenes on strong heating — a free radical reaction, and the basis for producing petrol/oil gas from kerosene.

Conformations: rotation about a C–C single bond, though hindered by a small energy barrier (torsional strain, 1–20 kJ/mol) due to weak repulsive interactions, is fast enough at ordinary temperature that conformers of ethane cannot be isolated separately. The two extreme conformations are eclipsed (hydrogens on adjacent carbons directly aligned; maximum torsional strain, least stable) and staggered (hydrogens maximally apart; minimum torsional strain, most stable — the preferred conformation). Conformations are depicted using Sawhorse projections (viewed along the molecular axis, both carbons shown as points on a diagonal line) or Newman projections (viewed head-on along the C–C bond; front carbon as a point, rear carbon as a circle).

3. Alkenes: Structure, Nomenclature & Isomerism

Alkenes (general formula CₙH₂ₙ, also called olefins) contain at least one C=C double bond. The double bond consists of one strong σ bond (sp²–sp² head-on overlap, bond enthalpy ~397 kJ/mol) and one weaker π bond (sideways 2p–2p overlap, bond enthalpy ~284 kJ/mol); the C=C bond (134 pm) is shorter than a C–C single bond (154 pm) but its total bond enthalpy (681 kJ/mol) exceeds that of a single C–C bond (348 kJ/mol). Because the π bond holds its electrons loosely, alkenes are attacked readily by electrophilic reagents.

Nomenclature: the longest chain containing the double bond is chosen as parent, numbered from the end nearer the double bond, and the suffix '-ene' replaces '-ane'.

Isomerism: alkenes show both structural isomerism (chain and position isomerism, e.g. the three structures of C₄H₈: but-1-ene, but-2-ene, 2-methylprop-1-ene) and geometrical (cis-trans) isomerism. When each doubly-bonded carbon bears two DIFFERENT groups, restricted rotation about C=C creates two distinct spatial arrangements: the cis isomer (two identical/similar groups on the same side) and the trans isomer (on opposite sides). Cis and trans isomers differ in physical properties: the cis form is generally more polar (has a net dipole moment) while the trans form has zero or a much smaller dipole moment (bond dipoles cancel by symmetry); in solids, the trans isomer usually has a higher melting point.

4. Alkene Preparation & Addition Reactions

Preparation:

1. From alkynes: partial hydrogenation with H₂ over Lindlar's catalyst (Pd partially deactivated with a sulphur compound/quinoline) gives the CIS alkene; reduction with Na in liquid ammonia gives the TRANS alkene.
2. From alkyl halides: heating with alcoholic KOH causes dehydrohalogenation (a β-elimination reaction — H is removed from the β-carbon, X from the carbon bearing the halogen). Rate: for halogens, I > Br > Cl; for alkyl groups, 3° > 2° > 1°.
3. From vicinal dihalides: treatment with zinc metal causes dehalogenation (loss of ZnX₂) to give the alkene.
4. From alcohols (acidic dehydration): heating with concentrated H₂SO₄ eliminates a water molecule (also a β-elimination reaction).

Addition reactions (electrophilic addition, via the loosely-held π electrons):

• Dihydrogen: with Ni/Pd/Pt catalyst → alkane.
• Halogens: Br₂/Cl₂ add to give vicinal dihalides; decolorisation of bromine water/CCl₄ solution is used as a test for unsaturation. Iodine does not add under normal conditions.
• Hydrogen halides: HI > HBr > HCl in reactivity; addition to a symmetrical alkene is straightforward, but to an UNSYMMETRICAL alkene follows Markovnikov's rule (see below).
• Sulphuric acid: cold conc. H₂SO₄ adds via Markovnikov's rule to form an alkyl hydrogen sulphate.
• Water: in the presence of a few drops of conc. H₂SO₄, water adds via Markovnikov's rule to give an alcohol.

5. Markovnikov's Rule & the Peroxide Effect

Markovnikov's rule (1869): when an unsymmetrical reagent HX adds to an unsymmetrical alkene, the negative part (X) of the addendum attaches to the carbon atom bearing the LESSER number of hydrogen atoms. Mechanistically, the electrophile H⁺ attacks the double bond to generate the MORE STABLE carbocation (e.g. the secondary carbocation from propene, rather than the primary), and this more stable carbocation is then attacked by X⁻ to give the major product.

Anti-Markovnikov addition / peroxide effect (Kharash effect): in the presence of peroxides, HBr (and ONLY HBr — not HCl or HI) adds to an unsymmetrical alkene contrary to Markovnikov's rule, via a free-radical chain mechanism: peroxide homolyses to generate radicals that abstract H from HBr to form a bromine radical, Br•, which then adds to the LESS substituted carbon of the double bond (to generate the more stable free radical, following the same "more stable intermediate" logic as Markovnikov's rule but now for radicals, not cations). This does not occur with HCl (the H–Cl bond, at 430.5 kJ/mol, is too strong for the radical chain to cleave) or with HI (the iodine atoms produced simply recombine rather than adding to the alkene, since the H–I bond at 296.8 kJ/mol is weak but the C–I bond formed would be similarly weak/reversible).

6. Oxidation & Ozonolysis of Alkenes

• Oxidation (Baeyer's reagent): cold, dilute, aqueous KMnO₄ converts alkenes to vicinal glycols (diols); decolorisation of KMnO₄ is also used as a test for unsaturation. Hot/acidic (concentrated) KMnO₄ or acidic K₂Cr₂O₇ cleaves the double bond entirely, giving ketones and/or carboxylic acids depending on the substitution pattern.
• Ozonolysis: ozone adds across the double bond to form an ozonide, which is then cleaved by Zn/H₂O (a reductive workup, preventing further oxidation) into two smaller carbonyl fragments (aldehydes and/or ketones). This reaction is extremely useful for locating the position of a double bond in an unknown alkene, since the carbon skeleton on each side of the original C=C can be reconstructed from the identity of the carbonyl products.
• Polymerisation: alkenes undergo addition polymerisation under high temperature/pressure with a catalyst — many small monomer units join to form large-molecular-mass polymers, e.g. ethene → polythene; propene → polypropene.

7. Alkynes: Structure, Preparation & Reactions

Alkynes (general formula CₙH₂ₙ₋₂) contain at least one C≡C triple bond, consisting of one sp–sp σ bond and two mutually perpendicular π bonds (bond enthalpy 823 kJ/mol, the strongest of the three; bond length 120 pm, the shortest).

Preparation: (1) from calcium carbide (CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂, industrial route to ethyne); (2) from vicinal dihalides by double dehydrohalogenation with alcoholic KOH followed by NaNH₂.

Acidic character: because sp-hybridised carbon has the highest s-character (50%) and hence the highest electronegativity, the C–H bond in terminal alkynes (≡C–H) holds its electrons more tightly, making that hydrogen appreciably ACIDIC — alkynes with a terminal triple bond react with strong bases like sodium metal or sodamide (NaNH₂) to form sodium/metal acetylides with the liberation of H₂. This behaviour is NOT shown by alkenes or alkanes and is used to distinguish terminal alkynes from internal alkynes (like but-2-yne) and from alkenes/alkanes. Acidic strength trend: HC≡CH > H₂C=CH₂ > CH₃–CH₃.

Addition reactions: alkynes add TWO molecules of dihydrogen, halogens, or hydrogen halides (following Markovnikov's rule for unsymmetrical alkynes) since two π bonds are available; water adds only in the presence of Hg²⁺/H⁺ catalyst at 333 K, giving (after tautomeric rearrangement) a carbonyl compound (ethyne → ethanal; other terminal alkynes → methyl ketones, via Markovnikov addition).

Polymerisation: (a) linear polymerisation of ethyne gives polyacetylene (used as electrodes in batteries, since it can conduct electricity); (b) cyclic polymerisation — passing ethyne through a red-hot iron tube at 873 K causes three molecules to cyclise into BENZENE, the single most important laboratory route from an aliphatic to an aromatic compound.

8. Aromatic Hydrocarbons & the Structure of Benzene

Aromatic hydrocarbons ("arenes") are named for their pleasant aroma; those containing a benzene ring are benzenoid, others (like tropone) are non-benzenoid. Benzene (C₆H₆) shows a high degree of unsaturation on paper, but its properties do not match those expected of a normal triene.

Kekulé's structure (1865) proposed a cyclic hexagon of six carbons with alternating single and double bonds, but this predicts TWO different 1,2-disubstituted isomers (depending on whether substituents attach to doubly- or singly-bonded carbons), while experimentally only ONE ortho-disubstituted product is ever found. Kekulé patched this with the idea of rapidly "oscillating" double bonds, but the modern and correct explanation is resonance: benzene is a resonance HYBRID of the two Kekulé structures (and other minor contributing structures), not oscillating between them.

Orbital picture: all six carbons are sp² hybridised, forming the hexagonal ring of C–C σ bonds and six C–H σ bonds, each carbon left with one unhybridised p orbital perpendicular to the ring. These six p orbitals overlap sideways equally in both directions around the ring, forming two continuous ring-shaped (doughnut) clouds of delocalised π electron density, one above and one below the plane of the ring — not three isolated, localised π bonds. X-ray diffraction confirms all six C–C bond lengths are identical (139 pm, intermediate between a C–C single bond at 154 pm and a C=C double bond at 133 pm), and the molecule is planar — direct evidence against the Kekulé picture of alternating bonds and in favour of complete delocalisation.

9. Aromaticity & the Hückel Rule

Aromaticity (originally applied only to benzene) is now applied to any ring system, benzenoid or not, that satisfies three conditions: (i) planarity; (ii) complete delocalisation of the π electrons in the ring; (iii) presence of (4n+2) π electrons in the ring, where n = 0, 1, 2, ... (Hückel's rule). Examples: benzene (n=1, 6π electrons), the cyclopentadienyl anion and cycloheptatrienyl cation (n=1, 6π electrons each, despite not being six-membered all-carbon rings), naphthalene (n=2, 10π electrons), anthracene and phenanthrene (n=3, 14π electrons each).

Preparation of benzene: (i) cyclic polymerisation of ethyne; (ii) decarboxylation of sodium benzoate with sodalime; (iii) reduction of phenol vapours passed over heated zinc dust.

10. Electrophilic Substitution Reactions of Benzene

Because addition would destroy the aromatic stabilisation, benzene strongly prefers electrophilic substitution, which restores the aromatic sextet after the reaction. Common substitution reactions:

• Nitration: benzene + conc. HNO₃/conc. H₂SO₄ (nitrating mixture) at 323–333 K → nitrobenzene. The electrophile is the nitronium ion, NO₂⁺, generated by an acid–base reaction in which H₂SO₄ protonates HNO₃ (H₂SO₄ acts as the acid, HNO₃ as the base).
• Halogenation: with Cl₂/Br₂ in the presence of a Lewis acid (anhydrous FeCl₃, FeBr₃ or AlCl₃) → haloarene.
• Sulphonation: heating with fuming sulphuric acid (oleum) → benzene sulphonic acid.
• Friedel–Crafts alkylation: benzene + alkyl halide + anhydrous AlCl₃ → alkylbenzene.
• Friedel–Crafts acylation: benzene + acyl halide/acid anhydride + anhydrous AlCl₃ → acyl benzene (a ketone), e.g. with acetyl chloride → acetophenone.

Mechanism (3 steps): (a) generation of the electrophile (E⁺, e.g. Cl⁺, R⁺, or NO₂⁺ — usually via a Lewis acid like AlCl₃ combining with the attacking reagent); (b) formation of the carbocation intermediate (arenium ion/σ-complex) — the electrophile attacks the ring, one ring carbon becomes sp³ hybridised, and the resulting positive charge is delocalised (resonance-stabilised) over the remaining ring carbons; (c) removal of a proton from the sp³ carbon (by [AlCl₄]⁻ or [HSO₄]⁻) restores the aromatic ring and full conjugation.

11. Directive Influence of Substituents

When a monosubstituted benzene undergoes further substitution, the group ALREADY on the ring (not the nature of the incoming group) determines whether the new substituent goes predominantly to the ortho/para or the meta position.

• Ortho/para-directing (activating) groups: e.g. –OH, –NH₂, –NHR, –NHCOCH₃, –OCH₃, –CH₃, –C₂H₅ — these are electron-donating by resonance (+R effect), increasing electron density specifically at the ortho and para positions (shown by drawing the resonance structures of, e.g., phenol), making the ring MORE reactive towards electrophiles at those positions.
• Halogens are a special case: moderately DEACTivating overall (due to a strong –I effect that withdraws electron density from the whole ring) but still ortho/para-DIRECTING (because resonance donation of a lone pair still concentrates the remaining electron density preferentially at the ortho/para positions relative to meta).
• Meta-directing (deactivating) groups: e.g. –NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H — these withdraw electron density from the ring by resonance (–R effect), leaving the meta position comparatively more electron-rich than ortho/para, so the electrophile attacks there instead.

12. Carcinogenicity & Toxicity

Benzene and polynuclear hydrocarbons containing more than two fused benzene rings (e.g. 1,2-benzanthracene, 1,2-benzpyrene, 1,2,5,6-dibenzanthracene, 3-methylcholanthrene) are toxic and possess carcinogenic (cancer-causing) properties. They form during the incomplete combustion of organic materials such as tobacco, coal and petroleum, enter the human body, undergo biochemical transformation, and finally damage DNA, causing cancer.