Chapter 9: Hydrocarbons | Summary

Alkanes are saturated open-chain hydrocarbons with the general formula CnH2n+2. Methane (CH4) is the first member. In alkanes, carbon atoms are sp3 hybridised and adopt a tetrahedral geometry with H-C-H bond angles of 109.5°. C-C and C-H bond lengths are 154 pm and 112 pm respectively. Alkanes were formerly called paraffins due to their chemical inertness.

Higher alkanes exhibit structural isomerism. C4H10 has two chain isomers (n-butane and 2-methylpropane), and C5H12 has three. Carbon atoms in alkanes are classified as primary (1°), secondary (2°), tertiary (3°), or quaternary (4°) based on the number of other carbon atoms attached to them. IUPAC naming follows the longest chain rule with the lowest possible locants for substituents.

Alkanes are prepared by hydrogenation of unsaturated hydrocarbons (using Pt, Pd, or Ni catalysts), reduction of alkyl halides with Zn/HCl, Wurtz reaction (treatment of alkyl halides with sodium in dry ether to give higher even-carbon alkanes), decarboxylation of sodium salts of carboxylic acids with soda lime, and Kolbe's electrolytic method.

Alkanes are non-polar due to the covalent nature of C-C and C-H bonds, and thus insoluble in water but soluble in non-polar solvents. The first four members (C1 to C4) are gases, C5 to C17 are liquids, and C18 and above are solids at 298 K. Boiling points increase with molecular mass due to stronger van der Waals forces. Branched alkanes have lower boiling points than their straight-chain isomers because branching reduces surface area. Chemical reactions include halogenation (free radical mechanism with steps: initiation, propagation, termination), combustion, controlled oxidation, isomerisation, aromatisation, reaction with steam, and pyrolysis.

Conformations are different spatial arrangements of atoms arising from rotation around a C-C single bond; they interconvert without bond breaking. Such arrangements are called conformers or rotamers. For ethane, two extreme conformations exist: eclipsed (H atoms on adjacent carbons are as close as possible) and staggered (H atoms are as far apart as possible). Intermediate arrangements are called skew conformations. These can be represented using Sawhorse or Newman projections.

In the staggered conformation, electron clouds of C-H bonds are maximally separated, resulting in minimum repulsion, minimum energy, and maximum stability. The eclipsed form has maximum torsional strain and is least stable. The energy difference between these two extremes is approximately 12.5 kJ mol-1, which is small enough that ethane molecules overcome it through intermolecular collisions at ordinary temperatures, making rotation around C-C practically free.

Alkenes are unsaturated hydrocarbons with the general formula CnH2n containing at least one C=C double bond. They are also called olefins. The double bond consists of one strong sigma bond (bond enthalpy ~397 kJ mol-1) formed by sp2-sp2 head-on overlap, and one weaker pi bond (bond enthalpy ~284 kJ mol-1) formed by lateral overlap of 2p orbitals. The C=C bond length is 134 pm, shorter than C-C (154 pm). The pi bond makes alkenes reactive towards electrophilic addition.

Alkenes exhibit structural isomerism (chain and position isomers) as well as geometrical (cis-trans) isomerism. Geometrical isomerism arises due to restricted rotation around the C=C bond when each doubly bonded carbon carries two different substituents. In the cis isomer, identical groups are on the same side; in the trans isomer, they are on opposite sides. Cis alkenes are generally more polar than trans alkenes. Trans isomers usually have higher melting points than cis isomers in solids.

Alkenes are prepared by: partial reduction of alkynes (using Lindlar's catalyst for cis-alkenes, or sodium in liquid ammonia for trans-alkenes), dehydrohalogenation of alkyl halides using alcoholic KOH (beta-elimination), dehalogenation of vicinal dihalides using zinc, and acidic dehydration of alcohols using concentrated H2SO4.

Alkenes undergo addition reactions due to their pi electrons. Key reactions include: addition of H2 (hydrogenation), addition of halogens (test for unsaturation using bromine water), addition of HX following Markovnikov's rule (negative part adds to the carbon with fewer H atoms), anti-Markovnikov addition of HBr in the presence of peroxides (Kharash effect, free radical mechanism), addition of H2SO4, and addition of water. Oxidation with cold dilute KMnO4 (Baeyer's reagent) gives vicinal glycols; ozonolysis cleaves the double bond giving aldehydes/ketones. Alkenes also undergo polymerisation.

Alkynes have the general formula CnH2n-2 and contain at least one C≡C triple bond. The first stable member is ethyne (acetylene). Each carbon in ethyne is sp hybridised, forming one C-C sigma bond, two C-H sigma bonds, and two C-C pi bonds. The H-C-C bond angle is 180°, making ethyne linear. The C≡C bond length is 120 pm and bond enthalpy is 823 kJ mol-1, making it shorter and stronger than C=C and C-C bonds.

Alkynes show acidic character: H atoms attached to sp hybridised triply bonded carbons are more acidic than those in alkenes or alkanes, because sp orbitals have 50% s-character and hence attract bonding electrons more strongly. Ethyne reacts with sodium metal and sodamide to liberate H2. Alkynes undergo addition reactions (H2, halogens, HX following Markovnikov's rule, and water with HgSO4/H2SO4 to form carbonyl compounds). Ethyne undergoes linear polymerisation to polyacetylene and cyclic polymerisation at 873 K over a red hot iron tube to form benzene.

Aromatic hydrocarbons (arenes) are characterised by a pleasant odour. Benzene (C6H6), isolated by Michael Faraday in 1825, is the parent aromatic compound. Kekulé proposed its cyclic structure with alternating single and double bonds in 1865. The modern understanding describes benzene as a resonance hybrid of two Kekulé structures, represented with a circle inside a hexagon. All six C-C bonds in benzene are equal at 139 pm, intermediate between single (154 pm) and double (133 pm) bonds, as confirmed by X-ray diffraction.

All six carbon atoms in benzene are sp2 hybridised. Six C-C sigma bonds and six C-H sigma bonds lie in the same plane. Each carbon also has one unhybridised p orbital perpendicular to the ring, and lateral overlap of all six p orbitals results in complete delocalisation of six pi electrons, forming two doughnut-shaped electron clouds above and below the ring. This delocalisation confers exceptional stability (aromaticity) and explains benzene's preference for substitution over addition reactions.

Aromaticity is not exclusive to benzene but applies to all ring systems that satisfy three criteria: planarity, complete delocalisation of pi electrons in the ring, and the presence of (4n+2) pi electrons where n is a non-negative integer (0, 1, 2, ...). This is known as Hückel's Rule. Examples of aromatic systems include benzene (6π electrons, n=1), cyclopentadienyl anion (6π electrons), cycloheptatrienyl cation (6π electrons), naphthalene (10π electrons, n=2), and anthracene and phenanthrene (14π electrons, n=3).

Aromaticity explains benzene's unusual stability and its preference for electrophilic substitution rather than addition reactions. Systems that lack planarity, have interrupted delocalisation, or do not satisfy the (4n+2) rule are not aromatic. For example, cyclooctatetraene (8π electrons) does not satisfy Hückel's rule and is not aromatic.

Benzene undergoes electrophilic substitution reactions as its primary chemical reactions: nitration (with conc. HNO3 and conc. H2SO4 to give nitrobenzene), halogenation (with Cl2 or Br2 in presence of anhydrous AlCl3 or FeCl3 as Lewis acid), sulphonation (with fuming H2SO4 to give benzene sulphonic acid), Friedel-Crafts alkylation (with alkyl halide and anhydrous AlCl3), and Friedel-Crafts acylation (with acyl halide and AlCl3). The mechanism involves generation of an electrophile, formation of a sigma complex (arenium ion), and restoration of aromaticity by proton removal.

Under special conditions, benzene also undergoes addition reactions: hydrogenation with H2 and Ni at high temperature gives cyclohexane, and addition of Cl2 under UV light gives benzene hexachloride (BHC/gammaxane). Benzene burns with a sooty flame in air.

When monosubstituted benzene undergoes further electrophilic substitution, the nature of the existing substituent determines whether the incoming group goes to ortho/para or meta positions. Ortho and para directing groups increase electron density at o- and p-positions through resonance (e.g., -OH, -NH2, -OCH3, -CH3). These groups generally activate the ring towards further substitution. Halogens are exceptions: they are o/p directors but moderate deactivators because their strong -I effect reduces overall electron density even though resonance still favours o/p positions.

Meta directing groups include electron-withdrawing groups such as -NO2, -CN, -CHO, -COOH, -SO3H, etc. These reduce electron density at o- and p-positions more than at the meta position through their strong -I (and sometimes -M) effects. Consequently, the electrophile attacks the comparatively electron-rich meta position. These groups deactivate the benzene ring towards further electrophilic substitution.

Benzene and polynuclear hydrocarbons containing more than two fused benzene rings are toxic and carcinogenic. These compounds are produced during incomplete combustion of organic materials including tobacco, coal, and petroleum. On entering the human body, they undergo biochemical reactions that damage DNA and cause cancer.

Examples of carcinogenic hydrocarbons include 1,2-benzanthracene, 3-methylcholanthrene, 1,2-benzpyrene, 1,2,5,6-dibenzanthracene, and 9,10-dimethyl-1,2-benzanthracene. Awareness of their toxicological properties is essential for environmental safety and public health.

Chapter 9 covers the vast and fundamentally important domain of hydrocarbons, spanning from simple alkanes to complex aromatic systems. The chapter establishes that the type of carbon-carbon bond determines classification into alkanes (single bonds), alkenes (double bonds), alkynes (triple bonds), and aromatic hydrocarbons. Reactivity patterns follow from bonding: alkanes are inert and react mainly by free-radical substitution; alkenes and alkynes with pi electrons undergo electrophilic additions; benzene with delocalised pi electrons prefers electrophilic substitution. Stereochemical concepts such as conformational isomerism in ethane and geometrical isomerism in alkenes provide insight into three-dimensional molecular behaviour.

For CBSE Board examinations, this chapter carries significant weightage. Students must be thorough with IUPAC nomenclature, all methods of preparation and their conditions and equations, the mechanisms of halogenation (free radical), electrophilic addition (Markovnikov and anti-Markovnikov), and electrophilic aromatic substitution. Aromaticity, Hückel's rule, and directive effects of substituents are high-priority theoretical topics. Conformations of ethane drawn using Newman and Sawhorse projections, cis-trans isomerism conditions, and the acidic nature of terminal alkynes are consistently tested. Understanding carcinogenicity of polynuclear hydrocarbons is also required as per the syllabus.