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Chemistry · 9 chapters
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Chapter 3: HydrocarbonsClass 11 Chemistry — summary, notes, extra questions & MCQ quiz

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The general formula of an alkane is:

Summary

Hydrocarbons, compounds of carbon and hydrogen only, are the principal sources of energy and the feedstock for polymers, dyes and drugs. They are classified as saturated alkanes (general formula \(C_nH_{2n+2}\), with single bonds), unsaturated alkenes (\(C_nH_{2n}\)) and alkynes (\(C_nH_{2n-2}\), with multiple bonds), and aromatic hydrocarbons. Alkanes are prepared by hydrogenation of unsaturated hydrocarbons, reduction of alkyl halides, the Wurtz reaction, decarboxylation and Kolbe electrolysis; they are fairly unreactive but undergo free-radical halogenation, combustion, controlled oxidation, isomerisation and pyrolysis, and their staggered conformation is more stable than the eclipsed because of lower torsional strain. Alkenes contain a sigma plus pi double bond, show chain, position and geometrical (cis-trans) isomerism, and undergo electrophilic addition of hydrogen, halogens, hydrogen halides (Markovnikov rule, with the anti-Markovnikov peroxide effect for HBr), water and sulphuric acid, as well as oxidation, ozonolysis and polymerisation. Alkynes have a triple bond between sp-hybridised carbons, terminal alkynes showing acidic hydrogens. Aromatic chemistry centres on benzene, whose unusual stability is explained by resonance delocalisation of its six pi electrons and the Hückel \((4n+2)\) rule; benzene undergoes electrophilic substitution — nitration, halogenation, sulphonation and Friedel-Crafts reactions — with substituents directing further substitution to ortho/para or meta positions. Polynuclear aromatic hydrocarbons are noted as carcinogenic and toxic.

Classification of hydrocarbonsPreparation and reactions of alkanesConformations of ethaneAlkenes and the double bondMarkovnikov and peroxide effectsAlkynes and acidity of terminal hydrogenStructure and aromaticity of benzeneElectrophilic substitution of benzeneCarcinogenicity and toxicity

Key terms

Conformation
One of the spatial arrangements of a molecule interconvertible by rotation about a single bond, such as the staggered and eclipsed forms of ethane.
Markovnikov rule
In the addition of an unsymmetrical reagent to an unsymmetrical alkene, the negative part adds to the carbon bearing fewer hydrogen atoms.
Peroxide effect
The anti-Markovnikov addition of HBr to alkenes in the presence of peroxide, via a free-radical mechanism.
Aromaticity
The special stability of a planar, cyclic, fully conjugated ring containing \((4n+2)\) pi electrons (Hückel rule).
Ozonolysis
The cleavage of an alkene by ozone, followed by reductive work-up, to give carbonyl compounds and locate the double bond.
Friedel-Crafts reaction
The electrophilic substitution of benzene by an alkyl or acyl group using an anhydrous aluminium chloride catalyst.

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One of the spatial arrangements of a molecule interconvertible by rotation about a single bond, such as the staggered and eclipsed forms of ethane.
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Hydrocarbons

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