Hydrocarbons: questions with answers

Hydrocarbons are classified as aliphatic or aromatic and as saturated or unsaturated. A defensible IUPAC name follows the longest chain containing the maximum number of multiple bonds, lowest locants, and alphabetised substituents; constitutional isomers differ in connectivity, not merely drawing orientation.

Hydrocarbons are classified as aliphatic or aromatic and as saturated or unsaturated. A defensible IUPAC name follows the longest chain containing the maximum number of multiple bonds, lowest locants, and alphabetised substituents; constitutional isomers differ in connectivity, not merely drawing orientation.

A substituent changes ring electron density and directs further electrophilic substitution. Electron-releasing groups such as alkyl activate and direct ortho/para; strongly electron-withdrawing groups such as nitro deactivate and usually direct meta. Halogens are deactivating yet ortho/para directing because their inductive withdrawal and resonance donation affect rate and orientation differently. Many fused aromatic hydrocarbons are toxic or carcinogenic.

A substituent changes ring electron density and directs further electrophilic substitution. Electron-releasing groups such as alkyl activate and direct ortho/para; strongly electron-withdrawing groups such as nitro deactivate and usually direct meta. Halogens are deactivating yet ortho/para directing because their inductive withdrawal and resonance donation affect rate and orientation differently. Many fused aromatic hydrocarbons are toxic or carcinogenic.

Alkynes contain a linear sp–sp C≡C bond with one sigma and two pi bonds. They are prepared by double dehydrohalogenation of vicinal or geminal dihalides. Terminal alkynes are weakly acidic because the conjugate-base charge resides in an orbital with 50% s character; additions occur stepwise and complete hydrogenation consumes two equivalents of H2.

Alkynes contain a linear sp–sp C≡C bond with one sigma and two pi bonds. They are prepared by double dehydrohalogenation of vicinal or geminal dihalides. Terminal alkynes are weakly acidic because the conjugate-base charge resides in an orbital with 50% s character; additions occur stepwise and complete hydrogenation consumes two equivalents of H2.

Alkanes are prepared by hydrogenation of unsaturated hydrocarbons, reduction of alkyl halides, Wurtz coupling, decarboxylation, and Kolbe electrolysis. Each method has a characteristic carbon-count consequence and substrate limitation; mixed Wurtz coupling gives mixtures and is unsuitable for a pure odd-carbon alkane.

Alkanes are prepared by hydrogenation of unsaturated hydrocarbons, reduction of alkyl halides, Wurtz coupling, decarboxylation, and Kolbe electrolysis. Each method has a characteristic carbon-count consequence and substrate limitation; mixed Wurtz coupling gives mixtures and is unsuitable for a pure odd-carbon alkane.

Alkanes are non-polar, water-insoluble fuels whose boiling points rise with molar mass and fall with branching at fixed formula. Their reactions include combustion, controlled oxidation, cracking and free-radical halogenation. Chlorination proceeds by initiation, propagation, and termination; radical recombination explains ethane during methane chlorination.

Alkanes are non-polar, water-insoluble fuels whose boiling points rise with molar mass and fall with branching at fixed formula. Their reactions include combustion, controlled oxidation, cracking and free-radical halogenation. Chlorination proceeds by initiation, propagation, and termination; radical recombination explains ethane during methane chlorination.

An alkene double bond contains one sigma and one pi bond between trigonal-planar sp2 carbons. Restricted rotation permits cis/trans or E/Z isomerism only when each double-bond carbon bears two different substituents. Cis isomers can be more polar and may boil higher, while trans isomers are often more symmetrical and may pack better.

Rotation about the ethane C–C sigma bond produces conformations represented by sawhorse or Newman projections. The staggered form is lowest in energy and the eclipsed form highest because eclipsing C–H bonds create torsional strain. Conformers interconvert without bond breaking and are not constitutional isomers.

Alkenes arise by dehydration of alcohols and dehydrohalogenation of haloalkanes. They undergo electrophilic additions, oxidation and ozonolysis. Markovnikov addition places H on the carbon already bearing more H; HBr with peroxide follows a radical anti-Markovnikov path. Ozonolysis cleaves C=C to carbonyls and can be reversed to reconstruct the alkene.

Benzene is a planar cyclic conjugated system with six delocalised pi electrons and equal C–C bonds. Its resonance hybrid is more stable than any localised Kekule form. Huckel aromaticity requires a planar, cyclic, continuously conjugated system with (4n+2) pi electrons; electron count alone is insufficient if planarity or conjugation is absent.

Benzene is prepared by cyclic polymerisation of ethyne, decarboxylation of sodium benzoate, and aromatisation of alkanes. Its electron-rich pi cloud reacts with electrophiles in nitration, sulfonation, halogenation and Friedel–Crafts reactions. Electrophile generation requires the stated acid or Lewis-acid catalyst; substitution restores aromaticity after sigma-complex formation.