Hydrocarbons

The One Thing

Hydrocarbon classification is determined by carbon–carbon bonding and aromatic structure. These structural features determine molecular formula, geometry, physical properties, and characteristic reactions.

Reactions, Processes and Experiments

Key Terms

• Hydrocarbon: An organic compound containing only carbon and hydrogen.
• Saturated hydrocarbon: A hydrocarbon containing only carbon–carbon single bonds; alkanes are saturated hydrocarbons.
• Unsaturated hydrocarbon: A hydrocarbon containing one or more carbon–carbon double or triple bonds; alkenes and alkynes are unsaturated.
• Alkane: A saturated open-chain hydrocarbon with the general formula CnH2n+2.
• Alkene: An unsaturated hydrocarbon containing at least one carbon–carbon double bond, with the general formula CnH2n for one open-chain double bond.
• Alkyne: An unsaturated hydrocarbon containing at least one carbon–carbon triple bond, with the general formula CnH2n-2 for one open-chain triple bond.
• Aromatic hydrocarbon: A hydrocarbon containing one or more aromatic rings with delocalized pi electrons, such as benzene.
• Homologous series: A family of compounds with the same functional pattern and general formula in which successive members differ by a -CH2- unit.
• Isomerism: The existence of compounds with the same molecular formula but different arrangements of atoms.
• Chain isomerism: Isomerism caused by different arrangements of the carbon skeleton.
• Position isomerism: Isomerism caused by different positions of a multiple bond or substituent on the same carbon skeleton.
• Geometrical isomerism: Cis–trans or E–Z isomerism caused by restricted rotation around a carbon–carbon double bond.
• Markovnikov's rule: During addition of an unsymmetrical reagent such as HBr to an unsymmetrical alkene, hydrogen generally attaches to the carbon already bearing more hydrogen atoms, while the other part attaches to the carbon with fewer hydrogen atoms.
• Peroxide effect: In the presence of peroxides, HBr adds to an unsymmetrical alkene by a free-radical mechanism in the anti-Markovnikov direction.
• Aromaticity: The special stability of a cyclic, planar, conjugated system containing 4n+2 pi electrons, according to Huckel's rule.
• Electrophilic addition: An addition reaction of alkenes or alkynes initiated by an electrophile attacking the electron-rich pi bond.
• Electrophilic substitution: A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring while the aromatic system is restored.
• Free-radical substitution: A reaction of alkanes with halogens under ultraviolet light or heat involving radical initiation, propagation, and termination steps.
• Polymerization: The joining of many small monomer molecules to form a large polymer molecule, often by addition across carbon–carbon double bonds.
• Cracking: The thermal or catalytic breakdown of larger hydrocarbons into smaller alkanes and alkenes.
• Carbon tetravalency: The ability of carbon to form four covalent bonds.
• Sigma bond: A covalent bond formed by direct orbital overlap; alkane carbon–carbon and carbon–hydrogen bonds are strong, nonpolar sigma bonds.
• Pi bond: A bond formed by sideways overlap of orbitals; a carbon–carbon double bond contains one pi bond and a triple bond contains two.
• Vicinal dihalogenoalkane: A compound in which two halogen atoms are attached to adjacent carbon atoms.
• Vicinal diol: A compound containing hydroxyl groups on adjacent carbon atoms.
• Aromaticity and Huckel's rule: Benzene has six pi electrons, satisfying 4n+2 with n=1.
• Lewis acid catalyst: An electron-pair acceptor, such as FeCl3 or AlCl3, used in benzene halogenation and Friedel-Crafts reactions.

Easily Confused

• Saturated and unsaturated hydrocarbons: Alkanes contain only carbon–carbon single bonds, whereas alkenes and alkynes contain double or triple bonds.
• Alkenes and alkynes: Alkenes contain carbon–carbon double bonds and have the formula CnH2n; alkynes contain triple bonds and have the formula CnH2n-2.
• Addition and substitution: Addition adds atoms across a multiple bond, whereas substitution replaces a hydrogen atom, particularly on an aromatic ring or in an alkane.
• Electrophilic addition and electrophilic substitution: Electrophilic addition consumes a pi bond in an alkene or alkyne, whereas electrophilic substitution restores and preserves aromaticity.
• Markovnikov's rule and the peroxide effect: HBr normally follows Markovnikov's rule, but in the presence of peroxides it adds in the anti-Markovnikov direction.
• Partial and complete hydrogenation of alkynes: Partial hydrogenation produces an alkene, whereas complete hydrogenation produces an alkane.
• Chain and position isomerism: Chain isomerism changes the carbon skeleton, whereas position isomerism changes the location of a multiple bond or substituent on the same skeleton.
• Benzene addition and substitution: Benzene mainly undergoes substitution because substitution preserves its aromatic stability; addition would disrupt aromaticity.
• Alkyl and acyl groups in Friedel-Crafts reactions: Friedel-Crafts alkylation introduces an alkyl group, whereas Friedel-Crafts acylation introduces an acyl group.
• Deactivating groups and directing effects: Halogens deactivate the benzene ring but remain ortho–para directing; strong electron-withdrawing groups generally deactivate and direct substitution to the meta position.

What Gets Asked

• State and apply the general formulae CnH2n+2, CnH2n, and CnH2n-2; marks are lost if the formula is assigned to the wrong hydrocarbon class.
• Identify geometry and hybridization: alkanes are approximately sp3 and tetrahedral with bond angles close to 109.5°; alkenes are approximately sp2 and trigonal planar with angles close to 120°; alkynes are sp and linear with angles of 180°.
• Distinguish alkane, alkene, alkyne, and aromatic reactions; a common error is describing alkene addition as substitution or failing to identify benzene substitution as electrophilic substitution.
• Predict products of hydrohalogenation and hydration using Markovnikov's rule, while recognizing the anti-Markovnikov peroxide effect specifically for HBr.
• Use observations to identify unsaturation: bromine solution is decolorized by an alkene, and cold dilute alkaline KMnO4 is decolorized during alkene oxidation.
• Explain benzene substitution conditions and directing effects, including concentrated H2SO4 in nitration, FeCl3 or AlCl3 in halogenation, and the distinction between ortho–para and meta direction.