Hydrocarbons

The One Thing

Hydrocarbon behaviour is determined primarily by carbon–carbon bonding and ring structure. Single bonds characterise relatively unreactive alkanes, multiple bonds enable addition reactions in alkenes and alkynes, and aromatic rings such as benzene undergo substitution while retaining aromatic stability.

Reactions, Processes and Experiments

Key Terms

• Hydrocarbon: An organic compound containing only carbon and hydrogen atoms.
• Alkane: A saturated hydrocarbon containing only carbon–carbon single bonds; its open-chain general formula is CnH2n+2.
• Alkene: An unsaturated hydrocarbon containing at least one carbon–carbon double bond; for one open-chain double bond, the general formula is CnH2n.
• Alkyne: An unsaturated hydrocarbon containing at least one carbon–carbon triple bond; for one open-chain triple bond, the general formula is CnH2n-2.
• Aromatic hydrocarbon: A hydrocarbon containing one or more aromatic rings with delocalised pi electrons; benzene is the simplest example.
• Saturated hydrocarbon: A hydrocarbon containing only single bonds between carbon atoms.
• Unsaturated hydrocarbon: A hydrocarbon containing one or more carbon–carbon double or triple bonds.
• Homologous series: A group of compounds with the same functional feature and general formula, in which successive members differ by a -CH2- unit.
• Structural 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 double bond, triple bond, or substituent.
• Geometrical isomerism: Isomerism in alkenes caused by restricted rotation around a carbon–carbon double bond, producing cis-trans or E-Z forms.
• Markovnikov's rule: In the addition of an unsymmetrical reagent to an unsymmetrical alkene, hydrogen generally adds to the carbon already having more hydrogen atoms.
• Peroxide effect: In the presence of peroxides, HBr adds to an unsymmetrical alkene by a free-radical mechanism, giving the anti-Markovnikov product.
• Electrophilic addition: A reaction in which an electrophile attacks the electron-rich pi bond of an alkene or alkyne.
• Substitution reaction: A reaction in which one atom or group in a molecule is replaced by another atom or group; common in alkanes and aromatic compounds.
• Aromaticity: The special stability of a cyclic, planar, fully conjugated system containing 4n+2 pi electrons, according to Huckel's rule.
• Resonance in benzene: The pi electrons in benzene are delocalised over the ring, making all six carbon–carbon bonds equivalent.
• Homolytic fission: Breaking of a covalent bond so that each atom receives one electron, forming free radicals.
• Cracking: The breaking of large hydrocarbon molecules into smaller alkanes and alkenes using heat, pressure, and sometimes catalysts.
• Catenation: The ability of carbon atoms to bond to one another to form chains, branched structures, and rings.
• Degree of unsaturation/index of hydrogen deficiency: For a hydrocarbon CxHy, the value given by (2x+2-y)/2.
• Aromaticity: The stability associated with a cyclic, planar, fully conjugated system containing 4n+2 pi electrons.
• Tetravalency: The ability of carbon to form four covalent bonds.

Easily Confused

• Alkanes and cycloalkanes: Open-chain alkanes have the formula CnH2n+2, whereas cycloalkanes generally have the formula CnH2n because ring formation introduces one degree of unsaturation.
• Saturated and unsaturated hydrocarbons: Saturated hydrocarbons contain only carbon–carbon single bonds; unsaturated hydrocarbons contain carbon–carbon double or triple bonds.
• Alkenes and alkynes: Alkenes contain double bonds and, for one open-chain double bond, have the formula CnH2n; alkynes contain triple bonds and, for one open-chain triple bond, have the formula CnH2n-2.
• Addition and substitution: Addition uses a multiple bond to incorporate new atoms or groups, whereas substitution replaces one atom or group with another.
• Markovnikov's rule and the peroxide effect: Markovnikov addition places hydrogen on the carbon already bearing more hydrogen atoms; the peroxide effect gives anti-Markovnikov addition specifically for HBr through a free-radical mechanism.
• Benzene addition and benzene substitution: Benzene mainly undergoes electrophilic substitution because substitution preserves aromatic stability, whereas ordinary addition would disrupt aromaticity.
• Chain and position isomerism: Chain isomerism changes the carbon skeleton; position isomerism changes the location of a multiple bond or substituent.
• Cis-trans and E-Z descriptions: Both describe geometrical isomerism caused by restricted rotation around a carbon–carbon double bond, but E-Z notation is used more generally where priority rules distinguish the groups.
• Complete and incomplete combustion: Complete combustion of an alkane produces carbon dioxide and water; incomplete combustion can produce carbon monoxide and soot.
• Straight-chain and branched alkanes: Branching usually lowers the boiling point because branched molecules have a smaller surface area and weaker intermolecular attractions.

What Gets Asked

• Classification and formula questions: Identify hydrocarbons as alkanes, alkenes, alkynes, cycloalkanes, or aromatic hydrocarbons and apply the formulae CnH2n+2, CnH2n, and CnH2n-2. Marks are lost by confusing the alkene and alkyne formulae or treating a ring as an open chain.
• IUPAC nomenclature questions: Select the longest parent chain or ring, number it to give multiple bonds the lowest possible locants, and use the suffixes -ane, -ene, and -yne. For compounds containing both types of multiple bond, use -en-yne and assign the lowest set of locants.
• Reaction-type questions: Distinguish alkane substitution and combustion from alkene or alkyne addition. A common error is describing bromine water decolourisation as substitution rather than addition.
• Mechanism and orientation questions: Apply Markovnikov's rule to acid-catalysed hydration and distinguish it from the peroxide effect, in which HBr gives the anti-Markovnikov product by a free-radical mechanism.
• Benzene reaction questions: Explain why benzene undergoes electrophilic substitution and identify nitration, sulphonation, halogenation, and Friedel-Crafts alkylation or acylation. Marks are lost by treating benzene as an ordinary alkene or omitting the concentrated nitric acid and concentrated sulphuric acid in nitration.
• Structure–property and environmental questions: Relate molecular structure to boiling point, solubility, volatility, and reactivity; explain why branching lowers boiling point and identify carbon dioxide, carbon monoxide, and soot as products or pollutants associated with hydrocarbon combustion.