Introduction
Hydrocarbons — compounds of only carbon and hydrogen — are organic chemistry’s simplest members and the foundation from which all functional groups (introduced from Chapter 14 onward) are built by substitution.
1. Alkanes and cycloalkanes
Alkanes (CₙH₂ₙ₊₂) contain only single (σ) bonds and are relatively unreactive (“saturated,” containing the maximum possible hydrogens), undergoing mainly free-radical substitution (e.g., halogenation) under forcing conditions like UV light or heat, since there is no π bond or polarised site for other reagents to attack easily. Cycloalkanes (CₙH₂ₙ) are ring-shaped alkanes; small rings (especially cyclopropane) experience significant ring strain — bond angles forced away from the ideal tetrahedral 109.5° — which makes them notably more reactive than their ring size would otherwise suggest.
2. Alkenes and alkynes
Alkenes (CₙH₂ₙ) contain at least one C=C double bond; alkynes (CₙH₂ₙ₋₂) contain at least one C≡C triple bond. Both are “unsaturated” (fewer hydrogens than the corresponding alkane) and, because the π bond(s) are weaker and more exposed than σ bonds, they react readily via addition reactions, in which the π bond breaks and new atoms/groups add across the former double or triple bond.
3. Preparation and characteristic reactions
Alkenes are commonly prepared by elimination reactions (dehydration of alcohols, dehydrohalogenation of alkyl halides — mechanistically developed fully in Chapter 15) or by partial reduction of alkynes. Characteristic alkene reactions include catalytic hydrogenation (addition of H₂ across the double bond, using a metal catalyst), halogenation (addition of X₂), and hydration (addition of water, acid-catalysed) — each converting the alkene into a saturated or partially saturated product.
4. Addition, substitution, and oxidation patterns
The reactivity pattern across hydrocarbons follows directly from bonding: alkanes undergo substitution (a hydrogen is replaced, since there is no π bond to add across); alkenes and alkynes undergo addition (across the π bond(s)); and all hydrocarbons can be oxidised, most completely via combustion (complete oxidation to CO₂ and H₂O, releasing energy) or more selectively via reagents like potassium permanganate, which can cleave or hydroxylate a double bond under controlled conditions — a useful diagnostic and synthetic tool.
5. Stability and orientation (Markovnikov’s rule)
When an unsymmetrical reagent (like HBr) adds across an unsymmetrical alkene, Markovnikov’s rule predicts that the hydrogen adds to the carbon already bearing more hydrogens, so the other group ends up on the more substituted carbon — this is explained mechanistically by the reaction proceeding through the more stable (more substituted) carbocation intermediate, since alkyl groups stabilise positive charge through both inductive donation and hyperconjugation (Chapter 11).
Key Takeaways
- Alkanes (saturated, only σ bonds) undergo substitution; alkenes and alkynes (unsaturated, containing π bonds) undergo addition — this single distinction predicts most of hydrocarbon reactivity.
- Cycloalkane ring strain (deviation from ideal tetrahedral angles) increases reactivity in small rings.
- Common alkene reactions (hydrogenation, halogenation, hydration) all add across the π bond, converting unsaturated starting material to a more saturated product.
- Markovnikov’s rule, explained by carbocation stability, predicts the regiochemistry of unsymmetrical addition to unsymmetrical alkenes.