What Are Hydrocarbons?
Hydrocarbons are organic compounds made exclusively of carbon and hydrogen atoms. They form the structural foundation of organic chemistry and serve as the primary components of fossil fuels. Despite their apparent simplicity, hydrocarbons display remarkable diversity in structure and properties.
Hydrocarbons are divided into two major categories: aliphatic (non-aromatic) and aromatic. Aliphatic hydrocarbons include alkanes, alkenes, alkynes, and cyclic variants. Aromatic hydrocarbons contain benzene rings and are covered separately.
Alkanes: The Saturated Hydrocarbons
Alkanes contain only single C–C and C–H bonds. They are called saturated because every carbon carries the maximum possible number of hydrogen atoms. The general formula for acyclic (open-chain) alkanes is CₙH₂ₙ₊₂.
Naming Alkanes (IUPAC Nomenclature)
The first ten straight-chain alkanes are:
| n | Name | Formula |
|---|---|---|
| 1 | Methane | CH₄ |
| 2 | Ethane | C₂H₆ |
| 3 | Propane | C₃H₈ |
| 4 | Butane | C₄H₁₀ |
| 5 | Pentane | C₅H₁₂ |
| 6 | Hexane | C₆H₁₄ |
| 7 | Heptane | C₇H₁₆ |
| 8 | Octane | C₈H₁₈ |
| 9 | Nonane | C₉H₂₀ |
| 10 | Decane | C₁₀H₂₂ |
Branched alkanes are named by identifying the longest continuous carbon chain (the parent chain) and numbering substituents from the nearest end.
Physical Properties of Alkanes
- Boiling point increases with chain length — more surface area means stronger London dispersion forces.
- Branching lowers boiling point — compact shapes reduce contact surface area.
- Alkanes are nonpolar and insoluble in water but soluble in nonpolar solvents.
- At room temperature: CH₄–C₄H₁₀ are gases; C₅H₁₂–C₁₇H₃₆ are liquids; C₁₈+ are solids (waxes).
Reactions of Alkanes
Alkanes are relatively unreactive — they don't react with strong acids, bases, or most oxidizing agents. Their main reactions are:
- Combustion: Complete combustion yields CO₂ and H₂O, releasing energy. Methane: CH₄ + 2O₂ → CO₂ + 2H₂O. This is the basis for natural gas as a fuel.
- Halogenation (free radical): In the presence of UV light, alkanes react with Cl₂ or Br₂ via a free-radical chain mechanism. CH₄ + Cl₂ → CH₃Cl + HCl.
Cycloalkanes
Cycloalkanes have the formula CₙH₂ₙ. Cyclohexane (C₆H₁₂) is particularly important — it adopts a chair conformation that minimizes ring strain and steric interactions.
Alkenes: The Unsaturated Hydrocarbons (One Double Bond)
Alkenes contain at least one carbon–carbon double bond (C=C). Because each double bond "uses up" two hydrogens compared to the saturated equivalent, alkenes are called unsaturated. The general formula for acyclic monoalkenes is CₙH₂ₙ.
The C=C double bond consists of: - A σ (sigma) bond: formed by head-on orbital overlap, allows free rotation in single bonds — but NOT in double bonds. - A π (pi) bond: formed by sideways p-orbital overlap, restricts rotation and creates the reactive site.
Naming Alkenes
The parent chain must include both carbons of the double bond. Number the chain so the double bond gets the lowest locant. Examples: - CH₂=CH₂ → ethene (also called ethylene) - CH₃CH=CH₂ → prop-1-ene - CH₃CH=CHCH₃ → but-2-ene
Reactions of Alkenes
The π bond is electron-rich and reactive toward electrophiles — species that seek electrons. Key reactions:
- Hydrogenation: H₂ adds across the double bond with a metal catalyst (Pt, Pd, Ni). CH₂=CH₂ + H₂ → CH₃CH₃. Used industrially to harden vegetable oils into margarine.
- Halogenation: Br₂ or Cl₂ adds across the double bond. CH₂=CH₂ + Br₂ → BrCH₂CH₂Br. The decolorization of bromine water is a classic test for unsaturation.
- Hydrohalogenation: HX (HCl, HBr) adds across the double bond following Markovnikov's Rule — the H goes to the carbon with more hydrogens.
- Hydration: Water adds across the double bond in the presence of acid catalyst to give an alcohol.
Alkynes: Triple Bond Hydrocarbons
Alkynes contain at least one carbon–carbon triple bond (C≡C). The general formula is CₙH₂ₙ₋₂. The triple bond consists of one σ bond and two π bonds, making it even more electron-rich and reactive than a double bond.
Key Alkynes
- Ethyne (acetylene, HC≡CH): the simplest alkyne. Burns with an intensely hot flame — used in oxyacetylene welding (temperatures up to 3500°C).
- Propyne (CH₃C≡CH): used as a propellant and chemical intermediate.
Reactions of Alkynes
Alkynes undergo similar addition reactions to alkenes, but two equivalents of reagent can add (one per π bond):
- Hydrogenation: Complete reduction with excess H₂ gives an alkane; partial reduction with a Lindlar catalyst (poisoned Pd) gives a cis-alkene.
- Hydrohalogenation: Two equivalents of HX can add to give a geminal dihalide (both halogens on the same carbon).
- Hydration: In the presence of H₂SO₄ and HgSO₄, water adds to give an enol, which tautomerizes to a ketone (or aldehyde from terminal alkynes).
Acidity of Terminal Alkynes
Terminal alkynes (R–C≡C–H) are surprisingly acidic for hydrocarbons (pKₐ ≈ 25). The sp-hybridized carbon holds the negative charge tightly, stabilizing the conjugate base (acetylide ion). This allows terminal alkynes to react with strong bases like NaNH₂ to form acetylide nucleophiles, which are useful in synthesis.
Comparing Alkanes, Alkenes, and Alkynes
| Property | Alkanes | Alkenes | Alkynes |
|---|---|---|---|
| Formula | CₙH₂ₙ₊₂ | CₙH₂ₙ | CₙH₂ₙ₋₂ |
| Bond type | Single | One double | One triple |
| Hybridization | sp³ | sp² | sp |
| Reactivity | Low | Moderate | High |
| Key reaction | Combustion | Electrophilic addition | Electrophilic addition |
Industrial and Everyday Applications
- Natural gas: primarily methane (CH₄), used for heating and electricity generation.
- Petroleum refining: crude oil is distilled into fractions — gasoline (C₅–C₁₂), kerosene, diesel, and fuel oil.
- Ethylene (CH₂=CH₂): the world's most produced organic chemical. Polymerization gives polyethylene (plastic bags, bottles). Also used to ripen fruit by naturally triggering plant hormones.
- Acetylene (HC≡CH): metal cutting and welding; precursor to vinyl chloride (PVC).
- Isoprene (2-methylbuta-1,3-diene): natural rubber is a polymer of isoprene.