Alkanes, Alkenes, and Alkynes: Hydrocarbons Explained

Alkanes: Saturated Hydrocarbons

Alkanes are the simplest hydrocarbon family, containing only carbon-carbon single bonds and carbon-hydrogen bonds. They are called "saturated" because every carbon atom is bonded to the maximum number of hydrogen atoms possible. The general formula for acyclic (non-ring) alkanes is CnH2n+2, where n is the number of carbon atoms. Methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10) are the first four members.

All carbons in alkanes are sp3-hybridized with tetrahedral geometry (109.5-degree bond angles). Free rotation around each C-C single bond gives alkanes conformational flexibility, meaning they can twist into different shapes without breaking any bonds. The physical properties of alkanes follow predictable trends: boiling points increase with chain length (methane boils at -162 degrees C, octane at 126 degrees C) because longer molecules have stronger London dispersion forces. Branching lowers the boiling point because compact molecules have less surface area for intermolecular contact.

Alkanes are relatively unreactive compared to other organic families because C-C and C-H bonds are strong and nonpolar. Their two main reactions are combustion (burning in oxygen to produce CO2 and H2O, releasing significant energy) and halogenation (substitution of a hydrogen atom with a halogen atom, initiated by UV light or heat). The combustion of alkanes powers internal combustion engines, furnaces, and gas stoves worldwide. Natural gas is primarily methane, and petroleum is a complex mixture of liquid alkanes and cycloalkanes.

Cycloalkanes are alkanes that form rings, with the general formula CnH2n. Cyclopentane and cyclohexane are the most common ring sizes. Cyclohexane adopts a puckered "chair" conformation that minimizes angle strain and steric strain, placing substituents in either axial (pointing up or down) or equatorial (pointing outward) positions. Larger substituents strongly prefer the equatorial position to avoid steric clashes with axial hydrogens on the same side of the ring.

Alkenes: Unsaturated with Double Bonds

Alkenes contain at least one carbon-carbon double bond and have the general formula CnH2n for simple acyclic alkenes. The double bond consists of one sigma bond and one pi bond. The two carbons of the double bond are sp2-hybridized with trigonal planar geometry (120-degree bond angles), and the pi bond prevents rotation around the C=C axis.

This restricted rotation gives rise to geometric (cis-trans) isomerism. In cis-2-butene, both methyl groups are on the same side of the double bond; in trans-2-butene, they are on opposite sides. These isomers have different physical properties (cis-2-butene boils at 3.7 degrees C, trans at 0.9 degrees C) and can have dramatically different biological activities.

Alkenes are much more reactive than alkanes because the pi bond is a region of high electron density that attracts electrophilic reagents. The characteristic reaction of alkenes is electrophilic addition, where atoms or groups add across the double bond, converting it to a single bond. Key addition reactions include hydrogenation (adding H2 with a metal catalyst to form an alkane), halogenation (adding Br2 or Cl2 to form a dihalide), hydrohalogenation (adding HBr or HCl following Markovnikov rule), and hydration (adding water in the presence of acid to form an alcohol).

Markovnikov rule states that when HX adds to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogen atoms already attached (the less substituted carbon), while the halide adds to the more substituted carbon. This regiochemistry produces the more stable carbocation intermediate. Anti-Markovnikov addition, achieved through radical mechanisms (using peroxides) or hydroboration-oxidation, reverses this selectivity.

Polymerization of alkenes produces some of the most important industrial materials. Ethylene polymerizes to polyethylene, propylene to polypropylene, styrene to polystyrene, and vinyl chloride to PVC. These polymers are manufactured on scales of tens of millions of tons per year and are ubiquitous in modern life.

Alkynes: Unsaturated with Triple Bonds

Alkynes contain at least one carbon-carbon triple bond, with the general formula CnH2n-2. The triple bond consists of one sigma bond and two pi bonds. The two carbons of the triple bond are sp-hybridized with linear geometry (180-degree bond angles). Acetylene (ethyne, C2H2) is the simplest and most commercially important alkyne, used as a welding fuel and as a feedstock for producing vinyl chloride, acrylic acid, and other chemicals.

Terminal alkynes (with the triple bond at the end of the chain, R-C triple bond C-H) have a weakly acidic hydrogen (pKa approximately 25) that can be removed by strong bases like sodium amide (NaNH2) to form acetylide anions (R-C triple bond C minus). These anions are excellent nucleophiles for forming new carbon-carbon bonds, a property that makes alkynes valuable in organic synthesis.

Alkynes undergo the same types of addition reactions as alkenes, but they can react with one or two equivalents of reagent. Adding one equivalent of H2 with a Lindlar catalyst (a poisoned palladium catalyst) selectively reduces the triple bond to a cis double bond. Adding two equivalents of H2 reduces it all the way to a single bond (alkane). Adding one equivalent of HBr gives a vinyl halide; adding two equivalents gives a geminal dihalide with both halogens on the same carbon.

Comparing the Three Families

The progression from alkanes to alkenes to alkynes represents increasing unsaturation and increasing reactivity. Alkanes are fuel sources and structural scaffolds but poor candidates for chemical transformation. Alkenes serve as versatile starting materials for synthesis because their double bonds react with many reagents under mild conditions. Alkynes offer even greater versatility, with the option of partial or complete reduction and the ability to form carbon-carbon bonds through acetylide chemistry.

Physical properties also differ systematically. For molecules of similar size, alkynes have slightly higher boiling points than corresponding alkenes, which in turn boil higher than the corresponding alkanes minus two hydrogens. All three families are nonpolar and insoluble in water but soluble in organic solvents. Their densities are less than that of water (they float), and they are all flammable.