3.5 Properties of Alkanes

Properties of Alkanes

Alkanes are the simplest organic compounds, built entirely from carbon-carbon and carbon-hydrogen single bonds. Understanding their physical and chemical properties comes down to one central idea: the nature of their bonds and the intermolecular forces between molecules.

Introduction to Alkanes

Alkanes are saturated hydrocarbons, meaning every carbon-carbon bond is a single bond and each carbon holds as many hydrogens as possible. They form a homologous series where each member differs from the next by one $CH_2$ group (methane, ethane, propane, butane, and so on).

Because alkanes contain only C–C and C–H bonds, they are nonpolar. This nonpolarity determines almost everything about how they behave physically and chemically.

Chemical Reactivity of Alkanes

Alkanes are often called "paraffins" (from Latin parum affinis, meaning "little affinity") because they're relatively unreactive. The C–C and C–H bonds are strong and nonpolar, so most reagents leave them alone under normal conditions.

That said, alkanes do undergo two important reactions:

Combustion is the most familiar. Alkanes burn in oxygen to produce carbon dioxide and water in a highly exothermic reaction:

$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

This is why alkanes (natural gas, propane, gasoline) are widely used as fuels.

Free radical halogenation occurs when alkanes react with halogens like $Cl_2$ under UV light or heat. A hydrogen atom is replaced by a halogen atom:

$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$

• The reaction proceeds through a free radical chain mechanism (initiation, propagation, termination).
• Further substitution can continue, producing dichloromethane ($CH_2Cl_2$), chloroform ($CHCl_3$), and carbon tetrachloride ($CCl_4$).

Alkane Melting and Boiling Points

Since alkanes are nonpolar, the only intermolecular forces holding them together are London dispersion forces (also called van der Waals forces). These are weak, temporary attractions caused by momentary uneven distributions of electrons.

Two factors control the strength of London dispersion forces:

• Number of electrons: More electrons means larger temporary dipoles. Bigger alkanes have more electrons, so they attract each other more strongly.
• Surface area of contact: Molecules that can line up closely along their length have more area for these interactions.

The result is a clear trend: boiling points and melting points rise as molecular weight increases.

Notice the jump from methane to decane: adding carbons dramatically raises the boiling point because London dispersion forces accumulate with each additional $CH_2$ group.

Straight-Chain vs. Branched Alkanes

Branching lowers the boiling point compared to a straight-chain isomer with the same molecular formula. The reason ties directly back to surface area.

A straight-chain alkane like n-pentane has an extended, linear shape. Molecules can pack closely together along their entire length, maximizing London dispersion forces.

A branched alkane like isopentane (2-methylbutane) is more compact and roughly spherical. This shape reduces the surface area available for contact between neighboring molecules, so the London dispersion forces are weaker.

Structural Variations of Alkanes

Alkanes with four or more carbons can exist as structural isomers (also called constitutional isomers). These are compounds that share the same molecular formula but differ in how the atoms are connected. For example, butane ($C_4H_{10}$) has two structural isomers: n-butane (a straight chain) and isobutane (2-methylpropane).

Alkanes also exist in different conformations, which are different spatial arrangements produced by rotation around C–C single bonds. Unlike isomers, conformations are not different compounds; they interconvert freely at room temperature. You'll explore conformational analysis (Newman projections, staggered vs. eclipsed arrangements) in detail elsewhere in this unit.