Saturated Hydrocarbons

From hydrocarbons to alkanes

A hydrocarbon is a compound made from carbon and hydrogen only. An alkane is a particular type of hydrocarbon: its atoms are joined entirely by single covalent bonds. This includes carbon–hydrogen bonds and, where there is more than one carbon atom, carbon–carbon bonds.

Carbon forms four covalent bonds. Hydrogen forms one. In methane, $\mathrm{CH_4}$, the carbon forms four single bonds to four hydrogen atoms. In ethane, $\mathrm{C_2H_6}$, each carbon uses one bond to join the other carbon and three bonds to hydrogen. Each carbon therefore still has four bonds altogether.

What “saturated” means

An alkane is saturated because its carbon atoms are bonded to as many hydrogen atoms as possible while retaining the same carbon skeleton. Every carbon already has four bonds, so another hydrogen atom cannot simply be attached: doing so would give a carbon atom more bonds than it can form.

The word does not mean that the substance has dissolved as much solute as possible. Here, saturation describes the bonding within each hydrocarbon molecule.

For comparison, an unsaturated hydrocarbon has a multiple bond between carbon atoms. That multiple bond can be changed during an addition reaction so that further atoms become attached. An alkane has no such carbon–carbon multiple bond: every carbon–carbon bond is single.

Connecting saturation to the general formula

Alkanes have the general formula

$\mathrm{C_nH_{2n+2}}$

where $n$ is the number of carbon atoms. For example, when $n=3$, the formula is $\mathrm{C_3H_8}$ because $2(3)+2=8$.

This formula reflects carbon’s four bonds. In an open chain of $n$ carbon atoms, the carbon–carbon single bonds use two carbon bonding positions each—one on either carbon. The remaining positions are filled by hydrogen, giving $2n+2$ hydrogen atoms.

Therefore, alkanes are classified as saturated hydrocarbons for two connected reasons: they contain carbon and hydrogen only, and they have only single bonds, leaving each carbon bonded to the maximum possible number of hydrogen atoms for that carbon skeleton.

Recall

• Hydrocarbons contain carbon and hydrogen only.
• Alkanes contain only single covalent bonds.
• Carbon forms four bonds; hydrogen forms one.
• Every carbon atom in an alkane already has four bonds, so further hydrogen cannot be added without changing existing bonds.
• Therefore, alkanes are saturated hydrocarbons.

Formula link

$\mathrm{C_nH_{2n+2}}$

This is the general formula for alkanes. It shows that an alkane has the maximum number of hydrogen atoms for its open-chain carbon skeleton.

Essential distinction

• Saturated: carbon–carbon single bonds only.
• Unsaturated: contains at least one carbon–carbon multiple bond.