3.1 : Structure des alcanes

Compounds containing carbon and hydrogen atoms are called hydrocarbons. The simplest hydrocarbon, methane, has a tetravalent carbon atom bonded to four hydrogen atoms.

If one of the hydrogen atoms of methane is replaced with a carbon atom bonded to three hydrogen atoms, the next hydrocarbon, ethane, is formed.

Methane and ethane belong to the family of hydrocarbons called alkanes. Since alkanes contain only single covalent bonds, they are saturated hydrocarbons.

In any alkane, for example ethane, the carbon atoms are sp³ hybridized, making them tetrahedral carbons. These singly occupied hybrid orbitals overlap with the sp³ hybrid orbitals of the neighboring carbon atom to form a carbon-carbon sigma bond.

The other hybrid orbitals of carbon form sigma bonds with the 1s orbitals of each of the hydrogen atoms. The bonded atoms are at an angle of 109.5° to each other.

Examine the structures of three simple alkanes — methane, ethane, and propane. Methane, CH₄, has one carbon and four hydrogen atoms; ethane, C₂H₆, has two carbons and six hydrogen atoms; and propane, C₃H₈, has three carbons and eight hydrogen atoms.

Notice that each succeeding formula has one carbon and two hydrogens — namely, a CH₂ group more than the preceding formula.

In general, for every n number of carbon atoms, there will be 2n+2 hydrogen atoms. Thus, alkanes have the characteristic molecular formula: C_nH_2n+2.

A condensed structural formula simplifies the Lewis structure of alkanes by omitting the bonds to hydrogens or even carbon-carbon bonds.

Hydrocarbons can also be represented using line-angle or skeletal structures. The lines denote carbon-carbon bonds, with each vertex and endpoint implying a carbon atom. The hydrogens, although not explicitly shown, are assumed to be bonded to the carbons, satisfying their valency.