9.1. Alkanes

Learning check

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Carbon binds with covalent bonds to other atoms

In methane, a central carbon atom binds to four hydrogen atoms.

• Carbon gets noble gas structure because it "borrows" four electrons from the four hydrogen atoms.
• Hydrogen gets noble gas structure because each hydrogen atom "borrows" one electron from the carbon atom.

Lewis structure of methane.

Methane and ethane: The two simplest hydrocarbons (alkanes)

Alkane:

• Only contains hydrogen and carbon.
• Only single bonds between the carbon atoms.

Name	Model	Structural formula	Molecular formula
Methane			CH4
Ethane			C2H6

Somewhat larger hydrocarbons

Often, the structural formula is simplified for larger molecules.

Name	Model	Structural formula	Simplified structure	Molecular formula
Propane			CH3−CH2−CH3	C3H8
n-butane			CH3−CH2−CH2−CH3	C4H10
n-pentane			CH3CH2CH2CH2CH3	C5H12

Even larger hydrocarbons

In a skeletal formula, the structure is simplified even more. Every line is a bond, and in every corner, there is a carbon atom. Everywhere where there's room, it is understood that there are hydrogen atoms.

Name	Model	Skeletal formula	Molecular formula
n-hexane			C6H14
n-heptane			C7H16
n-octane			C8H18
n-nonane			C9H20
n-decane			C10H22

Cycloalkanes

Carbon atoms joined in a circular structure.

Name	Model	Structure	Molecular formula
Cyclopropane			C3H6
Cyclobutane			C4H8
Cyclopentane			C5H10
Cyclohexane			C6H12

Cyclopropane and cyclobutane are both unstasble. This is because the binding angles differ a lot from 109°.

• In methane, binding angles are 109°.
• The hydrogen atoms end up in the corners of a tetrahedron.

A funny alkane

Cubane.

You can find more funny molecules on Molecules with Silly or Unusual Names!