The first four alkanes and their formulae
Definition
Alkane
A saturated hydrocarbon with the general formula CnH2n+2.
- Methane is CH4\text{CH}_4CH4, with a single carbon atom.
- Ethane is C2H6\text{C}_2\text{H}_6C2H6.
- Propane is C3H8\text{C}_3\text{H}_8C3H8.
- Butane is C4H10\text{C}_4\text{H}_{10}C4H10.
- Each fits the general formula CnH2n+2\text{C}_n\text{H}_{2n+2}CnH2n+2, so the count of hydrogens follows from the carbons.
Key Idea
Knowing the general formula makes any alkane's molecular formula available without recall.
Drawing the structure means showing every bond
Definition
Covalent bond
A shared pair of electrons between two atoms, attracted to the nucleus of each.
- A structure shows each atom and every covalent bond as a line between two atoms.
- Every carbon atom has four lines drawn to it, because carbon forms four bonds.
- Every hydrogen atom has exactly one line, because hydrogen forms one bond.
- The carbons are drawn in a chain, joined to one another by single lines.
- The remaining bonds on each carbon are filled with hydrogen atoms.
Example
- Methane: one carbon with four hydrogens attached, four lines in all.
- Ethane: two carbons joined by one line, each carrying three hydrogens.
- Propane: three carbons in a row; the middle one carries two hydrogens, the end ones three each.
Counting the bonds as a check
- The number of lines drawn to each carbon must come to four, every time.
- A carbon with only three lines has a missing hydrogen.
- The total number of hydrogen atoms drawn must match the molecular formula.
- Propane has eight hydrogens, arranged three, two and three along the chain.
- Butane has ten, arranged three, two, two and three.
Common Mistake
A structure with a carbon showing fewer than four bonds is wrong, however tidy it looks.
Alkanes are saturated
Definition
Saturated hydrocarbon
A hydrocarbon whose carbon atoms are joined only by single bonds.
- Every bond between carbon atoms in an alkane is a single bond.
- A molecule whose carbons are joined only by single bonds is saturated.
- Saturated means the molecule holds as many hydrogen atoms as it can.
- No more hydrogen can be added without breaking an existing bond.
- This is what separates the alkanes from the alkenes, which have a double bond.
Self review
- Give the molecular formulae of methane, ethane, propane and butane.
- How many bonds does each carbon atom form?
- Describe the structure of ethane, showing where each bond goes.
- What does saturated mean?
- Use the general formula to give the formula of the alkane with five carbons.
