Alkane Hydrocarbon Organic Molecules

What Are Alkanes?

Alkanes are hydrocarbons. This means molecules that only contain the elements C for carbon and H for hydrogen. No other elements.

Alkanes are the simplest organic molecules. Alkanes are often petrochemicals, derived from crude petroleum oil. Organic molecules are those based on carbon, and crude oil contains lots of carbon because it was a living plant or algae some millions of years ago. The carbon bonds have lots of energy, and alkanes are typical fuel molecules. For example, methane is commonly called natural gas, the gas that may heat your kitchen stove if it has a flame.

Alkanes normally have a carbon chain with only single bonds (not double or triple) between the carbon atoms. The general formula for alkanes is CxHx+2, where x is the number of carbon atoms in the carbon chain.

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Naming Alkanes

Counting to Ten

Naming alkanes is like counting to ten in chemistry.

Methane, ethane, propane, butane is like saying 1, 2, 3, 4 in alkane chemistry. Specifically, the prefixes meth-, eth-, prop-, and but- refer to molecules with 1 carbon atom, 2 carbons, 3 carbons, and 4 carbons respectively. You just have to memorize these first four prefixes.

Then is gets easy. Pent- means 5. Hex- means 6. Hept- means 7. Oct- means 8. Non- means 9 (non- not nin-). Finally, dec- means 10 as in the word “decade.”

Molecular Formula    (CnH2n+2)

methane CH4

ethane C2H6

propane C3H8

butane C4H10

pentane C5H12

hexane C6H14

heptane C7H16

octane C8H18

nonane C9H20

decane C10H22

Condensed Structural Formula

methane CH4

ethane CH3CH3

propane CH3CH2CH3

 butane CH3CH2CH2CH3

 pentane CH3CH2CH2CH2CH3

hexane CH3CH2CH2CH2CH2CH3

heptane CH3CH2CH2CH2CH2CH2CH3




Ex. What is the propane formula?

The prefix prop- means a 3 carbon chain. To get the number of hydrogen atoms in the propane molecule, apply the general formula for alkanes CxHx+2, where x is 3. Double 3 and add 2 more, for 6 + 2 = 8. So there are 8 hydrogen atoms. The propane formula is C3H8.

The condensed formula might be written as CH3CH2CHto better show the propane structure. The carbon atoms on the end of the carbon chain will always have 3 hydrogens attached, while those in the middle always have 2 hydrogens attached.

Propane is often used as the gas in a barbeque or outdoor grill, assuming it doesn’t use charcoal.

Ex. What is the butane formula?

The prefix but- means a 4 carbon chain. To get the number of hydrogen atoms in the butane molecule, apply the alkane formula CxHx+2, where x is 4. Double 4 and add 2 more, for 8 + 2 = 10. The butane formula is C4H10.

The condensed formula might be written as CH3CH2CH2CHto better show the butane structure. 

Butane is often used as lighter fuel or charcoal lighter fluid.

Ex. What is the hexane formula?

The alkane hexane has 6 carbons, just like a hexagon has 6 sides. Double 6 and add 2 to make 14 H atoms. The hexane formula is C6H14.

The hydrocarbon chain could also be written like CH3CH2CH2CH2CH2CH3.

Hexane, unlike the other alkanes, is rarely used as a fuel. It is an important solvent used in chemistry laboratories. It often dissolves chemicals that won’t dissolve in water.

Ex. What is the octane structure?

The alkane octane has 8 carbons, just like an octagon has 8 sides. 

The octane structure can be written as CH3CH2CH2CH2CH2CH2CH2CHto better illustrate the carbon chain. It adds up to C8H18.

You may see the word octane on gasoline pumps. Gasoline contains many chemicals, including octane and similar molecules.