17.5 The Alkanes Flashcards
Alkanes: a reminder
This is what you have learned about the alkanes so far: The alkanes are the simplest family of organic compounds.
They are hydrocarbons: they contain only carbon and hydrogen.
Their carbon – carbon bonds are all single bonds.
They form a homologous series, with the general formula Cn H2n12 .
Boiling point
Boiling point increases with chain length
What are alkanes found in?
They are found in petroleum and natural gas. Petroleum contains alkanes with up to 70 carbon atoms. Natural gas is mainly methane, with small amounts of ethane, propane, butane, and other compounds.
States of the alkanes
The first four alkanes are gases at room temperature. The next twelve are liquids. The rest are solids. Boiling points increase with chain length because attraction between the molecules increases – so it takes more energy to separate them.
Saturated
Since all their carbon – carbon bonds are single bonds, the alkanes are called saturated.
Reactivity of the alkanes
Generally, the alkanes are quite unreactive.
Flammability of the alkanes
But alkanes do burn well in a good supply of oxygen, forming carbon dioxide and water vapour, and giving out plenty of heat. So they are used as fuels. Methane burns the most easily. Like this: CH4 ( g) 1 2O2 ( g) CO2 ( g) 1 2H2 O (l) 1 heat energy
Not enough oxygen
If there is not enough oxygen, the alkanes undergo incomplete combustion, giving poisonous carbon monoxide.
Substitution reaction
Alkanes also react with chlorine in sunlight. This is called a substitution reaction, because a chlorine atom takes the place of a hydrogen atom. If there is enough chlorine, all four hydrogen atoms will be replaced, one by one.
Photochemical reaction
The reaction can be explosive in sunlight. But it will not take place in the dark, because it is also a photochemical reaction: light energy is needed to break the bonds in the chlorine molecules, to start the reaction off.
Isomers
isomers are compounds with the same formula, but different structures.
