Products from Crude Oil

Products from Crude Oil

Physical properties:

The chemistry of carbon compounds is called organic chemistry. There are millions of organic chemicals, but they can be divided into groups called homologous series. All members of a particular series will have similar chemical properties and can be represented by a general formula.

The alkane series is the simplest homologous series. The main source of alkanes is from crude oil.

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Alkanes are covalent compounds. They are hydrocarbons, which means they contain hydrogen and carbon. The general formula for an alkane is CnH2n+2.

Properties and uses of alkanes:

Name of alkane: Melting point oC: Boiling point oC: Density g/cm3: State at room temperature:
Methane CH4 -182 -162 0.42 Gas
Ethane C2H6 -183 -88 0.55 Gas
Propane C3H8 -188 -42 0.58 Gas
Octane C8H18 -57 126 0.72 Liquid

The first four alkanes are gases at room temperature.

Alkanes with 5-17 carbon atoms are liquids.

Alkanes with 18 or more carbon atoms are solids.

As the number of carbon atoms increases, the melting points, boiling points and densities increases.

They are insoluble in water but dissolve in organic solvents such as benzene.

Their chemical reactivity is poor. The C-C bond and C-H bond are very strong so alkanes are not very reactive.

They will carry out combustion. Burning alkanes in air (oxygen) produces water and carbon dioxide. The reactions are very exothermic (give out heat energy), so alkanes in crude oil and natural gas are widely used as heating fuels.

For example:

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If alkanes combust in too little air, carbon monoxide may form. This is dangerous and can cause death.

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The lighter fractions (for example, petrol) are in large demand. The heavier fractions are not so useful but unfortunately chemists have to be able to convert these heavier fractions into petrol and other useful products, due to supply and demand, by a method known as cracking.

Cracking breaks down molecules into smaller ones. Catalysts or heat may be used to crack the alkane chain into smaller ones.

Note, that one of the products that is formed when we crack naphtha contains a double bond between two carbon atoms. A hydrocarbon that possesses one double bond belongs to the next homologous series called alkenes.

Another reaction that often occurs after fractional distillation is reforming. Hydrocarbons of the same formula have different boiling points. Straight-chained alkanes have greater boiling points than the branched version. This means they catch light more easily - but this can be too much for the hot cylinder of the car engine. Reforming converts straight-chained alkanes to branched.

The members of this series contain a double bond. They are hydrocarbons.

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The general formula of the alkenes is CnH2n Most alkenes are formed when fractions from the fractional distillation of crude oil are cracked.

Properties of alkenes:

Like alkanes, the boiling point, melting point and densities increase with larger size molecules.

They are insoluble in water.

They combust like alkanes to produce carbon dioxide and water. However, they burn with sootier flames due to their higher percentage of carbon content to hydrogen.

Chemically, alkenes are more reactive than alkanes. This is because they possess a double bond that can be broken open and added to in a reaction.

For example:

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These reactions are called addition reactions.

Saturated and unsaturated:

Organic compounds, like alkanes, which have four single covalent bonds to all their carbon atoms are described as saturated.

Alkenes are hydrocarbons with a double bond between two carbon atoms and are described as unsaturated. This is because they do not have the maximum number of atoms attached to their four bonds, as one is double!

Polyunsaturated margarines and vegetable oils contain many C=C bonds.