Introduction to organic chemistry
CRUDE OIL consists of a mixture of hydrocarbons.
Hydrocarbons are organic compounds containing carbon and hydrogen only.
The compounds in crude oil can be separated by fractional distillation, where a fractionating column is used to separate the fractions based on boiling points and the length of the carbon chain.
Smaller molecules with lower boiling points are collected at the top of the column, while larger molecules with higher boiling points are collected at the bottom.
The fractions are mainly used as fuels with refinery gas, gasolene and diesel oil being the most important ones.
Crude oil is fed at the bottom. As you go up the column, the temperature decreases. Hydrocarbons with smaller carbon chains are very flammable and volatile (low boiling points) and are removed at the top.
Long-chain hydrocarbons are highly viscous and are not very flammable and volatile. They are collected nearer to the bottom of the column.
The smaller-chain hydrocarbons are more useful, so longer- chain hydrocarbons are usually cracked to produce these smaller chains.
Cracking – is the decomposition reaction of breaking up large hydrocarbons (alkanes) into smaller more useful products. Thermal cracking involves heating to very high temperatures. Catalytic cracking involves passing the hydrocarbon over hot ceramic catalyst. The products of cracking are smaller alkanes and an alkene.
Most of the hydrocarbons in crude oil are saturated compounds such as methane, ethane, propane and butane.
When hydrocarbons are completely burnt in oxygen, they produce carbon dioxide and water. Incomplete combustion leads to the formation of carbon monoxide. Carbon dioxide is a greenhouse gas and causes global warming.
Other products of burning fossil fuels and coal include oxides of sulphur and nitrogen. These pose several environmental risks, including the formation of acid rain.
A homologous series represents a family of organic compounds with its members having a general formula and showing similar physical and chemical properties.
Carbon forms a large variety of compounds based on its ability to bond with itself and form single, double and triple bonds. It can also form ring structures.
Alkanes, alkenes, alcohols and alkanoic (carboxylic) acids are some examples of different homologous series. Members are named based on the number of carbon atoms present (‘meth’ - 1 carbon, ‘eth’ - 2 carbons, ‘prop’ - 3 carbons) and ending in the parent name of the series (‘ane’ - alkane, ‘ene’ - alkene, ‘ol’ - alcohol, ‘oic’ - alkanoic acid).
The large variety of carbon compounds that exist is based on the presence of different homologous series. Members of the same series have similar chemical properties and the same functional group. Physical properties such as melting and boiling points increase as the size of the molecule increases (addition of a -CH2- group). Density also increases. For example, as you go down the alkane series, the compounds change from gas (methane) to liquid (hexane) to solid.
Isomers occur when compounds have the same general formula but differ in their structural formula.
Structural isomers differ in the arrangement and linkage of atoms. For example, let us examine the isomers of pentane C5H12.
The branching of groups from off the longest chain allow other isomers to be formed. They differ in how the atoms are displayed but they all have the same molecular formula of C5H . The first
12 molecule is called pentane, the second molecule is called 2methylbutane because the branch is a methyl group (CH3) placed on the second carbon in the longest chain. The third molecule has two methyl groups (CH3), both attached to the second carbon. It is named 2,2- dimethylpropane.
Please note that there is a specific method used to name organic compounds using the IUPAC system.
Other structural isomers differ in the position of the functional group. In alcohols, the functional group is -OH. When this group is placed in different positions, different isomers are formed as shown.
The positioning of substituents such as a halogen atom (Cl, Br) can lead to the formation of isomers.
These isomers normally differ in physical properties such as melting and boiling points and even physical state. However, they would all undergo the same chemical reactions.