The Hydrocarbons


The Hydrocarbons

Let us first look at a group of organic compounds known as the hydrocarbons. These molecules only contain carbon and hydrogen. The hydrocarbons that we are going to look at are called aliphatic compounds. The aliphatic compounds are divided into acyclic compounds (chain structures) and cyclic compounds (ring structures). The chain structures are further divided into structures that contain only single bonds (alkanes), those that contain double bonds (alkenes) and those that contain triple bonds (alkynes). Cyclic compounds include structures such as the benzene ring.  


Hydrocarbons that contain only single bonds are called saturated hydrocarbons because each carbon atom is bonded to as many hydrogen atoms as possible. Figure above shows a molecule of ethane which is a saturated hydrocarbon.

Hydrocarbons that contain double or triple bonds are called unsaturated hydrocarbons because they don’t contain as many hydrogen atoms as possible. Figure below shows a molecule of ethene which is an unsaturated hydrocarbon.


If you compare the number of carbon and hydrogen atoms in a molecule of ethane and a molecule of ethene, you will see that the number of hydrogen atoms in ethene is less than the number of hydrogen atoms in ethane despite the fact that they both contain two carbon atoms. In order for an unsaturated compound to become saturated, a double bond has to be broken, and another two hydrogen atoms added for each double bond that is replaced by a single bond.


Fat that occurs naturally in living matter such as animals and plants is used as food for human consumption and contains varying proportions of saturated and unsaturated fat. Foods that contain a high proportion of saturated fat are butter, ghee, suet, tallow, lard, coconut oil, cottonseed oil, and palm kernel oil, dairy products (especially cream and cheese), meat, and some prepared foods. Diets high in saturated fat are correlated with an increased incidence of atherosclerosis and coronary heart disease according to a number of studies. Vegetable oils contain unsaturated fats and can be hardened to form margarine by adding hydrogen on to some of the carbon=carbon double bonds using a nickel catalyst. The process is called hydrogenation


Functional groups

All organic compounds have a particular bond or group of atoms which we call its functional group. This group is important in determining how a compound will react.

 Functional group  is a specific group of atoms within molecules, that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of.

In one group of organic compounds called the hydrocarbons, the single, double and triple bonds of the alkanes, alkenes and alkynes are examples of functional groups. In another group, the alcohols, an oxygen and a hydrogen atom that are bonded to each other form the functional group for those compounds. All alcohols will contain an oxygen and a hydrogen atom bonded together in some part of the molecule

functional groups



Isomerism is a phenomenon where two or more compounds have the same molecular formula but different structural formula and properties.

It is possible for two organic compounds to have the same molecular formula but a different structural formula. Look for example at the two organic compounds below


If you were to count the number of carbon and hydrogen atoms in each compound, you would find that they are the same. They both have the same molecular formula (C4H10), but their structure is different and so are their properties. Such compounds are called isomers.

Isomers are molecules with the same molecular formula and often with the same kinds of chemical bonds between atoms, but in which the atoms are arranged differently.


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