Overview of the Chemistry of Unsaturated Organic Compounds

In this article, the subject of unsaturated organic compounds is discussed.  Usually organic compounds are by definition compounds which involve chemical bonding between carbon and hydrogen atoms.  Unsaturation of hydrocarbons refers to the presence of double bonds between two adjacent carbon atoms.  Organic chemicals are mostly naturally occurring in abundant quantities.  The molecular structure of all animals and plants in addition to humans is made of organic compounds that are the basic structure of all the cells in the body such as proteins and DNA molecules. 

Also naturally occurring organic compounds are present in the inner surface of the earth crust in the form of crude oil and natural gas.  These organic compounds are C-H bonds abundant  and are a valuable available source of chemical energy that can be obtained by combustion in the presence of oxygen.  A carbon atom is a very unique atom in nature,  and has remarkable chemical properties.  For example its compounds are very stable thermodynamically and are present in astronomical numbers of organic compounds. Carbon uniqueness is manifested for example by its ability to form polymers that are very stable and that have outstanding chemical and physical properties. 

Carbon itself can be present in nature in two forms that are completely different from its physical properties.  These are the graphite form and the diamond form.  These two structures are also used for completely different purposes.  Carbon chemistry is governed mainly by its electrons that are present in the outermost electronic shell or the valence shell.  In the valence electronic shell of carbon there are only four electrons that are distributed between one s orbital and three p orbitals.  These electrons are the dominating factor in determining the structure and bonding of the carbon atom in its organic compounds with hydrogen.  The highest coordination number of the carbon atoms in organic compounds is four in number.  Lower coordination number is possible for some extremely unstable electron deficient species of carbon atom such as the carbene species which can be generated in reactions in the gas phase as well as from the molecule of chloroform under basic conditions. 

Saturation of organic compounds refers also to the number of hydrogen atoms that are bound to the carbon atom.  Unsaturated organic compounds usually have less amounts of hydrogen atoms in its molecular structure.  In order to understand what is meant by unsaturated organic compounds it is useful to use the concept of orbitals hybridization.  In saturated organic compounds the hybridization mode around the carbon atom is of the sp3 type.  This means that there are one s orbital and three p orbitals that are mixed together to generate four identical hybridized orbitals with identical shape and energy. 

Unsaturated compounds on the other hand have different type of orbitals mixing or hybridization because of the fact that they possess double bonds between adjacent carbons.  The manner of hybridization in carbon carbon triple bonds is different than that which occur in double bonds only.  In  C-C double bond hybridization is of the sp2 type and not of the sp3 type.  This is due to the use of one p orbital to make the double bond between the two carbons.  In triple bonds between two carbons the manner of hybridization is of the sp type.  This is so due to the use of 2 p orbitals in order to make the two double bonds between the adjacent carbons.  As the s character is increased in the hybridization type the acidity of the hydrogen that is attached to the unsaturated carbon is increased.  This means that this hydrogen can be deprotonated easily using a relatively mild base of medium strength.

The double bond between two carbons can have two types of bonds.  These are sigma bond and pi bond.  Pi bond is usually higher in energy than the sigma bond and is of low symmetry in comparison with the sigma bond.  In triple bonds there are two pi bonds and one sigma bond.  Double bonds and triple bonds are higher energetically than the single bond molecule.  Hydrogenation of double and triple bonds is an exothermic process that is involved with release of energy in the form of heat.  The two pi bonds in acytelene are perpendicular to each other. 

A compound which possesses many conjugated double bonds is usually a very stable compound thermodynamically due to its stability energetically.  Conjugated double bonds in certain compounds are characterized by their absorption of light in the visible region.  Thus these compounds are usually pigments that have colors that depend on the length of the conjugated double bonds.  The structure that is responsible for conferring the color characteristics of the conjugated double bonds is called a chromophore.  Each pigment that includes different chromophore will have different color and different length of the conjugated double bonds.  Clothes  that become contaminated with pigments are usually treated with bleach which functions by oxidizing the double bonds.  Thus it interferes with the physical property of the chromophore by disrupting its continuity.