Existence of isomers is the characteristic of organic compounds. Isomers are compounds having same molecular mass but different properties.
The properties vary so much that it requires a special study to explain them. The varied physical properties like boiling points and melting points, density, stability, and optical activity and chemical properties like reactions with certain reagents, formation of derivatives and their properties, biological and clinical variations etc. pose a challenge for explanation.
Since the molecule formed with four valencies of carbon has a three dimensional orientation, the compounds formed out of the carbon atom also possess the three dimensional arrangement. This is called spatial distribution of atoms in a molecule and the entire study is termed as stereo chemistry of organic molecules.
Alkanes are the saturated compounds of carbon and hydrogen. They are least reactive because all the valences of carbon are saturated and they are also called as paraffin because of this non reactive nature. The catenation property of carbon in an order of increasing by one -CH2 unit gives the gradual change in the physical properties.
Study of Organic chemistry and the study of the compounds starts with
the study of alkanes. These are the most simple and less reactive
compounds which give a wide range of ideas about the properties of other
In order to study the nature of the element Carbon
in its covalent valency state, one has to study the variety of Alkanes.
- Methane CH4 is the simplest hydrocarbon and it is in the shape of a tetrahedron since all four hydrogen atoms are spread to the corners of a tetrahedron.
- If one of the hydrogen is replaced by another CH3 (methyl) unit ethane molecule is formed.
- As in the case of methane the C-H bonds result from overlapping of the sp3 orbitals with the s orbital of hydrogen atoms. The carbon carbon bond is formed by the overlapping of sp3 orbitals of carbon.
- There is one C-C bond and six C-H bonds. the C-C and C-H bonds have the same electron distribution and hence they are cylindrical and symmetrical about a line joining the two nuclei. These are termed as σ bonds.
- The C-C bond length is 1.54Å and all the bond angles between C and H in the molecule are 109.5º as in the case of methane. Thus in ethane two tetrahedrons are joined at one point and the remaining three points of each tetrahedron is spread in three directions.
Particular set of bond angles and bond lengths does not restrict the arrangement of atoms in ethane to one structure. The rotation of the molecule at C-C single bond gives different conformations.
- Since the relationship between the hydrogen of one carbon atom and the hydrogen of the other carbon atom is not specified these different structures are possible.
- If they are as shown in the diagram at different position, it is termed as staggered. In both the cases the distance between the hydrogen atoms of different carbon atoms are different.
- Thus the staggered position is most stable and so likely structure for ethane. This can be further explained by the study of certain physical properties which show that the rotation of the molecular C-C bond is not so simple. This requires to cross an energy barrier of about 3 Kcal/mol.
- The potential energy of ethane is minimum at the staggered position of conformation and increases by rotation reaching a maximum of 2.9 Kcal/mole as it reaches eclipsed conformation.
- Most ethane molecules exist in the most stable staggered form. However an energy of 2.9 kcal is not very high and the mutual collision of molecules rapidly even at room temperatures trigger the inter conversions.
- Thus in a quantity of ethane molecules the rotation along C-C will be occurring at different stages ranging from eclipsed to staggered conformations. However any shift from the staggered conformation results in torsional strain on the molecule.
Different arrangement of atoms in a molecule that can be converted by rotating about a single bond ( in this case C-C bond) to one another is called conformation. Depending on the positions of the groups that are bonded and the nature of the groups there are different conformations in for a structure. They are Eclipsed, Staggered, Skew, eclipsed skew, Staggered skew, Anti
Particular set of bond angles and bond lengths does not restrict the arrangement of atoms in ethane to one structure. The rotation of the molecule at C-C single bond gives different conformations. Since the relationship between the hydrogen of one carbon atom and the hydrogen of the other carbon atom is not specified these different structures are possible.
The Newman projections for ethane are Eclipsed and Staggered.
If the circle represents carbon atoms and the hydrogen atoms of both front and back carbon atoms are looking at same position the conformation is called eclipsed. If they are as shown in the diagram at different position, it is termed as staggered.
Eclipsed conformation of ethane is when the hydrogen atoms bonded to both carbon atoms lie in one line when looked from one end of the carbon atom. This can be better explained by the three different representations in diagram namely Sawhorse, wedge and Newman
as shown below.
In the wedge type the thick line represents the near to the plane and
dotted line away from the plane while the normal line in the plane of