The organic molecules are composed of carbon and hydrogen. Hydrocarbons are simple examples of organic molecules. Hydrocarbons are composed of carbon and hydrogen in which carbon atoms can be bonded in straight chain or branch manner. Hydrocarbons can be represented with the help of molecular formula and structural formulas. The molecular formula indicates the number of constituent elements whereas structural formula indicates the bonding of constituent elements.
Two or more hydrocarbons with the same molecular formula but different structural formula are known as isomers and this phenomenon is called as isomerism. Isomerism can be two types; structural and stereoisomerism. Structural isomers have the same molecular formula but different structural formulas whereas stereoisomer has different 3-dimensional arrangement of atoms that are bonded to the central atom.
One more type is conformation and these organic molecules are called as conformers. They are originated due to free rotation of carbon-carbon single covalent bond. For example, butane can exist in staggered and eclipsed forms which are inter-convertible into each other. Conformers have very little difference in their energy level therefore they can convert into each other with the free rotation of C-C bond. There are several ways to show the structures of conformers like Fisher projection, Newman projection and saw-horse projection. Let’s discuss Newman projection of alkanes with some appropriate examples.
Methane CH4 is the simplest hydrocarbon and it is in the shape of a tetrahedron. If one of the hydrogen is replaced by another CH3 (methyl) unit ethane molecule is formed. There is one c-c bond and six C-H 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. One can imagine either both tetrahedrons look eclipsed when looked from one end or be staggering.
Different arrangement of atoms in a molecule that can be converted by rotating about a single bond ( in this case C-C bond) in to the other is called conformation.
The Newman projections of propane shows that there is a CH3 group which replaces one hydrogen of one of the carbon atoms. There is a possibility of the following structures- Eclipsed and staggered.
The third carbon atom with three hydrogen atoms also prefer the position which is in staggered conformation with the middle carbon atom. When there is the unsymmetrical part in the shape of the molecule, there are certain changes in the properties related to the position of the non symmetrical portion. One of them is steric hindrance. when there is a density of atoms at a given space the bonds get distorted and weak. In case of eclipsed structure the space for the bonded electrons and the atoms of the adjacent atom is reduced and so the eclipsed structure is not preferred. In the staggered conformation the spatial distribution of the atoms is more uniform and so the staggered position is preferred.
Butane has molecular formula C4H10. All the four carbon atoms are in straight chain. CH3-CH2-CH2-CH3. There are a possible four Newman projections. Out of these four the first one shown is only theoretical which is absolute eclipsed. The third position which is a gauche in eclipse form is also very minimal in possibility.
In staggered position there are three conformations gauche(g+),(g(-), staggered 135º position and Anti conformation which is also known as 180º conformation. This is the most stable conformation of butane. The different positions are shown below. When atoms or branching is adjacent one another it is considered in gauche position. When they are 180 degrees apart, they are said to be in anti position
- In case of 2 methyl butane the structural formula is CH3-CH(CH3)-CH2-CH3. In this compound instead of CH3 group a C2H5 group is attached on second carbon atom.
- Even in this case the staggered anti conformation is stable although there is an increased possibility of the 135º conformation having stability because of the distance between the bulk groups.
- n-pentane is almost equally stable in eclipsed or staggered position of the terminal carbon atoms. This is attributed to the distance between the terminal carbon atoms.
- In general the staggered position has less potential energy. When a
molecule is rotated about the carbon-carbon single bond the potential
energy of the molecule increases by about 2-3 Kcal/mole. in other words
if a graph is plotted between the rotation on x axisand potential energy
on y axis, between two staggered positions there is a crest which is
ranging from 2-3 Kcal/mole.
- As the chain length becomes longer in
between the two terminal carbon atoms in a straight chain alkane, the
difference in potential energy reduces.The energy required to rotate the
ethane molecule about the single bond of C-C is called torsional
- The relative instability of the eclipsed or any other
intermediate skew conformations is due to the torsional strain.
Cyclohexane is a closed molecule with six CH2
groups joined together. In order to maintain the near tetrahedral structure the molecule is staggered in two different shapes. They are called chair conformation and Boat conformation.
Chair conformation is stable than the boat conformation which is explained as due to the better spatial distribution of groups and atoms.
Pentane exists in three isomers. Their Newman structures are