CBSE Class 11-science Answered

Two molecules which differ only by rotations about single bonds are said to be two different conformations of that molecules.

A representation of a molecule in which the atoms and bonds are viewed along the axix about which rotation occurs is called a Newman projection. The two carbon atoms forming the?-bond are represented by two circles; one behind the other so that only the front carbon is seen. The front carbon atom is shown by a point whereas the carbon further from the eye is represented by the circle. Therefore, the C - H bonds of the front carbon are depicted from the centre of the circle while C- H bonds of the back carbon are drawn from the circumference of the circle at an angle of 120° to each other.

 

The sawhorse projection represents a carbon-carbon bond that you are looking at neither exactly from the end nor from the side. You are looking at it at an angle such that all bonds and atoms are visible. The central carbon-carbon bond (C-C) is draw as a straight line slightly tilted to right for the sake of clarity. The front carbon is shown as the lower left hand carbon and the rear carbon is shown as the upper right hand carbon. The three bonds around each carbon atom ( C- H in ethane orC-C  in higher alkanes) are shown by three lines.