Chirality, Enantiomers and Optical Activity

When an sp3 hybridized tetrahedral carbon is attached to four different groups, as in 1 (see below), the molecule cannot be superimposed on its mirror image. This is the same property that a right hand, say, placed in front of a plane mirror possesses. Molecules such as 1 are said to be chiral (or handed), from the Greek word cheir for hand. 

Chirality then is the property of handedness, and the adjective chiral refers to the molecule as a whole, rather than to a particular atom. Molecules such as CC14 and CH2C12 that can be superimposed on their mirror images are said to be achiral . 

Molecules such as 1 and 2 are termed enantiomers , from the Greek enantio meaning opposite. Enantiomers are defined as a pair of molecules related as non-superimposable mirror images note that it is essential to include non-superimposable in the definition. It can be seen that enantiomers are a particular sub-class of stereoisomers. Each enantiomer of a pair has the same physical properties (e.g. m.p., b.p. and solubility) with one exception. This distinguishing feature arises when, in separate experiments, plane polarized light is passed through a solution of each 

A rowing boat is clearly not chiral, but note that it possesses a plane of symmetry. An engine, presumed symmetrical, fitted to this boat will have a propeller, almost certainly with three blades. The propeller itself is chiral (verify this by observation if possible) and after the engine with its propeller is fitted, the boat as a whole has now become chiral by virtue of its propeller, which has also caused the boat to lose its plane of symmetry. 

Some types of aircraft use one or more propellers that each have four blades each propeller is again chiral. Is a jet engine chiral (Consider the engine, not running, viewed from the front of the plane). 

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