Allenes and Related Molecules

In allenes fa) the adjacent ji bonds are orthogonal and (b) each terminal carbon and its two substituents lie in a plane, and these planes are orthogonal. 

Nearly all the examples of chiral molecules so far encountered possess one or more stereogenic carbons. In this chapter the scope is widened, and organic molecules are considered that are chiral but do not have stereogenic centres, as also are some molecules in which the stereogenic centre is not carbon. In all cases the condition for chirality, namely that a molecule and its mirror image are not superimposable, is met. 

The prediction that certain allenes could exist as enantiomers was made by van t Hoff in 1875 Maitland and Mills finally verified his prediction (in 1935/36) after intense effort in many laboratories. 

We consider the example of 1,3-dichloropropa-1,2-diene, which is shown with its mirror image in 4a and 4b. These two structures are related as enantiomers. 

If possible make a model to verify this, though not all model kits will have the facility to represent the central carbon. Take structure 4b and rotate it by 180° around a vertical axis through the central carbon this gives 4c, and neither by this, nor by any other manoeuvre, can 4b be superimposed on 4a. 

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