Discuss the anticipated stereochemistry of each of the following compounds.
a.`BrCH=C=C=CHBr`
b. `CH_(2)=C=C=CHBr`
c. `BrCH=C=C=CBr_(2)`

Explanation

All these moleucles are expected to be planar. Their stereochemistry is identical to that of the corresponding chloroethenes. Also (a) can exist as cis and trans isomers and show enantiomerism. Only one compound exists in the case of (b) and (C ).
Allenes are compounds whose molecules contain the following double bond sequence:

The planes of `pi` bonds of allenes are perpendicular to each other.

This geometry of `pi` bonds causes the group attached to the end carbon atoms to lie in perpendicular planes. Due to this, allenes with different substituents on the end carbon atoms are chiral as shown in the figure given below (allenes do not show cis-trans isomerism)

(Enantiomeric forms of `1,3` dibromoallence)
These two molecules are non-superimposable mirror images of each other and are, therfore, chiral. However. they do not posses a tetrahedral atom with four different groups.