The compound CH3CH(Br)CH=CHCH(Br)CH3 (2,5-dibromohex-3-ene) can exist in several stereoisomeric forms because of its two stereocentres and the double…

Both carbons bearing Br are stereocentres with identical substituents, and the C3=C4 bond can be E or Z. Because the two ends are identical, each geometry gives three stereoisomers: the (R,R) and (S,S) enantiomeric pair, and the achiral meso (R,S) form. In total there are $2\times3=6$ stereoisomers. The meso form of each geometry has an internal mirror plane (or centre of symmetry) and is optically inactive, while the (R,R) and (S,S) forms of each geometry are non-superimposable mirror images. Hence the optically active stereoisomers are E-(R,R), E-(S,S), Z-(R,R) and Z-(S,S): four in all.