Hex-3-ene-2,5-diol, $CH X_{3} - CH (OH) - CH = CH - CH (OH) - CH X_{3}$ , has two stereocentres and one stereogenic double bond. How many stereoisomers does it have in total (count all enantiomers, diastereomers, geometrical isomers and meso forms separately)? Give the answer as an integer.

Numerical answer type, tolerance 0.5.

Answer: 6

The molecule is symmetric about the middle of the

C = C

. For each double bond geometry (

E

Z

) the two identical stereocentres give

(R, R)

(S, S)

and the meso

(R, S)

form, i.e.

3

distinct compounds, because

(R, S)

and

(S, R)

coincide. Thus

2 \times 3 = 6

stereoisomers, of which

4

are chiral and

2

are meso.