The alkyne shown is hydrogenated over Lindlar catalyst to the alkene. Counting all configurational isomers (geometrical and optical together) of the constitution of the alkene formed, how many stereoisomers does that constitution have? Give the answer as an integer.

Numerical answer type, tolerance 0.5.

Answer: 8

The compound is 4,5-dimethylhept-2-yne. Lindlar reduction gives 4,5-dimethylhept-2-ene,

CH X_{3} CH = CHCH (CH X_{3}) CH (CH X_{3}) CH X_{2} CH X_{3}

. It has two stereocentres (C4, C5) and one stereogenic double bond (C2=C3), with no symmetry, so

2^{3} = 8

stereoisomers (checked by enumeration).