Stereospecific eliminations can give pure single isomers of alkenes

Stereospecific eliminations can give pure single isomers of alkenes 

You met a stereospecific elimination in Chapter 19. The requirement for the H and the Br to be anti-periplanar in the E2 transition state meant that the two diastereoisomers of this alkyl bromide eliminated to alkenes with different double bond geometries (p. 491). 

However, reactions like this are of limited use—their success relies on the base s lack of choice of protons to attack provide an alternative H and we are back with the situation in the reaction on p. 810. Logic dictates, therefore, that only trisubstituted double bonds can be made stereospecifically in this way, because the reaction must not have a choice of hydrogen atoms to participate in the elimination. The answer is, of course, to move away from eliminations involving H, as we did with the Julia olefination. We shall look at this type of reaction for much of the rest of this chapter. 

---