TBR Orgo seems to mention stereoselectivity with E/Z possible a lot but

[onedirection]

It never explains when it is E and when it is Z

is there a way to tell

or is it simply both

I know E is more stable

but that's about it

So is E alkenes more prevalent in reactions like

the Wittig Reaction

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[monkeyvokes]

trans or E should be preferentially created if both options are available. The only example I can think of when this may not occur is if you have two Z hydroxyl groups that get stabilized next to each other because of hydrogen bonding..

Sometimes reagents can't make both options, though. For instance, what if you treated an alkyne with a single round of H2 Pd/C? for sure you get cis addition of H2 forming a Z product..
Same rule goes for reactions like hydroboration oxidation (mark) and oxymercuration reduction (anti mark)..