[zeshkani]

whats the best way to tell how many possible cis or trans isomers a huge organic compound has?

[impulse29]

best in terms of:

fastest: internet
thoroughness: internet
learning: starting small, and moving up.

Just do a lot of smaller ones, and you'll become good at it.

[english]

There's really no formula to go by to tell how many cis/trans (E/Z) isomers you have. 

Other stereoisomers you can use an exponential relationship to find, where n is the number of asymmetrical centers:

# of Enantiomers = # of Diastereomers = 2ⁿ

For cis/trans (E/Z), use your fingers!

[movies]

That formula will give you the total number of stereoisomers, which includes enantiomers and all possible diastereomers.  the number of diastereomers and enantiomers is not necessarily equal.  An enantiomer is defined by its relationship to its mirror image, while a diastereomer is not.  Suppose you have three chiral centers in a molecule.  Pick one stereoisomer and you have one enantiomer of that compound and the rest of the stereoisomers are diastereomers relative to the compound you picked (in this case, 6 of them).

Also note that this formula only holds if there are no meso diastereomers.  If there is a meso form, then you subtract one.

[english]

That formula will give you the total number of stereoisomers, which includes enantiomers and all possible diastereomers. the number of diastereomers and enantiomers is not necessarily equal. An enantiomer is defined by its relationship to its mirror image, while a diastereomer is not. Suppose you have three chiral centers in a molecule. Pick one stereoisomer and you have one enantiomer of that compound and the rest of the stereoisomers are diastereomers relative to the compound you picked (in this case, 6 of them).

Also note that this formula only holds if there are no meso diastereomers. If there is a meso form, then you subtract one.

Oh OK.  I was thinking of a trivial example, like a compound with 2 asymmetric centers.  Glad you corrected me so I never make that mistake!

[movies]

I have been burned by the same mistake in the past!  In my undergrad thesis I made some compounds that had 4 chiral centers and a bunch of meso diastereomers.  I had to figure it all out before my defense!