[Donaldson Tan]

I am not sure how to apply it for naming enantiomers of cyclic structures and when one of the substituent is C=C or C=C. Any advice would be appreciated. I find no difficulty  identifying R or S compounds as long they arent cyclic or contain double/triple bonds.

[corey2]

For C=C or triple bond is very simple, just consider a vinyl substituent -C=C-H, it will be considered like -(C-C-H)2.
For cyclic compounds is not very problematic if you remember to consider only the first atom bonded to the chiral centre, and only if 2 substituents are identical, follow the ring only 1 more atom, then reapply the rules, go on until you find the correct stereochemistry. (in 99% of compounds you don't need to reach the 3rd atom for the determination of the stereochemistry).

I hope this will help you, i know this is not a great explanation, if you want i'll post 1 or 2 examples explaining this problems.

[Donaldson Tan]

just consider a vinyl substituent -C=C-H, it will be considered like -(C-C-H)2.

how do i consider -(CH2-CH3)₂ in the first place?

[corey2]

Let's see the example A, the numbers are the correct application of the rules: you can look on them clockwise and the less important substituent (hydrogen in this case) is behind the chiral centre --> (R)
1 is N atom, more important than C
2 is a CH2
3 is a CH2
4 a H

2 and 3 are the same so we look at the next adiacent atom
2 is CH2-CH3 again
3 is CH3

Example B is easy, but here the iso-propyl substituent is more important --> (S)
Example C: consider the vinyl like the obviously wrong molecule in the bottom right, but that's the point ! With a double bond you must consider an additional virtual CH2.
So is very similar to B in this case---> (S)

[dexangeles]

Corey got you covered  

this might help.....just like E-Z conformations on alkenes