[spirochete]

When assigning R/S configuration, why is an allyl Group -CH=CH2 higher priority than an isopropyl group?  Both are counted as carbons bonded to one hydrogen and two other carbons.

 Next up you are comparing carbons again.  Does the allyl carbon get to count its previous pi bond to carbon again?  That is the only reason I could think of that it's higher priority.

[azmanam]

First, -CH=CH₂ is a vinyl group, not an allyl group.  (-CH₂-CH=CH₂ is an allyl group)

But, between vinyl (-CH=CH₂) and isopropyl {CH(CH₃)₂}, here's the thinking:

1) For multiple bonds, break the pi bond(s) and fill the valence of the carbon atoms with generic carbon atoms (see picture)
2) look at atoms directly attached to the carbon atom of the chiral center: C vs C - tie.
3) look at what is attached to the carbon atoms: vinyl = C,C,H  isopropyl = C,C,H - tie.
4) look at what is attached to the next carbon atom in the chain: vinyl = C,H,H isopropyl = H,H,H - vinyl group wins.
5) label priorities, assign R/S - R in my example.

Hope this helps.

[spirochete]

Cool thanks for the explanation