R & S configuration versus trans and cis on double bond (E/Z)

[pandalove89]

When assigning R or S configuration to a chiral carbon which groups get priority? Is it by molecular weight or is it by how many molecules are on it?

For example if H, OH, Cl, and CH3 groups are on the chiral carbon what would be the ranking from first priority to last?


Also same question for double bonded compounds. When choosing trans versus cis or E versus Z, how do you pick priority?

Finally, what's the difference between E/Z and Trans/Cis? (not comparing trans to cis, but rather trans/cis as a group versus E/Z as a group)

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[LetsGo2DSchool]

For R and S, it's based on atomic weight of the individual atom (not group of atoms). So in your example of H, OH, Cl, CH3, the order would be Cl > OH > CH3 > H.

Cis/trans is for comparing only two non H groups, one on each side of the double bond. E/Z is when you have three or four non H groups.

For E/Z, compare the atoms to each other on each side of the double bond: R1 R2 = R3 R4

Compare R1 vs. R2 and R3 vs. R4. Again priority is done by atomic weight. If the two winning groups are on same side it's Z and E if they are on different sides. I know it's hard to follow in words and if I had attachment privileges, then I would show you a diagram.

A good mnemonic to remember is same side = Zame side

 

[pandalove89]

For R and S, it's based on atomic weight of the individual atom (not group of atoms). So in your example of H, OH, Cl, CH3, the order would be Cl > OH > CH3 > H.

Cis/trans is for comparing only two non H groups, one on each side of the double bond. E/Z is when you have three or four non H groups.

For E/Z, compare the atoms to each other on each side of the double bond: R1 R2 = R3 R4

Compare R1 vs. R2 and R3 vs. R4. Again priority is done by atomic weight. If the two winning groups are on same side it's Z and E if they are on different sides. I know it's hard to follow in words and if I had attachment privileges, then I would show you a diagram.


A good mnemonic to remember is same side = Zame side

So for cis/trans or E/Z would a CH3CH3CH3 group ave more priority than an OH group since the 3 CH3 groups weigh more than this one OH group?

 

[LetsGo2DSchool]

So for cis/trans or E/Z would a CH3CH3CH3 group ave more priority than an OH group since the 3 CH3 groups weigh more than this one OH group?

OH would have priority since the O outranks the highest of the CH3 groups which is one C. So when there are multiple atoms vs. multiple atoms, compare the highest ranking atom from each group.

 

[needzmoar]

OH would have priority since the O outranks the highest of the CH3 groups which is one C. So when there are multiple atoms vs. multiple atoms, compare the highest ranking atom from each group.

Yeah, the OH group gets priority but iirc, the rules state that you go by the the first atom that is bonded to the chiral carbon.

e.g. ~OH would have higher priority than ~CH3CH3OH

 

[LetsGo2DSchool]

This is the hardest thing to explain without a diagram.

 

[needzmoar]

This is the hardest thing to explain without a diagram.

Yeah, it is. To the OP, here's what I'm trying to say

View attachment pannnnnnnndadadadadada.gif

I was too lazy to draw the two R groups on the ring and assign them too so we'll just focus on the other two R groups.

edit: 1 = higher priority

 

[pandalove89]

Yeah, it is. To the OP, here's what I'm trying to say

View attachment 17507

I was too lazy to draw the two R groups on the ring and assign them too so we'll just focus on the other two R groups.

edit: 1 = higher priority

so this is the rule for everything (cis/trans.....E/Z......R vs S) ?