Rank priority!!!???

[feri]

1. Rank priority to designate R/S Chiral center
- The chiral center has following 4 groups
1) CHO
2) CH2OH
3) CH=CH2
4) C (triple bond) CH

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[Baylor2011]

Ranking priority you just look at atomic weight...so just add them up. If you have a tie, you look at the next atom.

CHO - 29
CH2OH - 31
CHCH2-27
CCH - 25

lowest priority will be c-ch, then itll be chch2, then cho, and highest priority is ch2oh.

I'm pretty sure that's correct, branching and bonding shouldnt have anything to do with ranking priorities for R/S designation...or at least I wasn't told.

 

[sfoksn]

1. Rank priority to designate r/s chiral center
- the chiral center has following 4 groups
1) cho
2) ch2oh
3) ch=ch2
4) c (triple bond) ch

1 > 2 > 4> 3 ?

Unless I forgot everything already 😛 Isn't this Cahn Ingold Prelog?

 

[Baylor2011]

so you're saying:

triple bonds have higher priority than double bonds, double bonds higher priority than single bonds

then regarding the first two: an aldehyde group has a higher priority than hydroxyl group

is this right? lol

 

[sfoksn]

You know, DAT material really goes down the drain quickly 😛

I thought we are supposed to look at the connectivity (counting the multiple bonds)..

So CHO has C connected with O, O, H.

CH2OH has C connected with O, H, H

CH=CH2 has C connected with C, C, H

C(triple)CH has C connected with C, C, C.

OOH > OHH > CCC> CCH....

No? In reality I am not even sure if it's right haha

 

[Baylor2011]

I just looked it up...you're almost right, math error haha

so CHO = has O, O, H = 33
CH2OH = has O, H, H = 18
CH=CH2 = has C, C, H = 25
C (trip bond) CH = has C, C, C = 36

so, triple bond > CHO > CH=CH2 > CH2OH

 

[sfoksn]

Awesome. Lucky I didn't get one on my DAT lol

 

[UCB05]

/agree with sfoksn on this one.

When dealing with priority, a double or triple bond means you count that connectivity twice.

C=O is the same as O-C-O

So an aldehyde carbon with two O bonds will have a higher priority than an alcohol with only one O subsituent.

The O's have higher priority than C's, so the aldehyde and alcohol are ranked above the alkene and alkyne. Of those, the alkyne connectivity is C(CH-CC)3 while the alkene connectivity is C(CH2-C)2H. The alkyne has higher priority.

You can't just add atomic masses. You have to look at the atomic mass at the first point of difference.
-CFH2 = 33amu -C(CH3)3 = 57amu.
-CFH2 is higher priority because F>CH3

I've seen that add-up-the-atomic-masses method before. Whoever it is that does this should stop teaching it, because it's hardly ever right.

 

[ScratchMan]

Clear answer by UCB05: "You can't just add atomic masses. You have to look at the atomic mass at the first point of difference."

 

[sfoksn]

Yeah. UCB is never wrong. Seriously. I don't know how.

 

[feri]

Thanks a lot guys...I totally agree with skofsn 🙂)) 1>2>4>3

 

[tommyngu]

1>2>4>3 is correct. look at the atom connected directly to the stereocenter. determine what atoms that atom is connected to and rank according to cahn ingold prelog convention. do this for each of the 4 groups. this way you can easily rank to determine R/S.
check this out too: http://www.youtube.com/watch?v=raJ4UMQaX8U&feature=related

i've never heard of adding up atomic masses to determin R/S. the method described above is much simpler

 

[Baylor2011]

That link is excellent, thank you very much!

 

[tommyngu]

That link is excellent, thank you very much!

glad to have helped. i haven't looked at Chad's videos too much yet, but freelanceteach seems more concise and organized. (and free!)