Orgo Question

[yalla22]

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OKay this is really primitive..but when you have to designate R/S with diastiomers you have to prioritize groups..so in one example I had---ch3, cooh, h, and oh
I thought that the order of priority would be--
1-oh
2-cooh
3-ch3
4-h
but apparently that is wrong. it is really--
1-cooh
2-ch3
3-oh
4-h

Why is that? I thought that you always compare the atomic number of the first atom bound to the asymetric carbon--so oxygen is higher than c and so OH would have higher priority??

please help! thanks🙂

 

[PianoGirl04]

Well, my track record for answering orgo questions on SDN isn't too great (right, Nutmeg? 😉 ), but I'll give it a shot.

Who told you your answer was wrong? As far as I know, it goes by atomic number/mass of the first atom attached, so oxygen should always be higher than carbon. Anyone wanna back me up on this?

 

[lyragrl]

If these are the four groups attached to a single carbon then the OP's idea is right. Pianogirl is also right, you do go by atomic number. Oxygen's number is higher than carbon's, so OH must be the highest priority, followed by COOH, then CH3, then H.

 

[sounddiver]

I agree.... but then again, I purged my mind of all that orgo nonsense about an hour after the mcat.

 

[lyragrl]

I just thought of another thing: In your diagram, is the lowest priority molecule (H) in front? If so, you have to do the prioritization, assign R or S, and then choose the opposite of your original answer. If this doesn't make sense, then PM me and I'll explain further.

 

[Brickhouse]

Your answer is actually correct - higher priority is always assigned to the more oxidized group, in this case oxygen is obviously more oxidized. Whoever told you otherwise is WRONG and deserves to be punished for spreading such horrible lies!

 

[hmno82]

OK, so I just talked to someone who recently got their PhD in organic chem. He said that you were right if they asked you about the priority off of a chiral carbon. If the actual answer is what you stated, then there must be more to the question, or there had to be different substituents involved. Otherwise, you were correct.

BUT, if this was about naming the compound and assigning priority that way, it would be named as a caboxylic acid. That should saitisfy the IUPAC man below.😉

 

[yalla22]

My book was wrong?!😕

 

[bobby_chamo]

Originally posted by Brickhouse
Your answer is actually correct - higher priority is always assigned to the more oxidized group, in this case oxygen is obviously more oxidized. Whoever told you otherwise is WRONG and deserves to be punished for spreading such horrible lies!

I think IUPAC would have a few words with you!

 

[lyragrl]

Originally posted by yalla22
My book was wrong?!😕

Are you sure you're reading the answer in your book right? Is it a particular problem that you're doing or is it a passage in the book? Seriously, if those are the four groups off the chiral carbon, OH is the top priority then COOH, etc.

 

[Bones2008]

Yup, -OH is the highest. No question. Could be a misprint?