O Chem Question

[happyana]

Does anyone know why an OH group would be lower in priority than a CHO group? This goes with the absolute configuration stuff.

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[Andre3k]

I don't know. Does anybody know what the priorities are for naming a compound? Like if the compound has a carbonyl, a hydroxyl, and a amino group which gets the highest priority? is there a list for this?

 

[doc3232]

Does anyone know why an OH group would be lower in priority than a CHO group? This goes with the absolute configuration stuff.

Sure, this is because you look at the first atom. That is the most important. If the two first atoms are the same then and only then do you look at the next atom.
In this case O > C, so case closed. Hence, OH > CHO

I just reread your question...I disagree with your statement. I think OH is bigger. Someone correct me if I am wrong.

 

[DROCKINDAHOUSE]

Does anyone know why an OH group would be lower in priority than a CHO group? This goes with the absolute configuration stuff.

It really has to do with the number of bonds that carbon has with an electronegative atom. For instance, in the hydroxyl group the carbon has one bond with an oxygen, whereas in the aldehyde group there is a carbonyl carbon which has two bonds to an oxygen.

I'm a chem major, maybe I know what I'm talking about.

 

[doc3232]

Are you asking about configuration (like R or S) or about naming?
CHO is more oxidized for one thing.

 

[water88]

Pretty sure OH is higher priority then CHO.

 

[happyana]

Ii'm asking about the R/S configurations. I'm doing some problems and came across this one. I got it wrong and wasn't sure why. It just doesn't make sense since the rule states that if the first element is the most electronegative element then it would have the highest priority.

 

[DROCKINDAHOUSE]

pretty sure not guy

 

[happyana]

do you guys think this is an error? it's from examcrackers. argh.

 

[DROCKINDAHOUSE]

Maybe you should try maybe...i dont know...probably...OPENING an organic book

 

[Albuterol]

pretty sure not guy

lol
yea but he is right...carbonyls are higher priority groups than alcohols

 

[soopasteve]

CHO group is lower priority than OH if you're referring to R/S configurations. If you're talking about naming then I'm not sure why. I know alot of priority for naming is assigned based on reactivity of the group. a CHO group could react in a whole slew of reactions so I could see it being given higher priority in naming. I havent taken time to memorize which groups are given highest priority in naming aside from OH and alkene/alkyne, etc, so hopefully someone can respond and clear that up.

 

[PreDent2009]

The CHO group would have higher priority than the -OH group. When doing R/S nomenclature you go bond by bond to see what group will win out. In the -OH groups case, it would be C-O. In the CHO groups case it would be C-O, C-O, then C-H. You count each one of the carbon-oxygen bonds once. When you get to the first bond counted they are tied, when you get to the second C-O of the carbonyl you have your winner.

Does anyone know why an OH group would be lower in priority than a CHO group? This goes with the absolute configuration stuff.

 

[happyana]

bump

 

[water88]

Are we talking about assigning R/S or nomenclature? For nomenclature, yes, CHO would have priority over OH. But, according to the Cahn-Ingold-Prelog, you look at the first atoms attached to the chiral carbon and evaluate priority based on atomic number. Therefore OH is higher priority then CHO for assigning R/S.

There is an example with a stereocenter with the groups H, OH, CHO, and CH2OH on this website, and according to them, the OH is higher priority then CHO.


http://www.usm.maine.edu/~newton/Chy251_253/Lectures/CarbohydratesI/Carbo1FS.html

 

[nixon13]

I agree with water. In nomenclature CHO is higher priority than OH. In R/S you look at the first atom bonded to the chiral carbon. In this case it would be C-O for OH and C-C for CHO groups. Therefore the OH would have higher priority. You go by atomic number when assigning prority and double and triple bonds count as 2 or 3 serparate bonds. You probably got the question wrong because you didnt orient the molecule properly.

 

[PreDent2009]

It all just depends on what molecule you are looking at. For instance if we wanted to name and assign stereochemistry to something like
2-hydroxypropanal then OH would get higher priority than the CHO. In something like 3-hydroxy-2-methylpropanal now the CHO gets the higher priority.

I agree with water. In nomenclature CHO is higher priority than OH. In R/S you look at the first atom bonded to the chiral carbon. In this case it would be C-O for OH and C-C for CHO groups. Therefore the OH would have higher priority. You go by atomic number when assigning prority and double and triple bonds count as 2 or 3 serparate bonds. You probably got the question wrong because you didnt orient the molecule properly.

 

[water88]

It all just depends on what molecule you are looking at. For instance if we wanted to name and assign stereochemistry to something like
2-hydroxypropanal then OH would get higher priority than the CHO. In something like 3-hydroxy-2-methylpropanal now the CHO gets the higher priority.

3-hydroxy-2-methylpropanal doesn't have an -OH attached to the chiral carbon. What we're debating is whether an OH gets priority over a CHO when both groups are attached DIRECTLY to a chiral carbon. And it does. In your example, we would be comparing CH2OH to CHO.

 

[PreDent2009]

Yeah in my first post I was not thinking about them directly attached to the chiral carbon. I'll have to agree that OH would get top priority in the situation your discussing.

3-hydroxy-2-methylpropanal doesn't have an -OH attached to the chiral carbon. What we're debating is whether an OH gets priority over a CHO when both groups are attached DIRECTLY to a chiral carbon. And it does. In your example, we would be comparing CH2OH to CHO.