(S)-2,3- Dihydroxypropanal priority

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nomadreid
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TL;DR
In (S)-2,3- Dihydroxypropanal, the (2,3) indicates that the numbering starts at O, but doesn't OH have higher priority than O?
The question in the summary refers to
Skeletal structural formula of a three-carbon aldehyde: a CH2OH group at left, a stereocentre bearing a wedged OH below it, and a CHO aldehyde group at right with its C=O oxygen labelled O.

While I am here, I might as well be hanged for a sheep as for a lamb, so another question that will reveal the fact that I am relatively new to organic chemistry nomenclature refers to the following diagram:
Skeletal structure of a stereocentre with four groups numbered by CIP priority: OH (priority 1, red), CHO aldehyde carbon with C=O (priority 2, blue), CH2OH (priority 3, green), and H (priority 4, black, hashed wedge).

Why is one hydroxide given priority over the carboxyl group (if that's what the priority #2 group is), and the carboxyl group is given priority over the other hydroxide? And why couldn't the one that now has priority #3 get priority #1 and vice-versa?
Thanks for any help.
 
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#3 is a -CH2OH group, not an -OH group. And the priorities for the purpose of assigning R or S stereochemistry have nothing to do with numbering along a carbon chain.
 
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Many thanks, mjc123. Your answer to my second question helps me adjust my thinking to looking at the groups. That's definite progress. So now I need to figure out why the priorities appear to be: hydroxide>carboxyl>hydroxymethyl >Hydrogen

Also, the top diagram in my above post, I read, is (S)-2,3- Dihydroxypropanal, and the second one glyceraldehyde. They are isomers, both having the formula C3H6O3
, but I don't see the difference in the structures.

Thanks again for your patience and help.
 
2,3-dihydroxypropanal is glyceraldehyde. There are two enantiomers; the top one in your post is the S isomer and the bottom one the R isomer (also known as L- and D-glyceraldehyde respectively). They differ in the stereochemistry of the chiral carbon atom (no.2).

You have obviously come across the concept of priorities of groups in assigning stereochemistry. I suggest you review what you have learned and come back with specific questions if there's something you don't understand. (Basically it goes by the highest atomic number of the atom adjacent to the chiral centre, then the one after that and so on.)
 
Thanks very much, mjc123. Super. Excellent information, followed by sage advice that I will indeed follow.