Determining R/S configuration

[nwu]

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I just wanted to confirm if how I am assigning the R/S configuration for D-erythrose is correct:

http://chemwiki.ucdavis.edu/@api/de...?size=bestfit&width=211&height=123&revision=1

Answer for the R/S configuration based on the website is 2R, 3R
I understand how to find R for the C2 carbon but a little vague for the C3 carbon.

We look at C3 carbon, we can immediately assign the hydrogen as #4 and the alcohol oxygen as #1
Moving outward we focus on the C2 and C4 carbons.

So C2 and C4 are both carbons so we must look at what is attached to them to break the tie. C2 is attached to OHH while C4 is also attached to OHH. So still a tie, we move outward again. So on the left side it kinda of runs out of atoms, I picked Hydrogen on the alcohol as the last atom to compare against while on the right side I went to the carbonyl carbon. So here it is H vs C. Is this correct? As in we run out of atoms on the very left side so H is the correct comparative atom to look at while the carbon carbonyl atom is the next one to compare against as well?

Thanks!

 

[Czarcasm]

I just wanted to confirm if how I am assigning the R/S configuration for D-erythrose is correct:

http://chemwiki.ucdavis.edu/@api/de...?size=bestfit&width=211&height=123&revision=1

Answer for the R/S configuration based on the website is 2R, 3R
I understand how to find R for the C2 carbon but a little vague for the C3 carbon.

We look at C3 carbon, we can immediately assign the hydrogen as #4 and the alcohol oxygen as #1
Moving outward we focus on the C2 and C4 carbons.

So C2 and C4 are both carbons so we must look at what is attached to them to break the tie. C2 is attached to OHH while C4 is also attached to OHH. So still a tie, we move outward again. So on the left side it kinda of runs out of atoms, I picked Hydrogen on the alcohol as the last atom to compare against while on the right side I went to the carbonyl carbon. So here it is H vs C. Is this correct? As in we run out of atoms on the very left side so H is the correct comparative atom to look at while the carbon carbonyl atom is the next one to compare against as well?

Thanks!

Usually if you run into a complication like this, you went wrong somewhere. It's a good idea to backtrack and approach this methodically to avoid careless error. C2 is attached to: O, C, H ; C4 is attached to O, H, H. Hence C2 is 2nd priority, C4 is third (OH is first) - collectively this forms the R configuation since the H is pointing away from you.