Stereochemistry

[NA19]

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Why isn't this an S stereocenter?

 

[FeinMS]

OH gets 1
CHO which is carbon connected to O,O,H (two O because of double bond) gets 2
CH2OH which is carbon connected to O,H,H gets 3
H gets 4 and it's properly in the back. Get the arc connecting 1,2,3. It will turn clockwise so R.

 

[sazerac]

The carbon on top has two bonds to (the same) oxygen. That beats the carbon on the right, which has only one bond to an oxygen.

They both lose to an actual oxygen of course.

 

[NA19]

Oh, forgot the double bond counts as two! Thanks!