organic chemistry - designation R, S

[joonkimdds]

what are the designations of configuration for the molecule shown below?

a) 2R, 3R
b) 2R, 3S
c) 2S, 3S
d) 2S, 3R

how do i solve this? i know that R means rotate clockwise and S is counterclockwise but I still don't get it.
please explain to me please?

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[cdmOMR]

what are the designations of configuration for the molecule shown below?

a) 2R, 3R
b) 2R, 3S
c) 2S, 3S
d) 2S, 3R

how do i solve this? i know that R means rotate clockwise and S is counterclockwise but I still don't get it.
please explain to me please?

You need to rank the groups off of the carbon you are defining. The ranking goes by highest in molecular weight. For the #2 carbon the O bonded to the carbon has the highest mole. weight so it is the highest priority. The #3 carbon has an O bonded to it and the number 1 carbon has 3 H's bonded to it so the #3 carbon (plus all of its constituants) is the second priority group. and the carbon #1 is the third priorty group. The hydrogen is always last. It needs to rotate from 1st priority to 2nd priority to 3rd priority with the hydrogen acting as an axis. So it must go from the Oxygen to the #3 carbon to the #1 carbon...which looks like its going counterclockwise, but dont forget that the H is facing towards you so you have to imagine the H facing away so in reality its clockwise and R. The number 3 Carbon is S...but im not going to type it out..

You have to think of it as a 3D model, its hard to explain in writing.

 

[joonkimdds]

awesome! thanks!
but I don't understand the last part.
H is facing toward me and u said it's actually away from me?
I don't get that part.

 

[Ex_EE]

In order to determine R or S you need to orient the model such the lowest ranking group (usually H) is pointing away from you. In the current drawing the H on C2 is pointing towards you. So, you have to mentally rotate it to where it points away from you. This is not necessary for C3 since it is already pointing away from you.

 

[joonkimdds]

In order to determine R or S you need to orient the model such the lowest ranking group (usually H) is pointing away from you. In the current drawing the H on C2 is pointing towards you. So, you have to mentally rotate it to where it points away from you. This is not necessary for C3 since it is already pointing away from you.

SUPER!