[AdiDex]

I have a Doubt say if I have to compare -CHO ad -Ch₂SH , then which will have highest priority .
See the attached image .

I have another logic if we add all the β  position atom's atomic no. then for CHO , 2 O and one H = 17
For CH2SH then 1 S and 2 H = 18

Then -CH2SH will have higher priority .
What is the criteria for such problems ?? Do we compare the highest atomic no. atom or we just add ??  or we See in how much case which substituent is winning ??

[kriggy]

Well, highest priority is the atom with higher atomic number. Therefore, S has priority over O even if its double bonded. The single bonded/double bonded matters only for same atoms. For example (hypoteticaly) in CH3-COOH, carbonyl would be priority 1, OH would be priority 2 and CH3 would be last (COOH is planar so there is no point in using CIP rules here) Is it more clear?

You can also look here https://en.wikipedia.org/wiki/Cahn%E2%80%93Ingold%E2%80%93Prelog_priority_rules its pretty good description

[Dan]

Then -CH2SH will have higher priority .
What is the criteria for such problems ?? Do we compare the highest atomic no. atom or we just add ??  or we See in how much case which substituent is winning ??

The highest atomic weight single atom always wins. 1xS always beats multiple O, you don't add up the substituents. You only consider multiple atoms if there is a tie (e.g. alcohol vs aldehyde, in which case 2xO beats 1xO)

[AdiDex]

Thanks for your reply . ^_^