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Check first the priority of substituents. Here Cl, C2H5, CH3, H
Then use

https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/CIP-Diagramm.svg/450px-CIP-Diagramm.svg.png

Quote from: apurva on September 07, 2022, 08:21:29 AM
Question: [attachment id=0 msg=3310]

We are going to use CIP rules (Cahn-Ingold -Prelog) to assign configurations to chiral centers.
According to this rule, priority is assigned based on the atomic mass of the atom bonded directly to chiral carbon. If the atoms are same, then to break the tie check the next atom and when you see first difference in atomic mass assign priority.
In our example -Groups directly bonded to carbon are
1)H  least atomic mass -priority 4
2) CH3 and C2H5 has a tie because in both cases it is carbon bonded to chiral carbon.
So check next atoms - in CH3 there are 3 H next to carbon and in CH2CH3(C2H5) there are two hydrogens and one carbon. So winner is C2H5 -Priority -2
3) methyl (CH3) as decided above.-Priority -3
4)Cl is of highest atomic mass so highest priority- 1
Once you assign priority, then check as you go from no 1 to 3 -if your eyes rotate "anticlockwise" then it is S and if rotate "clockwise" then it is R
Another important rule which we have to use in these Fischer projection formula is to place least priority group on the vertical line which means it is pointing away or behind the plane. If it is not on vertical line then we can always give pinwheel motion or umbrella motion or exchange of three groups to move it behind the plane or vertical line.
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Question : why is second ecample S even the arrow goes clockwise. The same for third example ,arrow goes antclockwise but it is given a R?

Quote from: chenbeier on September 10, 2022, 08:19:44 AM
Question : why is second example S even the arrow goes clockwise. The same for third example ,arrow goes anticlockwise but it is given a R?

ohh thanks -you are right
I have done the correction.