Question

According to the R, S convention, which of the groups has highest priority?
(A) $C{H_2}OH$
(B) COOH
(C) $COC{H_3}$
(D) $COOC{H_3}$

Answer 1

The priority order of the groups is decided on the basis of the atomic number of the atom or the substituent group which is directly attached to the stereocenter. If the directly attached atom is the same for two or more groups we move on to the next atom attached to this atom along the chain of this substituent until we find a difference. The substituent chain having first connection to an atom of highest priority will be the chain of higher priority.

Complete step by step answer:
-First of all let us see what the R, S convention is.
The R, S convention is basically a method which specifies the absolute configuration of the chiral centres in the organic molecules. We should also remember that the corresponding chiral centres in enantiomers have opposite absolute configuration.
The enantiomers of any chiral compound are either “right handed” or “left handed” and are named as R and S stereocenters.
-The priority order of the groups is decided on the basis of the atomic number of the atom which is directly attached to the stereocenter of the compound. The substituent with a higher atomic number is more prior as compared to the substituent with lower atomic number. If the substituents directly attached to the stereocenter are of the same atomic number then we move or proceed along the substituent chains, until we find a point of difference. For them as we move along the chain from one atom to another, the chain having first connection to an atom of highest priority will be the chain of higher priority.
-This is how we will now see all the groups given in the options for their priority order:
All the 4 groups have the first atom which is directly attached to the stereocenter to be the same: Carbon (C). So, we will now look at the next atoms attached to them.

For (A) $C{H_2}OH$: Here the C atom is attached to 2 hydrogen atoms (At.no. = 1) and one oxygen atom (At.no. = 8).

For (B) COOH: Here the C atom is attached to 3 oxygen atoms (At.no. = 8). It is 3 because of a double and a single bond. One of the oxygen atoms is further attached to a hydrogen atom.

For (C) $COC{H_3}$: Here the C atom is attached to 2 oxygen atoms (At.no. = 8) and one carbon atom (At.no. = 6).

For (D) $COOC{H_3}$: Here the C atom is attached to 3 oxygen atoms (At.no. = 8). One of these oxygen atoms is further attached to a carbon atom.
-We know that the atom having higher atomic number is of higher priority and so the atoms involved in the given compounds will have the following priority order:
Oxygen > Carbon > Hydrogen

-From the above discussion we can conclude that $COOC{H_3}$ will be the compound of highest priority since the carbon atom (C) will be attached to 3 oxygen atoms (O), which is further attached to a carbon atom (C). This is the highest priority group in the given options.
So, the correct answer is “Option D”.

Note: We should know that optical isomers or enantiomers have the same sequence of atoms and bonds but are different in their 3D shape. The enantiomers are non-superimposable mirror images of one another. We can thus call them chiral.