Question

According to Cahn-Ingold-Prelog sequence rules, the correct order of priority for the given groups is:
(A) $- COOH > - C{H_2}OH > - OH > - CHO$
(B) $- COOH > - CHO > - C{H_2}OH > - OH$
(C) $- OH > - C{H_2}OH > - CHO > - COOH$
(D) $- OH > - COOH > - CHO > - C{H_2}OH$

Answer 1

Cahn- Ingold- Prelog sequence rules generally deal with the concept of the atomic number of the atoms. These rules are used in organic chemistry to name a stereoisomer of a molecule. The main purpose of these rules is to assign a R/S descriptor to each stereocenter, so that the molecule can be named uniquely.

Complete step by step answer:
The naming of the enantiomers of a compound R/S nomenclature is followed.
Such a sequence of rules was designed by three chemists; R.S Cahn, C. Ingold and V. Prelog. That’s why this sequence of rules is known as Cahn- Ingold- Prelog sequence of rules or CIP rules.
The steps that are included while naming a molecule according to the CIP rules are:
Identification of stereocenters and double bonds.
Assignment of priorities to the groups attached to each stereocenter or double-bonded atom
Assignment of R/S and E/Z descriptors.
Before naming stereocenters according to R and S nomenclature, the substitutes are prioritized according to sequence rules. Sequence rules are basically for ranking priority of the groups attached to each stereocenter. Following are the sequence rules:
Rule (I): The Substituent directly attached to the stereocenter of a compound having higher atomic number is given priority over the other. That’s why hydrogen is the lowest possible priority substituent.
Rule (II): If there are substituents of the same rank, then determine which chain has first connection to an atom with highest priority (high atomic number).
Rule (III): If the same kind of atom is connected to a chain twice or thrice, then check if there is any atom having greater atomic number than any of atoms of the competing chain.

For the order given in the question,
$- OH > - COOH > - CHO > - C{H_2}OH$ is correct according to $CIP$ rules.

So, the correct answer is Option D.

Note:
Note that with the groups of comparable atomic numbers and having double or triple bonds in them then, the one having double or triple bond has higher priority than those having a single bond.