What is the order of priority for the set of substituents below according to the Cahn-Ingold-Prelog sequence rules?
Ranking Substituents According to Cahn-Ingold-Prelog Sequence Rules
According to the Cahn-Ingold-Prelog (CIP) sequence rules, we need to consider the atomic number of the atoms directly bonded to the central carbon atom. The higher the atomic number, the higher the priority. Additionally, if there is a tie in atomic number, we look at the atomic numbers of the atoms bonded to those atoms until a difference is found.
Let's analyze each substituent:
(a) -CH=CH₂:
The carbon-carbon double bond contains two carbon atoms on both sides. We need to consider the atoms bonded to each of those carbon atoms. Since hydrogen (H) has the lowest atomic number, we need to look at the second atom. In this case, both carbon atoms are bonded to another carbon atom, so we need to proceed to the next level. Comparing the second-level atoms, we find that both carbons are bonded to hydrogen (H), which has the lowest atomic number. Thus, the priority is determined by the atom bonded to the second-level carbon atom. Since both carbon atoms in the double bond are identical, the priority for this substituent is the same as the next substituent.
(b)-CH₂CH₃:
The carbon chain contains two carbon atoms. Both are bonded to three hydrogen atoms (H), which have the lowest atomic number. Since the atoms are identical, we need to compare the second-level atoms. In this case, both carbon atoms are bonded to another carbon atom. Since the atoms are identical again, we proceed to the next level. Finally, the third-level atoms are both hydrogen (H). Since there is no difference at this point, the substituent has the same priority as (a).
(c)-CH₂OCH₃:
The carbon atom is bonded to two hydrogen atoms (H) and an oxygen atom (O). Oxygen has a higher atomic number than hydrogen, so we assign higher priority to the oxygen atom. The remaining atom is hydrogen (H) bonded to the second carbon atom. Since hydrogen has a lower atomic number than carbon, the priority of this substituent is higher than (a) and (b).
(d)—CH₂OH:
The carbon atom is bonded to three hydrogen atoms (H) and one oxygen atom (O). Again, oxygen has a higher atomic number than hydrogen, so we assign higher priority to the oxygen atom. The remaining atom is hydrogen (H) bonded to the second carbon atom. Since hydrogen has a lower atomic number than carbon, the priority of this substituent is higher than (a), (b), and (c).
The substituents can be ranked in order of priority as follows:
1 —CH₂OH
2 –CH₂OCH₃
3 —CH=CH₂
4 —CH₂CH₃