Consider the chirality center in the compound below. What is the highest priority substituent group? (CH3)CCl Incorrect: C(OH)CH2CH3 Substituent priority is determined by atomic numbers, working outward from the chirality center. (CH3)Br -(CH3)CH(CH3)2 not by the highest total mass of the entire substituent group. OH OH What is the second highest priority substituent group? (CH3)CCl C(OH)CH2CH3 (CH3)Br -(CH3)CH(CH3)2
The Correct Answer and Explanation is:
To determine the priority of substituent groups attached to a chirality center, we use the Cahn-Ingold-Prelog (CIP) priority rules. These rules assign priority based on atomic number, not mass or bulk.
Step 1: List the Substituents
Let’s identify the first atoms directly attached to the chiral center in each group:
- (CH₃)CCl
First atom: Carbon (C)
Attached to: Cl, CH₃, and H - C(OH)CH₂CH₃
First atom: Carbon (C)
Attached to: OH, CH₂CH₃, and H - (CH₃)Br
First atom: Carbon (C)
Attached to: Br, H, and CH₃ - (CH₃)CH(CH₃)₂
First atom: Carbon (C)
Attached to: CH₃, CH₃, and CH(CH₃)
Now apply the CIP rule:
Step 2: Compare the atoms directly attached to the chiral center.
All first atoms are carbon, so we move to the atoms attached to these carbons:
- (CH₃)CCl: This carbon is bonded to Cl, which has an atomic number of 17. Highest among all.
- (CH₃)Br: The carbon is bonded to Br (atomic number 35) — highest so far.
- C(OH)CH₂CH₃: The carbon is bonded to OH (Oxygen has atomic number 8) — lower than Cl and Br.
- (CH₃)CH(CH₃)₂: This carbon is bonded only to other carbons and hydrogens — no heteroatoms.
Final Priority Ranking:
- (CH₃)Br – due to Br (atomic number 35)
- (CH₃)CCl – due to Cl (atomic number 17)
- C(OH)CH₂CH₃ – due to OH (Oxygen, atomic number 8)
- (CH₃)CH(CH₃)₂ – only carbon and hydrogen attached
Correct Answers:
- Highest priority substituent: (CH₃)Br
- Second highest priority substituent: (CH₃)CCl
Explanation Summary:
Substituent priority is not about the total weight or size of the group. It is determined by the atomic number of atoms directly bonded to the chiral center, and if necessary, by atoms in the next bonding layer. Using this rule, (CH₃)Br takes the top spot due to the bromine atom’s high atomic number, followed by (CH₃)CCl with chlorine.
