Question

Which group in each pair is assigned the higher priority in R,S nomenclature?

1. ${\mathbf{\text{-CD}}}_{\mathbf{3}\mathbf{,}}{\mathbf{\text{-CH}}}_{\mathbf{3}}$
2. localid="1648446360910" ${\mathbf{\text{-CH(CH}}}_{\mathbf{3}}{\mathbf{\text{)}}}_{\mathbf{2}}{\mathbf{\text{,-CH}}}_{\mathbf{2}}\mathbf{\text{OH}}$
3. localid="1648447310055" ${\mathbf{\text{-CH}}}_{\mathbf{2}}{\mathbf{\text{Cl,-CH}}}_{\mathbf{2}}{\mathbf{\text{CH}}}_{\mathbf{2}}{\mathbf{\text{CH}}}_{\mathbf{2}}\mathbf{\text{Br}}$
4. ${\mathbf{\text{-CH}}}_{\mathbf{2}}{\mathbf{\text{NH}}}_{\mathbf{2}}{\mathbf{\text{,-NHCH}}}_{\mathbf{3}}$

Short answer

The following group among the pairs will get the higher priority:

1. ${\text{-CD}}_3$
2. ${\text{-CH}}_2\text{OH}$
3. ${\text{-CH}}_2\text{Cl}$
4. ${\text{-NHCH}}_3$

Step-by-step solution

Assigning the R and S nomenclature

The enantiomers of a compound are two separate compounds. According to the rules of IUPAC, the enantiomers are named by adding the prefixes R or S to their IUPAC names.

The priority is assigned to each group that is connected to the stereogenic center. With the help of these priorities, the enantiomers are labeled as R or S.

Rules to assign priority

The priority is assigned to various groups based on the following rules:

• The directly bonded atom, which has the highest atomic number, gets the highest priority.
• The isotopes with the highest mass number get the highest priority.
• The multiple bonds are considered as the equal number of singly bonded atoms.
• If the atoms bonded to the stereocenter are the same, the next bonded atom is considered for assigning priority.

Assigning priority to the given group of atoms

For the R and S nomenclature, the priority is assigned in the following way:

a.${\text{-CD}}_3{\text{,-CH}}_3$${\text{-CD}}_3{\text{,-CH}}_3$

Among the above pair, the group ${\text{-CD}}_3$is given the higher priority. Both these groups will be bonded to the stereogenic center by the carbon atom. After the carbon atom, the next atom is considered to assign the priority.

The next atom after the carbon is deuterium in the first group and hydrogen in the second group. Since deuterium has the highest mass number, the ${\text{-CD}}_3$group will get higher priority.

b.${\text{-CH(CH}}_3{\text{)}}_2{\text{,-CH}}_2\text{OH}$

Among the above pair, ${\text{-CH}}_2\text{OH}$ will get the higher priority. In this group, the carbon atom is bonded to the oxygen atom.

In ${\text{-CH(CH}}_3{\text{)}}_2$ the carbon is bonded to another carbon atom. An oxygen atom has a higher atomic number than carbon; it will get priority over the carbon atom.

c. role="math" localid="1648447325968" ${\text{-CH}}_2{\text{Cl,-CH}}_2{\text{CH}}_2{\text{CH}}_2\text{Br}$

Among the above pair, the ${\text{-CH}}_2\text{Cl}$group will get the higher priority. In this group, the carbon atom is bonded to the chlorine atom which has a higher atomic number.

In ${\text{-CH}}_2{\text{CH}}_2{\text{CH}}_2\text{Br}$, the carbon atom is bonded to another carbon atom.

d.${\text{-CH}}_2{\text{NH}}_2{\text{,-NHCH}}_3$

Among the above pair, ${\text{-NHCH}}_3$ will get the higher priority because the nitrogen atom has a higher atomic number than the carbon atom.