Question

Rank the following groups in order of decreasing priority.

1. ${\mathbf{\text{-F,-NH}}}_{\mathbf{2}}{\mathbf{\text{,-CH}}}_{\mathbf{3}}\mathbf{\text{,-OH}}$
2. ${\mathbf{\text{-CH}}}_{\mathbf{3}\mathbf{,}}{\mathbf{\text{-CH}}}_{\mathbf{2}}{\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{\text{,-CH}}}_{\mathbf{2}}{\mathbf{\text{CH}}}_{\mathbf{2}}{\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{\text{,-(CH}}}_{\mathbf{2}}{\mathbf{\text{)}}}_{\mathbf{3}}{\mathbf{\text{CH}}}_{\mathbf{3}}$
3. ${\mathbf{\text{-NH}}}_{\mathbf{2}}{\mathbf{\text{,-CH}}}_{\mathbf{2}}{\mathbf{\text{NH}}}_{\mathbf{2}}{\mathbf{\text{,-CH}}}_{\mathbf{3}}{\mathbf{\text{,-CH}}}_{\mathbf{2}}{\mathbf{\text{NHCH}}}_{\mathbf{3}}$
4. ${\mathbf{\text{-COOH,-CH}}}_{\mathbf{2}}\mathbf{\text{OH,-H,-CHO}}$
5. ${\mathbf{\text{-Cl,-CH}}}_{\mathbf{3}}\mathbf{\text{,-SH,-OH}}$
6. ${\mathbf{\text{-C≡CH,-CH(CH}}}_{\mathbf{3}}{\mathbf{\text{)}}}_{\mathbf{2}}{\mathbf{\text{,-CH}}}_{\mathbf{2}}{\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{\text{,-CH=CH}}}_{\mathbf{2}}$

Short answer

1. ${\text{-F>-OH>-NH}}_2{\text{>-CH}}_3$
   
2. ${\text{-(CH}}_2{\text{)}}_3{\text{CH}}_3{\text{>-CH}}_2{\text{CH}}_2{\text{CH}}_3{\text{>-CH}}_2{\text{CH}}_3{\text{>-CH}}_3$
3. ${\text{-NH}}_2{\text{>-CH}}_2{\text{NHCH}}_3{\text{>-CH}}_2{\text{NH}}_2{\text{>-CH}}_3$
4. ${\text{-COOH>-CHO>-CH}}_2\text{OH>-H}$
5. ${\text{-Cl>-SH>-OH>-CH}}_3$
6. ${\text{-C≡CH>-CH=CH}}_2{\text{>-CH(CH}}_3{\text{)}}_2{\text{>-CH}}_2{\text{CH}}_3$
   

Step-by-step solution

Assigning the R and S nomenclature

The R and S nomenclature is used to name the enantiomers. The enantiomers of a compound are considered separate compounds. Each group that is directly bonded to the stereocenter is assigned a priority.

Based on this priority, the enantiomers are named R and S along with their IUPAC names.

Rules to assign priority

There are rules used to assign priority to each group connected to the stereogenic center. By using this, the enantiomers are labeled as R or S.

Following are the rules to assign the priorities to various groups:

• The atom with the highest atomic number, directly bonded to the stereogenic center, gets the highest priority.
• For isotopes, the isotope with the highest mass number gets priority over the other isotopes.
• In the case of the multiple bonds, they are counted as the equal number of singly bonded atoms.
• In the case of the same atoms bonded to the stereocenter, the atom bonded next is considered for assigning priority.

Ranking of the groups in orders of the decreasing priority

a.${\text{-F,-NH}}_2{\text{,-CH}}_3\text{,-OH}$

The order of decreasing priority for the above groups is as follows:

${\text{-F>-OH>-NH}}_2{\text{>-CH}}_3$

The fluorine atom has the highest atomic number. Hence, it gets the first priority. The atomic number of the carbon atom is the lowest among the group. It gets the least priority.

b.${\text{-CH}}_{3,}{\text{-CH}}_2{\text{CH}}_3{\text{,-CH}}_2{\text{CH}}_2{\text{CH}}_3{\text{,-(CH}}_2{\text{)}}_3{\text{CH}}_3$

The order of decreasing priority for the above groups is as follows:

${\text{-(CH}}_2{\text{)}}_3{\text{CH}}_3{\text{>-CH}}_2{\text{CH}}_2{\text{CH}}_3{\text{>-CH}}_2{\text{CH}}_3{\text{>-CH}}_3$

The group which has more carbon atoms gets the highest priority as compared to the other groups.

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

The order of decreasing priority for the above groups is as follows:

${\text{-NH}}_2{\text{>-CH}}_2{\text{NHCH}}_3{\text{>-CH}}_2{\text{NH}}_2{\text{>-CH}}_3$

The nitrogen atom has the highest atomic number and gets the highest priority. In the second group, the secondary amine gets the second priority, followed by the primary amine.

In the case of secondary amine, the nitrogen atom is bonded to the carbon atom and gets priority over the primary amine.

d.${\text{-COOH,-CH}}_2\text{OH,-H,-CHO}$

The order of decreasing priority for the above groups is as follows:

${\text{-COOH>-CHO>-CH}}_2\text{OH>-H}$

The carboxylic acid gets the highest priority because the carbon atom is bonded to two other oxygen atoms.

The aldehyde group gets priority over the alcohol because, in aldehyde, the carbon atom forms a double bond with an oxygen atom.

e.localid="1648450198972" ${\text{-Cl,-CH}}_3\text{,-SH,-OH}$

The order of decreasing priority for the above groups is as follows:

localid="1648450206131" ${\text{-Cl>-SH>-OH>-CH}}_3$

The chlorine atom has the highest atomic number, followed by sulfur, oxygen, and a carbon atom.

f.localid="1648450216329" ${\text{-C≡CH,-CH(CH}}_3{\text{)}}_2{\text{,-CH}}_2{\text{CH}}_3{\text{,-CH=CH}}_2$

The order of decreasing priority for the above groups is as follows:

localid="1648450223735" ${\text{-C≡CH>-CH=CH}}_2{\text{>-CH(CH}}_3{\text{)}}_2{\text{>-CH}}_2{\text{CH}}_3$

The group in which the carbon atom is forming a triple bond gets the highest priority, followed by the group with a carbon-carbon double bond.