Chapter 3, Problem 3.20P

Assign priorities to the groups in each set.

1. (a) —H —CH₃ —OH —CH₂OH
2. (b) —CH₂CH=CH₂ —CH=CH₂ —CH₃ —CH₂COOH
3. (c) —CH₃ —H —COO^–—NH₃⁺
4. (d) —CH₃ —CH₂SH —NH₃⁺ —CHO

Interpretation Introduction

Interpretation:

Priorities to the given groups has to be assigned.

Concept Introduction:

Priority rules:

• Priorities based on atomic number

$\begin{array}{l}\stackrel{\text{-}\text{H,}{\text{-CH}}_{\text{3}}\text{,}{\text{-NH}}_{\text{2}}\text{,}\text{-OH,}\text{-SH,}\text{-Cl,}\text{-Br,}\text{-I}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• When priority cannot be assigned on the basis of the atoms bonded directly to the chiral center

$\begin{array}{l}\stackrel{{\text{-CH}}_{\text{2}}\text{H,}{\text{-CH}}_{\text{2}}{\text{CH}}_{\text{3}}\text{,}{\text{-CH}}_{\text{2}}{\text{NH}}_{\text{2}}\text{,}{\text{-CH}}_{\text{2}}\text{OH,}{\text{-CH}}_{\text{2}}\text{Cl}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of phantom atoms.

For example,

• Priority assignment is made at the first point of difference between groups (should not be based on the larger group).

Explanation of Solution

Given groups,

$\text{-H}\text{,}\text{-}{\text{CH}}_{\text{3}}\text{,}\text{-}\text{OH}\text{and}\text{-}{\text{CH}}_{\text{2}}\text{OH}$

Assign priorities to the given groups based on the priority rules,

$-OH>-C{H}_{2}OH>-C{H}_3>-H$

Interpretation Introduction

Interpretation:

Priorities to the given groups has to be assigned.

Concept Introduction:

Priority rules:

• Priorities based on atomic number

$\begin{array}{l}\stackrel{\text{-}\text{H,}{\text{-CH}}_{\text{3}}\text{,}{\text{-NH}}_{\text{2}}\text{,}\text{-OH,}\text{-SH,}\text{-Cl,}\text{-Br,}\text{-I}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• When priority cannot be assigned on the basis of the atoms bonded directly to the chiral center

$\begin{array}{l}\stackrel{{\text{-CH}}_{\text{2}}\text{H,}{\text{-CH}}_{\text{2}}{\text{CH}}_{\text{3}}\text{,}{\text{-CH}}_{\text{2}}{\text{NH}}_{\text{2}}\text{,}{\text{-CH}}_{\text{2}}\text{OH,}{\text{-CH}}_{\text{2}}\text{Cl}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of phantom atoms.

For example,

• Priority assignment is made at the first point of difference between groups (should not be based on the larger group).

Explanation of Solution

Given groups,

${\text{-CH}}_{\text{2}}{\text{CH=CH}}_{\text{2}}\text{,}\text{-}{\text{CH=CH}}_{\text{2}}\text{,}\text{-}{\text{CH}}_{\text{3}}\text{and}\text{-}{\text{CH}}_{\text{2}}\text{COOH}$

Assign priorities to the given groups based on the priority rules,

$-CH=C{H}_2>-C{H}_{2}COOH>-C{H}_{2}CH=C{H}_2>-C{H}_3$

Interpretation Introduction

Interpretation:

Priorities to the given groups has to be assigned.

Concept Introduction:

Priority rules:

• Priorities based on atomic number

$\begin{array}{l}\stackrel{\text{-}\text{H,}{\text{-CH}}_{\text{3}}\text{,}{\text{-NH}}_{\text{2}}\text{,}\text{-OH,}\text{-SH,}\text{-Cl,}\text{-Br,}\text{-I}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• When priority cannot be assigned on the basis of the atoms bonded directly to the chiral center

$\begin{array}{l}\stackrel{{\text{-CH}}_{\text{2}}\text{H,}{\text{-CH}}_{\text{2}}{\text{CH}}_{\text{3}}\text{,}{\text{-CH}}_{\text{2}}{\text{NH}}_{\text{2}}\text{,}{\text{-CH}}_{\text{2}}\text{OH,}{\text{-CH}}_{\text{2}}\text{Cl}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of phantom atoms.

For example,

• Priority assignment is made at the first point of difference between groups (should not be based on the larger group).

Explanation of Solution

Given groups,

$\text{-H}\text{,}\text{-}{\text{CH}}_{\text{3}}\text{,}\text{-}{\text{COO}}^{\text{-}}\text{and}\text{-}{\text{NH}}_{\text{3}}{}^{\text{+}}$

Assign priorities to the given groups based on the priority rules,

$-N{H}_3{}^{+}>-CO{O}^{-}>-C{H}_3>-H$

Interpretation Introduction

Interpretation:

Priorities to the given groups has to be assigned.

Concept Introduction:

Priority rules:

• Priorities based on atomic number

$\begin{array}{l}\stackrel{\text{-}\text{H,}{\text{-CH}}_{\text{3}}\text{,}{\text{-NH}}_{\text{2}}\text{,}\text{-OH,}\text{-SH,}\text{-Cl,}\text{-Br,}\text{-I}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• When priority cannot be assigned on the basis of the atoms bonded directly to the chiral center

$\begin{array}{l}\stackrel{{\text{-CH}}_{\text{2}}\text{H,}{\text{-CH}}_{\text{2}}{\text{CH}}_{\text{3}}\text{,}{\text{-CH}}_{\text{2}}{\text{NH}}_{\text{2}}\text{,}{\text{-CH}}_{\text{2}}\text{OH,}{\text{-CH}}_{\text{2}}\text{Cl}}{\to }\\ \text{Increasing}\text{priority}\end{array}$

• Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of phantom atoms.

For example,

• Priority assignment is made at the first point of difference between groups (should not be based on the larger group).

Explanation of Solution

Given groups,

$\text{-}{\text{CH}}_{\text{3}}\text{,}\text{-}{\text{CH}}_{\text{2}}\text{SH}\text{,}\text{-}{\text{NH}}_{\text{3}}{}^{\text{+}}\text{and}\text{-}\text{CHO}$

Assign priorities to the given groups based on the priority rules,

$-N{H}_3{}^{+}>-C{H}_{2}SH>-CHO>-C{H}_3$