Chapter 13, Problem 43P

Interpretation Introduction

Interpretation:

The compound, which reacts faster with sodium methoxide in methanol in each of the given pair of compounds is to be determined and the chemical equation for the faster reaction is to be written.

Concept introduction:

Nucleophilic aromatic substitution reactions follow an addition-elimination mechanism and follow the second order rate law.

In nucleophilic aromatic substitution reactions, the nucleophile substitutes a leaving group from the aryl ring.

Aryl halides bearing an electron withdrawing substituent undergo nucleophilic substitution rapidly.

The substituents attached ortho and para with respect to the halogen atom in the aryl halide react at similar rates. The substituents attached at meta position in the aryl halide react at slower rates than ortho and para substituents.

Electron withdrawing substituents stabilize the intermediate carbanion formed and thus are strongly activating substituents in the nucleophilic aromatic substitution reactions.

Electron donating substituents destabilize the intermediate carbanion formed and thus are strongly deactivating substituents in the nucleophilic aromatic substitution reactions.

Answer to Problem 43P

Solution:

a) $\text{o-chloronitrobenzene}$ reacts faster with sodium methoxide in methanol than chlorobenzene.

The reaction is as follows:

b) In between $\text{o-chloronitrobenzene}$ and $\text{m-chloronitrobenzene}$, $\text{o-chloronitrobenzene}$ reacts faster with sodium methoxide in methanol.

The reaction is as follows:

c) In between $\text{4-chloro-3-nitroacetophenone}$ and $\text{4-chloro-3-nitrotoluene}$, $\text{4-chloro-3-nitroacetophenone}$ reacts faster with sodium methoxide in methanol.

The reaction is as follows:

d) In between $\text{2-fluoro-1,3-dinitrobenzene}$ and $\text{1-fluoro-3,5-dinitrobenzene}$, $\text{2-fluoro-1,3-dinitrobenzene}$ reacts faster with sodium methoxide in methanol.

The reaction is as follows:

e) In between $\text{1,4-dibromo-2-nitrobenzene}$ and $\text{1-bromo-2,4-dinitrobenzene}$, $\text{1,4-dibromo-2-nitrobenzene}$ reacts faster with sodium methoxide in methanol than.

The reaction is as follows:

Explanation of Solution

a)

In this nucleophilic aromatic substitution reaction, sodium methoxide is a source of the nucleophile ${\text{CH}}_{\text{3}}{\text{O}}^{-}$, which displaces chlorine from the corresponding aryl chloride. The presence of strongly electron withdrawing substituents at ortho and para positions with respect to the halogen atom in aryl halides increases the rate towards the nucleophilic aromatic substitution reaction. The structures for two given aryl chlorides are as below:

In chlorobenzene, a chlorine atom is attached to the benzene ring while in $\text{o-chloronitrobenzene}$ a strong electron withdrawing nitro substituent is attached at the ortho position with respect to the chlorine atom. Due to the presence of strong electron withdrawing nitro substituent, $\text{o-chloronitrobenzene}$ reacts faster with sodium methoxide in methanol than chlorobenzene.

The reaction of $\text{o-chloronitrobenzene}$ with sodium ethoxide is shown below:

b)

In this nucleophilic aromatic substitution reaction, sodium methoxide is a source of the nucleophile ${\text{CH}}_{\text{3}}{\text{O}}^{-}$, which displaces chlorine from the corresponding aryl chloride. The presence of strongly electron withdrawing substituents at ortho and para positions with respect to the halogen atom in aryl halides increases the rate towards the nucleophilic aromatic substitution reaction. The structures for the two given aryl chlorides are as below:

In both the given aryl halides, a strong electron withdrawing substituent is attached on the ring. In $\text{o-chloronitrobenzene}$, a strong electron withdrawing nitro substituent is attached at the ortho position with respect to the chlorine atom. In $\text{m-chloronitrobenzene}$, a strong electron withdrawing nitro substituent is attached at the meta position with respect to the chlorine atom. A strong electron withdrawing substituent at ortho and para positions reacts at a faster rate than the substituent on the meta position. Thus, in between $\text{o-chloronitrobenzene}$ and $\text{m-chloronitrobenzene}$, $\text{o-chloronitrobenzene}$ reacts faster with sodium methoxide in methanol.

The reaction of $\text{o-chloronitrobenzene}$ with sodium ethoxide is shown below:

c)

In this nucleophilic aromatic substitution reaction, sodium methoxide is a source of the nucleophile ${\text{CH}}_{\text{3}}{\text{O}}^{-}$, which displaces chlorine from the corresponding aryl chlorides. The presence of strongly electron withdrawing substituents at ortho and para positions with respect to the halogen atom in aryl halides increases the rate towards the nucleophilic aromatic substitution reaction. Structures for $\text{4-chloro-3-nitroacetophenone}$ and $\text{4-chloro-3-nitrotoluene}$ are as below:

In $\text{4-chloro-3-nitroacetophenone}$, an acyl substituent is attached to the para position and a nitro substituent is attached to the ortho position with respect to the chlorine atom in the ring. Both the attached substituents are strong electron withdrawing substituents and will increase the rate of the nucleophilic aromatic substitution.

In $\text{4-chloro-3-nitrotoluene}$, a methyl substituent is attached to the para position and a nitro substituent is attached to the ortho position with respect to the chlorine atom in the ring. Methyl substituent is the electron donating substituent while nitro substituent is the electron withdrawing substituent. Presence of electron donating substituents at ortho and para positions deactivates the rate of nucleophilic aromatic substitution.

Thus, in between $\text{4-chloro-3-nitroacetophenone}$ and $\text{4-chloro-3-nitrotoluene}$, $\text{4-chloro-3-nitroacetophenone}$ reacts faster with sodium methoxide in methanol

The reaction of $\text{4-chloro-3-nitroacetophenone}$ with sodium ethoxide is shown below:

d)

In this nucleophilic aromatic substitution reaction, sodium methoxide is a source of the nucleophile ${\text{CH}}_{\text{3}}{\text{O}}^{-}$, which displaces fluorine atom from the corresponding aryl fluoride. The presence of strongly electron withdrawing substituents at ortho and para positions with respect to the halogen atom in aryl halides increases the rate towards the nucleophilic aromatic substitution reaction. Structures for $\text{2-fluoro-1,3-dinitrobenzene}$ and $\text{1-fluoro-3,5-dinitrobenzene}$ are as below:

Nitro substituents are strong electron withdrawing substituents.

In $\text{2-fluoro-1,3-dinitrobenzene}$, two nitro substituents are attached at ortho positions with respect to the fluorine atom. In $\text{1-fluoro-3,5-dinitrobenzene}$, two nitro substituents are attached at meta positions with respect to the fluorine atom.

Electron withdrawing substituents at ortho and para positions activate the ring more than the electron withdrawing substituents at meta positions.

Thus, in between $\text{2-fluoro-1,3-dinitrobenzene}$ and $\text{1-fluoro-3,5-dinitrobenzene}$, $\text{2-fluoro-1,3-dinitrobenzene}$ reacts faster with sodium methoxide in methanol.

The reaction of $\text{2-fluoro-1,3-dinitrobenzene}$ with sodium ethoxide is shown below:

e)

In this nucleophilic aromatic substitution reaction, sodium methoxide is a source of the nucleophile ${\text{CH}}_{\text{3}}{\text{O}}^{-}$, which displaces fluorine atom from the corresponding aryl bromides. The presence of strongly electron withdrawing substituents at ortho and para positions with respect to the bromine atom in aryl bromide increases the rate towards the nucleophilic aromatic substitution reaction. Structures for $\text{1,4-dibromo-2-nitrobenzene}$ and $\text{1-bromo-2,4-dinitrobenzene}$ are as below:

In $\text{1,4-dibromo-2-nitrobenzene}$, one nitro group is attached at ortho to one bromine atom and meta to the other bromine atom. In $\text{1-bromo-2,4-dinitrobenzene}$, there are two nitro substituents attached at ortho and para positions with respect to the bromine atom. As the number of electron withdrawing substituents attached at ortho and para positions increases, the rate of nucleophilic aromatic substitution increases. Thus, in between $\text{1-bromo-2,4-dinitrobenzene}$ and $\text{1,4-dibromo-2-nitrobenzene}$, $\text{1-bromo-2,4-dinitrobenzene}$ reacts faster with sodium methoxide in methanol.

The reaction of $\text{1-bromo-2,4-dinitrobenzene}$ with sodium ethoxide is shown below: