Chapter 3: Problem 29
The deactivating as well as meta directing group(s) among the following is/are (a) \(-\mathrm{CH}_{2} \mathrm{Cl}\) (b) \(-\mathrm{CCl}_{3}\) (c) \(-\stackrel{\oplus}{\mathrm{NR}}_{3}\) (d) \(-\mathrm{C}_{6} \mathrm{H}_{5}\)
Short Answer
Expert verified
Groups (b) \(-\mathrm{CCl}_{3}\), (c) \(-\stackrel{\oplus}{\mathrm{NR}}_{3}\), and (d) \(-\mathrm{C}_{6} \mathrm{H}_{5}\) are both deactivating and meta directing. Group (a) \(-\mathrm{CH}_{2} \mathrm{Cl}\) is only deactivating.
Step by step solution
01
Group (a) - \(-\mathrm{CH}_{2} \mathrm{Cl}\)
This group is considered mildly electron-withdrawing through the inductive effect provided by the electronegativity of the chlorine atom. It slightly slows down the rate of electrophilic aromatic substitution reactions. Therefore, it is a deactivating group. However, it has no significant resonance effect and directs the incoming electrophile to the ortho/para position, so it is not a meta directing group.
02
Group (b) - \(-\mathrm{CCl}_{3}\)
The \(-\mathrm{CCl}_{3}\) group is considered electron-withdrawing through the strong inductive effect provided by three electronegative chlorine atoms. It decreases the rate of electrophilic aromatic substitution reactions and is a deactivating group. The group has a resonance effect that directs the electrophile to the meta position more efficiently, making it meta directing as well.
03
Group (c) - \(-\stackrel{\oplus}{\mathrm{NR}}_{3}\)
This group has a positive charge on the nitrogen, making it highly electron-withdrawing. It is a strong deactivating group due to the unstable nature of a charged species. As a positively charged group, it exhibits a strong resonance effect, which is directing the electrophile to attack at the meta position relative to the group. Thus, this group is both deactivating and meta directing.
04
Group (d) - \(-\mathrm{C}_{6} \mathrm{H}_{5}\)
The phenyl group (\(-\mathrm{C}_{6}\mathrm{H}_{5}\)) acts as an electron-withdrawing group through the resonance effect, as the electrons are delocalized throughout the aromatic ring. As a result, it slows down electrophilic aromatic substitution reactions, making it a deactivating group. Since it exhibits a resonance effect directing the electrophile to the meta position, it is considered a meta directing group.
In conclusion, groups (b) \(-\mathrm{CCl}_{3}\), (c) \(-\stackrel{\oplus}{\mathrm{NR}}_{3}\), and (d) \(-\mathrm{C}_{6} \mathrm{H}_{5}\) are deactivating as well as meta directing groups, while group (a) \(-\mathrm{CH}_{2} \mathrm{Cl}\) is only deactivating.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Deactivating Groups
In the context of electrophilic aromatic substitution (EAS), a deactivating group is one that decreases the rate of the reaction when attached to an aromatic ring. These groups are electron-withdrawing, meaning they pull electron density away from the ring, making it less reactive towards electrophiles.
Electron-withdrawing can occur through two main effects: the inductive effect and the resonance effect. Groups like \texttt{\text{-CH\(_2\)Cl}} have a deactivating influence because the chlorine atom is electronegative and pulls electron density away through the sigma bonds (inductive effect). When you have groups such as \texttt{\text{-CCl\(_3\)}}, the electron-withdrawing power is amplified due to the presence of multiple electronegative atoms, leading to a stronger deactivating capability.
Therefore, in the provided exercise, the groups \texttt{\text{-CCl\(_3\)}}, \texttt{\text{-NR\(_3^+\)}}, and \texttt{\text{-C\(_6\)H\(_5\)}} act as deactivators of EAS due to their electron-withdrawing characteristics.
Electron-withdrawing can occur through two main effects: the inductive effect and the resonance effect. Groups like \texttt{\text{-CH\(_2\)Cl}} have a deactivating influence because the chlorine atom is electronegative and pulls electron density away through the sigma bonds (inductive effect). When you have groups such as \texttt{\text{-CCl\(_3\)}}, the electron-withdrawing power is amplified due to the presence of multiple electronegative atoms, leading to a stronger deactivating capability.
Therefore, in the provided exercise, the groups \texttt{\text{-CCl\(_3\)}}, \texttt{\text{-NR\(_3^+\)}}, and \texttt{\text{-C\(_6\)H\(_5\)}} act as deactivators of EAS due to their electron-withdrawing characteristics.
Meta Directing Groups
Meta directing groups are a specific type of deactivating groups that not only reduce the reactivity of the aromatic ring but also influence the position where the electrophilic substitution will occur. In benzene derivatives, these groups direct the incoming electrophile to the meta position relative to themselves. This is important in predicting the structure of the product in EAS reactions.
The meta position is the carbon two positions away from the substituent group. Not all deactivating groups are meta directing, but all meta directing groups are deactivating. In the exercise, the groups \texttt{\text{-CCl\(_3\)}}, \texttt{\text{-NR\(_3^+\)}}, and \texttt{\text{-C\(_6\)H\(_5\)}} are examples of meta directing groups. Their ability to direct the electrophile to the meta position can be attributed to either their inductive or resonance effects, which destabilize the ortho and para carbocations that form during the reaction.
The meta position is the carbon two positions away from the substituent group. Not all deactivating groups are meta directing, but all meta directing groups are deactivating. In the exercise, the groups \texttt{\text{-CCl\(_3\)}}, \texttt{\text{-NR\(_3^+\)}}, and \texttt{\text{-C\(_6\)H\(_5\)}} are examples of meta directing groups. Their ability to direct the electrophile to the meta position can be attributed to either their inductive or resonance effects, which destabilize the ortho and para carbocations that form during the reaction.
Inductive Effect
The inductive effect refers to the transmission of electron density through sigma bonds in a molecule due to the electronegativity of atoms or the presence of a polar bond. Electronegative atoms pull electron density toward themselves, leading to a partial positive charge on the atom directly bonded to it.
In the context of EAS, substituents that wield a strong inductive effect, such as \texttt{\text{-CCl\(_3\)}}, withdraw electron density from the aromatic ring, making it less nucleophilic and consequently less reactive to attack by electrophiles. This withdrawal can decrease the rate of EAS reactions. With each additional electronegative atom, like chlorine in this example, the inductive effect is enhanced, exemplifying the cumulative nature of this effect.
In the context of EAS, substituents that wield a strong inductive effect, such as \texttt{\text{-CCl\(_3\)}}, withdraw electron density from the aromatic ring, making it less nucleophilic and consequently less reactive to attack by electrophiles. This withdrawal can decrease the rate of EAS reactions. With each additional electronegative atom, like chlorine in this example, the inductive effect is enhanced, exemplifying the cumulative nature of this effect.
Resonance Effect
The resonance effect involves the delocalization of electrons within a molecule and can either donate to or withdraw electron density from the aromatic ring based on the nature of the substituent group.
Electron-withdrawing groups (EWGs) that are capable of resonance will pull electron density out of the aromatic ring through pi bonds. This can be seen with substituents like \texttt{\text{-NR\(_3^+\)}} and \texttt{\text{-C\(_6\)H\(_5\)}} in the exercise, where the EWGs stabilize the ring's electron density through overlapping p-orbitals, which affects the location and rate of electrophilic attack. In contrast to the inductive effect, the resonance effect can either enhance or reduce the reactivity of the ring, depending on whether the substituent is electron-donating or electron-withdrawing.
Electron-withdrawing groups (EWGs) that are capable of resonance will pull electron density out of the aromatic ring through pi bonds. This can be seen with substituents like \texttt{\text{-NR\(_3^+\)}} and \texttt{\text{-C\(_6\)H\(_5\)}} in the exercise, where the EWGs stabilize the ring's electron density through overlapping p-orbitals, which affects the location and rate of electrophilic attack. In contrast to the inductive effect, the resonance effect can either enhance or reduce the reactivity of the ring, depending on whether the substituent is electron-donating or electron-withdrawing.