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Mobile App Chapter 3: Problem 124
Give the major product of mono nitration of the following compounds. Justify each case.
Short Answer
Expert verified
#Answer#: The major product of the mononitration of toluene is a mixture of ortho-nitrotoluene (about 57%) and para-nitrotoluene (about 40%).
Step by step solution
01
Nitration of Benzene
The nitration of benzene involves the attack of the electrophile (nitronium ion, NO2+) on the aromatic ring. In the case of benzene, the six carbon atoms are equivalent and provide equal electron density for the electrophile. Both ortho, meta, and para positions are all equally reactive. However, the major product of mononitration is the one and only product - nitrobenzene.
02
Nitration of Toluene
In the case of toluene (methylbenzene), the aromatic ring contains a methyl group, which is an electron-donating group (EDG) and can release electron density through the inductive effect. This electron density preferentially activates both ortho and para positions, meaning the electrophile is more likely to attack these positions. The major product of toluene nitration would be a mixture of ortho-nitrotoluene (about 57%) and para-nitrotoluene (about 40%).
03
Nitration of Nitrobenzene
Nitrobenzene already contains a nitro group on the aromatic ring. Nitro group is a strong electron-withdrawing group (EWG), which means it reduces electron density through the resonance effect. This deactivates the ortho and para positions of the aromatic ring, making the meta position relatively more reactive. Therefore, the major product of mononitration of nitrobenzene would be meta-dinitrobenzene.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Electrophilic Aromatic Substitution
One of the most significant reactions involving aromatic compounds is electrophilic aromatic substitution (EAS). This process allows us to introduce a variety of different functional groups into an aromatic system. In EAS, an aromatic compound undergoes a substitution where one of its hydrogen atoms is replaced by an electrophile.
EAS occurs in several steps. Initially, the aromatic ring, rich in \(\pi\)-electrons, forms a bond with the incoming electrophile, generating an intermediate called an 'arenium ion' or 'sigma complex'. This step is energetically costly because it disrupts the aromaticity of the ring. The arenium ion then swiftly loses a proton to a base, typically the counterion of the electrophile, restoring the aromaticity of the system. The stabilization provided by aromaticity is the driving force that makes aromatic compounds reactive towards electrophiles despite their relatively stable nature.
In the case of nitration, as mentioned in the exercise, the specific electrophile is the nitronium ion \(NO_2^+\). It is generated in situ from the reaction between nitric acid and a strong acid like sulfuric acid. The electrophile attacks the \(\pi\)-electron-rich aromatic ring and, following the loss of a proton, yields the nitro-substituted aromatic compound.
EAS occurs in several steps. Initially, the aromatic ring, rich in \(\pi\)-electrons, forms a bond with the incoming electrophile, generating an intermediate called an 'arenium ion' or 'sigma complex'. This step is energetically costly because it disrupts the aromaticity of the ring. The arenium ion then swiftly loses a proton to a base, typically the counterion of the electrophile, restoring the aromaticity of the system. The stabilization provided by aromaticity is the driving force that makes aromatic compounds reactive towards electrophiles despite their relatively stable nature.
In the case of nitration, as mentioned in the exercise, the specific electrophile is the nitronium ion \(NO_2^+\). It is generated in situ from the reaction between nitric acid and a strong acid like sulfuric acid. The electrophile attacks the \(\pi\)-electron-rich aromatic ring and, following the loss of a proton, yields the nitro-substituted aromatic compound.
Electron-Donating Groups
When we talk about substituents on an aromatic ring, their influence on the reactivity and orientation of electrophilic aromatic substitution is pivotal. Electron-donating groups (EDGs) are functional groups attached to the aromatic ring that can donate electron density into the system.
EDGs make the aromatic ring more nucleophilic, which, in simple terms, makes it hungrier for electrophiles. They activate the aromatic ring towards EAS reactions. Common examples of EDGs include alkyl groups – like the methyl group in toluene – and groups with lone pair of electrons such as methoxy (–OCH_3) or amino (–NH_2) groups.
EDGs enhance the electron density particularly at the ortho and para positions relative to their location on the ring. This is why, when a compound like toluene undergoes nitration, the major products are ortho-nitrotoluene and para-nitrotoluene, as the methyl group pushes electron density towards these sites. Being aware of the presence of EDGs and their effects is crucial for predicting the outcome of an EAS reaction.
EDGs make the aromatic ring more nucleophilic, which, in simple terms, makes it hungrier for electrophiles. They activate the aromatic ring towards EAS reactions. Common examples of EDGs include alkyl groups – like the methyl group in toluene – and groups with lone pair of electrons such as methoxy (–OCH_3) or amino (–NH_2) groups.
EDGs enhance the electron density particularly at the ortho and para positions relative to their location on the ring. This is why, when a compound like toluene undergoes nitration, the major products are ortho-nitrotoluene and para-nitrotoluene, as the methyl group pushes electron density towards these sites. Being aware of the presence of EDGs and their effects is crucial for predicting the outcome of an EAS reaction.
Electron-Withdrawing Groups
In contrast to EDGs, we have electron-withdrawing groups (EWGs) which exert the opposite effect. EWGs are groups that pull electron density away from the aromatic ring, thus making it less reactive towards electrophiles.
An aromatic ring with an attached EWG is described as deactivated, indicating its reduced reactivity. EWGs include nitro (–NO_2), carbonyl (–COR), cyano (–CN), and sulfonic acid (–SO_3H) groups. These groups usually draw electron density away through resonance or inductive effects, as is particularly evident with the strong deactivating influence of the nitro group in nitrobenzene.
Due to the electron-withdrawing nature of these groups, the ortho and para positions are less electron-rich and therefore less likely to react with electrophiles. This is why, in nitration of nitrobenzene, the meta position is favored, as indicated in the exercise. Understanding the influence of EWGs is equally important for predicting where an electrophile will attack an aromatic compound.
An aromatic ring with an attached EWG is described as deactivated, indicating its reduced reactivity. EWGs include nitro (–NO_2), carbonyl (–COR), cyano (–CN), and sulfonic acid (–SO_3H) groups. These groups usually draw electron density away through resonance or inductive effects, as is particularly evident with the strong deactivating influence of the nitro group in nitrobenzene.
Due to the electron-withdrawing nature of these groups, the ortho and para positions are less electron-rich and therefore less likely to react with electrophiles. This is why, in nitration of nitrobenzene, the meta position is favored, as indicated in the exercise. Understanding the influence of EWGs is equally important for predicting where an electrophile will attack an aromatic compound.
