Question

Explain why the \(-\mathrm{CF}_{3},-\mathrm{NO}_{2}\), and -CHO groups should be meta-orienting with deactivation.

Short answer

The \(-CF_3\), \(-NO_2\), and \(-CHO\) groups are meta-orienting and deactivating in electrophilic aromatic substitution reactions due to their electron-withdrawing nature, which decreases the overall electron density in the benzene ring and makes it less reactive towards electrophilic attack. Their orientation preference arises from the resonance structures of the intermediates, which show that the meta position has the lowest electron density and therefore the most stable intermediate for electrophilic attack.

Step-by-step solution

Identifying the effect of the groups on electron density

First, we need to understand the nature of these groups. All three groups, \(-CF_3\), \(-NO_2\), and \(-CHO\), are electron-withdrawing groups (EWGs). They withdraw electron density from the benzene ring, leading to a decrease in electron density and making the ring less susceptible to electrophilic attack.

Examining the resonance structures

Next, we'll analyze the resonance structures of these groups with the benzene ring. When an electrophile attacks the benzene ring, the stability of the intermediate formed will be important for determining the regioselectivity. Draw the resonance structures for these groups attached to the benzene ring, and observe how the electron density is distributed in the ortho, meta, and para positions.

Understanding the deactivation effect

Since these groups are EWGs, they have a deactivating effect on the benzene ring, making it less reactive toward electrophilic attack. They stabilize the intermediate formed during the attack by sharing their electron-withdrawing ability. The increased stability of the intermediate means that it is less likely to react with an electrophile, leading to slower overall reaction rates.

Importance of meta-orientation

Considering the resonance structures of the ring with these groups attached, we find that the electron density is lowest at the meta positions. When an EWG is present on the ring, the intermediate is stabilized when the electrophile attacks the meta position because the electron-withdrawing effect of the group is not disturbed. In contrast, attacking the ortho or para position will disrupt the electron-withdrawing effect of the group, destabilizing the intermediate and making it less likely to form.

Conclusion

Based on the electronic properties of the \(-CF_3\), \(-NO_2\), and \(-CHO\) groups, we can conclude that they are meta-orienting during electrophilic aromatic substitution reactions. Their electron-withdrawing nature decreases the overall electron density in the benzene ring, making them deactivating groups. Additionally, the orientation preference arises from the resonance structures of the intermediates, which show that the meta position has the lowest electron density and therefore the most stable intermediate for electrophilic attack.

Key concepts

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are fascinating participants in many chemical reactions, especially in the context of electrophilic aromatic substitution. These groups, like \(-\text{CF}_3\), \(-\text{NO}_2\),and \(-\text{CHO}\), have a unique ability to pull electron density away from the benzene ring to which they are attached. This effect results in a decrease in the electron density of the overall benzene ring. When EWGs are present, the ring becomes less reactive to electrophiles, which are species that seek electrons. Therefore, electron-deficient rings, as a result of EWGs, tend to react less swiftly in such substitutions. This withdrawal of electron density by EWGs makes them critically important in determining the chemical reactivity and orientation during reactions.

Regioselectivity

Regioselectivity is a concept that describes the preference of a chemical reaction to occur at one location over another on a molecule. In electrophilic aromatic substitution, the presence of electron-withdrawing groups significantly influences regioselectivity. With EWGs attached to the benzene ring, the electron density is not evenly distributed around the ring. This uneven distribution directs the incoming electrophile to particular positions on the ring.

• Ortho: Directly adjacent to the EWG.
• Meta: One carbon removed from the EWG.
• Para: Directly opposite the EWG.

Among these, EWGs advance the likelihood of meta substitution, where the electrophile bonds to the carbon one position removed from the group. This meta orientation maximizes stability through minimal disruption of the EWG's electron-withdrawing capability.

Resonance Structures

Resonance structures are alternative structures used to represent the delocalization of electrons within a molecule. In the case of benzene rings with electron-withdrawing groups, resonance structures play a crucial role in determining where electrophilic substitution is most favorable. By drawing and analyzing these structures, we can observe how electron density shifts across the ring. Electrophilic attack causes the formation of a resonance-stabilized cationic intermediate. The distribution of this positive charge is significantly influenced by resonance structures. When EWGs are present, the resonance structures show that the meta position retains the least disruption to the flow of electron-withdrawing effects from the group. Thus, the most stable intermediate during an electrophilic attack forms when the substitution occurs at the meta position. This resonance-backed stability guides the reaction outcome.

Meta-Oriented Substitution

Meta-oriented substitution is a specific form of electrophilic aromatic substitution where the incoming electrophile attacks the meta position relative to an existing substituent on the benzene ring. When the substituent is an electron-withdrawing group, like \(-\text{CF}_3\), \(-\text{NO}_2\), and \(-\text{CHO}\),the result is a strong preference for meta substitution.This preference exists because the electron deficiency created by EWGs is least disturbed at the meta position. Electrophiles, seeking the path of least resistance in terms of electron rearrangement, find the most stable situation for reaction at this position. The resultant intermediates formed by meta attack are more stable due to less interference with the EWG, ensuring the highest probability of successful reaction and therefore directing the regioselectivity toward meta.