$Br^+$

Definition

$Br^+$ is a positively charged bromine cation that is a key intermediate in nucleophilic aromatic substitution reactions. It serves as an electrophile that can attack and displace substituents on aromatic rings, facilitating the substitution of new functional groups.

5 Must Know Facts For Your Next Test

1. $Br^+$ is a bromonium ion that acts as an electrophile in nucleophilic aromatic substitution reactions.
2. The formation of $Br^+$ is the rate-determining step in S$_N$Ar reactions, as it creates a partially positively charged carbon that is susceptible to nucleophilic attack.
3. $Br^+$ is typically generated in situ from aryl halides (e.g., bromobenzene) through the action of a strong base or other activating agents.
4. The presence of electron-withdrawing groups on the aromatic ring can stabilize the $Br^+$ intermediate and facilitate the S$_N$Ar reaction.
5. Nucleophiles can then attack the $Br^+$ intermediate, displacing the bromine and forming a new carbon-nucleophile bond.

Review Questions

• Explain the role of $Br^+$ in the mechanism of nucleophilic aromatic substitution (S$_N$Ar).
  • $Br^+$ is a key intermediate in the S$_N$Ar mechanism, acting as an electrophile that is attacked by a nucleophile. The formation of $Br^+$ is the rate-determining step, as it creates a partially positively charged carbon that is susceptible to nucleophilic attack. The presence of electron-withdrawing groups on the aromatic ring can stabilize the $Br^+$ intermediate and facilitate the substitution reaction. Once formed, the nucleophile can then attack the $Br^+$ intermediate, displacing the bromine and forming a new carbon-nucleophile bond.
• Compare and contrast the mechanisms of nucleophilic aromatic substitution (S$_N$Ar) and electrophilic aromatic substitution (S$_E$Ar), and explain how $Br^+$ is involved in each.
  • In S$_N$Ar, $Br^+$ acts as an electrophile that is attacked by a nucleophile, whereas in S$_E$Ar, an electrophile directly attacks the aromatic ring, displacing a hydrogen. The formation of $Br^+$ is the rate-determining step in S$_N$Ar, as it creates a partially positively charged carbon that is susceptible to nucleophilic attack. In contrast, S$_E$Ar does not involve the formation of a bromonium ion intermediate like $Br^+$. Instead, the electrophile directly attacks the aromatic ring, leading to the substitution of a new functional group. While both mechanisms involve the substitution of functional groups on aromatic rings, the specific intermediates and pathways differ.
• Discuss the factors that influence the stability and reactivity of the $Br^+$ intermediate in nucleophilic aromatic substitution reactions, and explain how these factors impact the overall reaction.
  • The stability and reactivity of the $Br^+$ intermediate in S$_N$Ar reactions are influenced by several factors. The presence of electron-withdrawing groups on the aromatic ring can stabilize the $Br^+$ intermediate by delocalizing the positive charge, making the carbon more susceptible to nucleophilic attack. Additionally, the nature of the leaving group (e.g., halide) can affect the ease of $Br^+$ formation, with better leaving groups (e.g., iodide) facilitating the generation of the $Br^+$ intermediate. The stability of the $Br^+$ intermediate, in turn, impacts the rate and efficiency of the overall nucleophilic aromatic substitution reaction, as a more stable $Br^+$ species is more readily attacked by the nucleophile, leading to faster and more favorable substitution.

Related terms

Nucleophilic Aromatic Substitution (S$_N$Ar): A mechanism where a nucleophile attacks an aromatic ring, displacing a leaving group and forming a new carbon-nucleophile bond.

Electrophilic Aromatic Substitution (S$_E$Ar): A mechanism where an electrophile attacks an aromatic ring, displacing a hydrogen and forming a new carbon-electrophile bond.

Aryl Halides: Aromatic compounds containing a halogen substituent, such as chloro-, bromo-, or iodo-benzene, which can undergo nucleophilic aromatic substitution.