What Is the Diazotization Mechanism in Organic Chemistry?

TL;DR

Diazotization converts an amino group into an aryl diazonium salt by reacting aniline with sodium nitrite and hydrochloric acid at a very low temperature, around 5 degrees C, giving an N2+ group complexed with a chloride ion. This four-step electrophilic aromatic substitution sequence runs nitration, reduction, diazotization, and diazo coupling, and the diazonium salt then opens the door to Sandmeyer reactions. Read on for the full mechanism from benzene to substituted product.

Transcript

in this video we're going to talk about electrophilic aromatic substitution reactions but specifically nitration reduction dizotization and a semi reaction so let's start with benzene we're going to go over the mechanism of these reactions but let's just write an overview of each step now the first step is nitration once we add nitric acid and sulf... Read More

Key Insights

• 🫀 Nitration involves replacing a hydrogen atom in benzene with a nitro group using nitric acid and sulfuric acid.
• 👥 Reduction converts the nitro group to an amino group using iron metal and acid.
• 👥 Diazotization converts the amino group to an aryl diazonium salt using sodium nitrite and hydrochloric acid.
• 😋 The semi reaction, or diazo coupling, connects two benzene rings together to form azo dyes.
• 🖐️ Copper salts can play a role in the reactions, potentially through radical intermediates.
• ➕ Copper can exist in different oxidation states, such as copper metal (zero), copper plus one, and copper plus two.
• 🎮 Careful control of reaction conditions, such as temperature, is needed for successful reactions.

Questions & Answers

Q: What is the mechanism of aniline diazotization to form a diazonium salt?

Aniline is treated with sodium nitrite and hydrochloric acid at a very low temperature, around 5 degrees C. The NH2 group is converted into an N2 group, which is an arene diazonium salt. Because the nitrogen has four bonds it carries a positive formal charge, so it is complexed with a chloride ion.

Q: What is diazotization?

Diazotization is the process of converting an amino (NH2) group into an aryl diazonium salt. It is carried out by reacting aniline with sodium nitrite and hydrochloric acid, producing an arene diazonium ion with a positive nitrogen center.

Q: What is the Sandmeyer reaction?

The Sandmeyer reaction replaces the N2 group of the diazonium salt with a group from a copper salt. Adding copper chloride replaces N2 with a Cl group, copper bromide gives bromobenzene, and copper cyanide installs a CN group. Copper can exist in different oxidation states such as copper metal (zero), copper plus one, and copper plus two, and the reactions may proceed through radical intermediates.

Q: How is aniline made from benzene before diazotization?

First, nitration adds nitric acid and sulfuric acid to benzene, replacing a hydrogen with a nitro group to form nitrobenzene. Then reduction with iron metal and acid converts the nitro group into an amino group, giving aniline. Iron acts as a reducing agent, donating electrons to the nitrogen.

Q: How is the nitro group reduced to an amino group?

Nitrobenzene is reacted with iron metal and hydrochloric acid. Iron is a reducing agent that donates electrons to the nitrogen atom, and the electrons can flow one or two at a time. Reduction removes oxygen and adds hydrogen, converting the NO2 group into an NH2 group to give aniline.

Q: What is diazo coupling and what does it produce?

Diazo coupling, described as the semi reaction, connects two benzene rings together using an aryl diazonium salt. The result is the formation of azo compounds, or azo dyes.

Q: What is the nitronium ion and its role in nitration?

In nitration, nitric acid reacts with sulfuric acid: an oxygen of nitric acid grabs a hydrogen and leaves as water, generating the nitronium ion. The nitronium ion is the electrophile, while benzene acts as the nucleophile, so electrons flow from benzene to the nitronium ion to install the nitro group.

Q: Why is diazotization run at low temperature?

The reaction of aniline with sodium nitrite and hydrochloric acid is done at a very low temperature, maybe 5 degrees C. Careful control of conditions such as temperature is needed for the diazonium salt to form successfully.

Summary & Key Takeaways

• Nitration is the process of replacing a hydrogen atom in benzene with a nitro group using nitric acid and sulfuric acid.

• Reduction converts the nitro group into an amino group using iron metal and acid, resulting in the formation of aniline.

• Diazotization involves the conversion of the amino group to an aryl diazonium salt using sodium nitrite and hydrochloric acid.

• The semi reaction, or diazo coupling, connects two benzene rings together using an aryl diazonium salt.