The Balz Schiemann reaction is a type of reaction that is dependent on certain reagents or reactants. The reactants that take part in the Balz Schiemann reaction are aromatic amines, nitrous acid, and fluoroboric acid. The Schiemann reaction is a chemical reaction in which the main aromatic amine is converted into a chemical compound famously known as aryl fluoride. Diazonium Tetrafluoroborate is an intermediate in this chemical reaction. Certain other chemical compounds such as fluorobenzene and certain related derivatives, including 4 - fluorobenzoic acid, can be obtained with the help of this reaction.
The Schiemann reaction is named after the scientists who carried out the reaction for the first time. Their names are Gunther Balz and Gunther Schiemann, thus the name Balz Schiemann was coined for this reaction. Both of them contributed to the reaction in their own way, thus laying out an established framework in which substitution reaction can successfully occur.
Balz Schiemann Reaction and Its Mechanisms
The Balz Schiemann reaction mechanism is closely linked with the process known as diazotization. Therefore, to understand this chemical reaction, we will first take a look at this process before we proceed into the chemical foundations of the Schiemann reaction.
The interesting aspect of the Balz Schiemann mechanism is the way fluorine reacts with diazonium salts of the amine molecules present within the confines of the Schiemann reaction. The Balz – Schiemann reaction normally ends up incorporating HBF4 into its chemical reaction and requires the separation of a diazonium salt. The intricate mechanisms involved in this reaction allow the mentioned chemical species to be changed, making it profitable to use in adherence to the Schiemann reaction.
Other counter-ions have been used instead of trifluoroborates, including hexafluorophosphate (PF6−) and hexafluoroantimonate (SbF6−), which have shown results with better yields for certain substrates, expanding the field of the mentioned reaction. Furthermore, diazotization reactions can be conducted with nitrosonium salts like [NO]SbF6 without isolating the diazonium intermediate.
Aryl fluorides can also conveniently be prepared by the Balz-Schiemann reaction. If we take into account the aggressive nature of the reaction and the difficulties of regulating the process, it is useful to create aryl fluorides by direct fluorination of aromatic hydrocarbons. This is the the reason why the Balz-Schiemann reaction is considered to be the favoured way of producing aryl fluorides.
What is Diazotization?
The chemical method processing and refining the main aromatic amine into the subsequent diazonium salt of the amine is referred to as 'diazotization' process. The preparation of the mentioned diazonium salts generally ends up in the reaction of the aromatic amine and nitrous acid. It is pertinent that during this reaction, the two chemical compounds are both exposed to another acid.
Now, let us learn about the Balz Schiemann reaction mechanisms. Basically, Balz Schiemann reaction of diazonium salt is itself functionally equivalent or similar to the reaction of Sandmeyer, where diazonium salts come into play and transform into aryl halides (ArCl, ArBr).
However, while the reaction of Sandmeyer specifically requires a copper reagent or catalyst and certain intermediates, the heating of diazonium tetrafluoroborate can continue without a catalyst. Thus, it is creates completely volatile aryl cations (Ar+) that further leads to the generation of aryl fluoride, boron trifluoride is a by-product of this process. Fluorine is one of the essential elements required to complete the Balz Schiemann reaction mechanism.
Schiemann Reaction’s Development Over The Years
Over the years, the Schiemann reaction mechanism has been centered around the thermal decomposition of an aryl diazonium salt. The Balz Schiemann reaction has a long history with the concept of introducing a fresh new fluorine atom into an aromatic ring. The theory backing Balz Schiemann mechanisms have been ongoing since the late 1800s and has been studied since 1967 in chemistry. According to the historical use of fluorine-18, Balz Schiemann reaction is known to facilitate the development of aryl cation by thermal decomposition, which then reacts with solvents or even other reactants for the generation of a substituted aromatic compound. Moreover, the use of fluorine-18-labeled tetrafluoroborate anion as a counter-ion for diazonium salt has contributed to the development of the intended aryl fluoride.
FAQs on Balz Schiemann Reaction
1. What is the Balz-Schiemann reaction as per the Class 12 syllabus 2025-26?
The Balz-Schiemann reaction is a key chemical process covered in the 'Haloalkanes and Haloarenes' chapter for preparing aryl fluorides (fluorobenzene). In this reaction, a primary aromatic amine is first converted into its diazonium salt, which is then treated with fluoroboric acid (HBF₄). The resulting diazonium fluoroborate precipitate is isolated and heated to produce the final aryl fluoride.
2. What are the essential steps involved in the mechanism of the Balz-Schiemann reaction?
The mechanism of the Balz-Schiemann reaction proceeds in three main steps:
- Step 1: Diazotization: A primary aromatic amine, such as aniline, reacts with nitrous acid (generated in-situ from NaNO₂ and HCl) at a low temperature (0-5 °C) to form an aryl diazonium salt (e.g., benzene diazonium chloride).
- Step 2: Formation of Diazonium Fluoroborate: The aryl diazonium salt is then treated with fluoroboric acid (HBF₄). This results in the precipitation of a stable intermediate, aryl diazonium fluoroborate (ArN₂⁺BF₄⁻).
- Step 3: Thermal Decomposition: The isolated diazonium fluoroborate salt is gently heated. It decomposes to yield the desired aryl fluoride, releasing nitrogen gas (N₂) and boron trifluoride (BF₃) as byproducts.
3. Can you provide a specific example of the Balz-Schiemann reaction?
A classic example is the synthesis of fluorobenzene from aniline. The reaction is represented as follows:
C₆H₅NH₂ (Aniline) + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ (Benzene diazonium chloride) + NaCl + 2H₂O
C₆H₅N₂⁺Cl⁻ + HBF₄ → C₆H₅N₂⁺BF₄⁻ (Benzene diazonium fluoroborate) + HCl
C₆H₅N₂⁺BF₄⁻ (on heating) → C₆H₅F (Fluorobenzene) + N₂↑ + BF₃↑
4. How does the Balz-Schiemann reaction differ from the Sandmeyer reaction?
The primary difference lies in the specific halogen they introduce and the reagents used:
- Product: The Balz-Schiemann reaction is used exclusively to prepare aryl fluorides. The Sandmeyer reaction is used to prepare aryl chlorides, bromides, and cyanides.
- Reagent: The Balz-Schiemann reaction uses fluoroboric acid (HBF₄) and heat. The Sandmeyer reaction uses a cuprous salt (CuCl, CuBr, or CuCN) as a catalyst.
- Intermediate: While both start with a diazonium salt, the Balz-Schiemann reaction involves isolating a stable diazonium fluoroborate intermediate, which is not typically done in the Sandmeyer reaction.
5. Why is the Balz-Schiemann reaction considered a superior method for introducing a fluorine atom to an aromatic ring?
The Balz-Schiemann reaction is preferred because direct fluorination of aromatic compounds is an extremely violent and explosive reaction, making it impractical and dangerous. This reaction provides a controlled and reliable pathway to synthesise aryl fluorides. By converting the amine to a stable, isolable diazonium fluoroborate salt, the introduction of fluorine can be achieved safely and with a good yield simply by applying heat.
6. What are the key reagents required, and what is the important intermediate formed during the Balz-Schiemann reaction?
The key reagents required for the Balz-Schiemann reaction are:
- A primary aromatic amine (e.g., aniline).
- Sodium Nitrite (NaNO₂) and a strong acid (like HCl) for the initial diazotization step.
- Fluoroboric Acid (HBF₄), which is the defining reagent for this reaction.
