Balz schiemann reaction mechanism


The Balz-Schiemann (BS) reaction was one of the first procedures for producing fluorinated aromatic compounds but is still commonly used today. The thermal breakdown of diazonium tetrafluoroborate by diazotization of the corresponding aromatic amines introduces a fluorine atom into aromatic rings in this process. This reaction is notably essential for the synthesis of fluorinated aromatic compounds, whose breakdown in the lack of solvent generally yields a reasonably excellent yield of aromatic fluorides. The BS reaction has been thoroughly studied or shown to be beneficial in the synthesis of aromatic halides.

What is Balz Schiemann Reaction ?

The BS reaction is a reversible reaction that requires specific reagents as well as reactants. Aromatic amines, nitrous acid, plus fluoroboric acid are the reactants in this reaction. This reaction is a chemical process that converts the primary aromatic amine into the chemical substance aryl fluoride. Diazonium Tetrafluoroborate is a chemical intermediate in this process. This reaction may be used to produce additional chemical compounds such as fluorobenzene as well as made in various derivatives such as 4 – fluorobenzoic acid.

The reaction was called after the scientists that performed it for the first time. Gunther Balz as well as Gunther Schiemann represent the names, hence the name Balz Schiemann was developed for this response. They each assisted in the reaction in very own unique ways, establishing the groundwork for a successful substitution reaction.

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Balz Schiemann Reaction Mechanism

The BS reaction mechanism is intimately related to the diazotization process. To comprehend this chemical reaction, we will examine the procedure before delving into the chemical underpinnings of the reaction.

The way fluorine interacts with diazonium salts of the amine molecules contained inside the boundaries of the BS mechanism is an intriguing component of the process. Normally, the BS reaction incorporates ${HBF_4}$ into its chemical reaction thus necessitating the separation of a diazonium salt.

Because of the complicated mechanics included in this, the listed chemical species may be modified, making it useful for adhering to the BS reaction. Other ions, such as hexafluorophosphate ${(PF_6)}$ as well as hexafluoroantimonate ${(SbF_6)}$, have been employed instead of trifluoroborates, yielding superior results for some substrates thus broadening the field of the described reaction.

Additionally, diazotization processes involving nitrosonium salts such as ${[NO]SbF_6}$ may be carried out without isolating the diazonium intermediate.

This reaction may also be used to make aryl fluorides. Despite the aggressive character of the reaction as well as the challenges in controlling the procedure, direct fluorination of aromatic hydrocarbons is beneficial for producing aryl fluorides. This is why the BS reaction is regarded as the preferred method of generating aryl fluorides.

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Examples of Balz Schiemann Reaction

The transformation of phenylamine (aniline) to phenyl fluoride (fluorobenzene) utilizing nitrous acid, fluoroboric acid, plus heat is an example of a BS reaction.

This process, for instance, was utilized to create fluoro-substituted epibatidine analogue 79 (epibatidine is a high-affinity nonselective ligand for nicotinic cholinergic receptors.

What is Diazotization?

The 'diazotization' procedure refers to the chemical processing techniques as well as purifying of the principal aromatic amine into the following diazonium salt of the amine. The reaction between aromatic amine with nitrous acid is commonly used in the synthesis of the aforementioned diazonium salts.

It is important to note that the 2 chemical compounds are both exposed to yet another acid throughout this reaction.

Let us now look at the BS reaction mechanisms. Essentially, the Balz Schiemann reaction of diazonium salt is functionally equal to or comparable to the Sandmeyer reaction, where diazonium salts enter the picture that changes into aryl halides ${ArCl,\:ArBr}$.

While the Sandmeyer reaction requires a ${Cu}$ reagent with catalyst plus particular intermediates, the heating of diazonium tetrafluoroborate can proceed without even a catalyst. As a result, volatile aryl cations ${(Ar^+)}$ are formed, which leads to the formation of aryl fluoride; boron trifluoride is just another of this process. Fluorine ${(F)}$ is one of the necessary ingredients for this reaction process to be completed.

Schiemann reactions development over the years

The BS reaction mechanism has been concentrated on the heat breakdown of an aryl diazonium salt over the years. This reaction, which involves inserting a new fluorine ${(F)}$ atom into an aromatic ring, has a lengthy history. The theory behind BS reaction has been investigated in chemistry since 1967 but has been ongoing since the late 1800s. As per the previous use of fluorine - 18, the BS reaction is proven to boost the thermal breakdown of aryl cation, which further combines with solvents and possibly other reactants to produce a substituted aromatic molecule. Furthermore, the use of fluorine - 18 labelled tetrafluoroborate anion as a refute for diazonium salt has aided in the formation of the desired aryl fluoride.


Due to the obvious availability of isoquinoline derivatives, the BS reaction remains among the most frequent procedures for direct ring fluorination. So although numerous modified processes have been described, the original circumstances, which require the use of tetrafluoroboric acid (fluoroboric acid), are still often used. We researched this reaction, and its mechanism, including its evolution over time. This reaction is widely used in the synthesis of aromatic halides.

Frequently Asked Questions

Q1. Why is the Balz Schiemann (BS) reaction employed in fluorine 18?

Per the past use of fluorine - 18 this reaction is known to enhance the thermal breakdown of aryl cation, which then combines with solvents and also other reactants to produce a substituted aromatic molecule.

Q2. Is aryl diazonium water soluble?

Colourless crystalline solids, aryl diazonium salts Water dissolves benzenediazonium chloride. However, it only reacts to it when it is warmed. Water does not dissolve benzenediazonium fluoroborate.

Q3. How is diazohydroxide converted to diazonium?

To produce the necessary aryl diazonium ion, diazohydroxide is protonated then water is pumped out from the molecule (which can easily be converted into a diazonium salt).

Q4. What was Sandmeyer's reaction?

The Sandmeyer reaction is a popular chemical process whose origins may be found in its usage of copper ${(Cu)}$ salts as reagents and any other catalysts to synthesize aryl halides from aryl diazonium salts. This is an illustration of the radical-nucleophilic aromatic substitution phenomena.

Q5. What is Gattermann's response?

This reaction is an aromatic ring compound formylation process. In this process, benzene is combined with hydrocyanic acid plus Friedel craft's ${AlCl_3}$ catalyst to make benzene diazonium chloride, which can be hydrolyzed to give aldehyde. ${Cu}$ functions as a catalyst in such a process.

Q6. The N exits as_____in diazonium salt processes the displacement of the diazonium group.

When diazonium salts are prepared with appropriate reagents to substitute the ${N_2[(^ +)]\:[(X^ -)]^(X\:=\:anion)}$

with an anionic group, nitrogen gas is released. It may or may not entail the production of another by-product.

Q7. Determine the best reagent to use for turning benzene diazonium chloride to chlorobenzene.

In the appearance of ${Cu(I)}$ ion generated by ${Cu_2Cl_2}$ in hydrochloric acid, the ${Cl^{-}}$ nucleophile may be inserted into the benzene ring. When this is heated with the diazonium salt, it produces chlorobenzene.


  • Ni, C., Xing, B., & Hu, J. (2020). Balz-Schiemann Reaction. In Fluorination (pp. 31-36). Springer, Singapore.

  • Park, N. H., Senter, T. J., & Buchwald, S. L. (2016). Rapid synthesis of aryl fluorides in a continuous flow through the Balz–Schiemann reaction. Angewandte Chemie, 128(39), 12086-12090.

  • Butler, R. N. (1975). Diazotization of heterocyclic primary amines. Chemical Reviews, 75(2), 241-257.

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Updated on: 13-Oct-2022


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