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Why do some aromatic rings suddenly become super reactive? The secret lies in the SNAr reaction!
In the world of organic chemistry, aromatic rings are usually considered to be stable structures. However, there are some aromatic compounds that suddenly become extremely reactive under certain conditions. The secret of all this lies in the nuclear affinity of aromatics. Substitution reaction (SNAr).
Nucleophilic aromatic substitution reaction refers to the reaction in which a nucleophilic substance displaces a good leaving group (such as halogen) in an aromatic ring.
Mechanism of nuclear affinity aromatic substitution reaction
There are many ways for nuclear affinity substitution of aromatic rings, but the most important one is the SNAr mechanism. When the aromatic ring has an electron-attracting group, such as a nitro group (NO₂), it will significantly promote the attack of the nucleophile. For example, if the nitro group is located in the ortho or para position to the halogen, the chances of this reaction are greatly increased.
In basic solution, when the hydroxide nucleophore attacks dinitrochlorobenzene, the Masonheimer complex formed makes the reaction more feasible because it stabilizes the extra electron density.
Key steps of the reaction
Take the SNAr reaction of dinitrochlorobenzene (2,4-dinitrochlorobenzene) in a basic solution of water as an example. The reaction steps are as follows:
- The hydroxide core parent attacks the aromatic ring with concentrated electron density to form a Masonheimer complex.
- In this complex, the halogen group (such as chlorine) will gradually leave to form a new product (such as 2,4-dinitrophenol).
- The reaction will eventually reach chemical equilibrium, and the stable products will not return to the state of the reactants.
During this process, the formation of the Masonheimer complex is slow because the aromaticity is lost due to the attack of the nuclear affinity body; however, the departure of chlorine or hydroxide is relatively rapid. Because the aromatic ring has a lower recovery energy state.
Recent studies have shown that the Masonheimer complex is not necessarily a true intermediate, which may depend on the stability of the electron attracting group.
Characteristics of SNAr reaction
Most important properties in the SNAr reaction include: different leaving groups will affect the reaction rate, and fluorine is in some cases more reactive than iodine, which is exactly the opposite in the SN2 reaction. In addition, in addition to ammonia, alcohols, sulfides, etc., stable carbanions are also common nucleophiles.
In SNAr, the reaction rate changes with the strength of the electron-attracting group, which greatly increases the reactivity of certain aromatic rings.
Applications and future development
Nucleophilic aromatic substitution is not limited to traditional aromatic molecules, but is also effective on some isocyclic rings, such as pyridine. Such reactions have shown their potential in the synthesis of chiral molecules, drugs, and functional materials, and have also opened up new avenues for chemical synthesis. Over the past few years, scientists have reported a variety of strategies for the synthesis of chiral molecules using the SNAr reaction, demonstrating its importance in organic synthesis.
With the development of science, our understanding of the SNAr reaction is getting deeper and deeper, and its application in more complex synthesis may be further expanded in the future.
With the in-depth study of nuclear affinity aromatic substitution reactions, can we find more similar innate reactions to promote the exploration and application of new materials?
