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From A to X: Do you know how chemical reaction intermediates are born in a two-step reaction?
In chemical reactions, there are often many processes that are not easy to detect, among which chemical reaction intermediates play a key role. It refers to an intermediate molecular entity produced in a sequenced chemical reaction and consumed after a certain stage of reaction. These intermediates do not appear in the chemical equation of the overall reaction, but their presence is crucial to understanding the overall reaction process.
"The reaction intermediate is a unique molecule. Its life cycle is longer than the vibration time of the molecule, but it is not as durable as the reaction product."
Take an imaginary reaction as an example: when the reaction A + B is converted into C + D, if the overall reaction can be divided into two basic steps, namely A + B → X, and then X → C + D, then X is the intermediate in this reaction. The concept of intermediates is important because it not only affects the formation of the main products, but also helps us understand the mechanism of the reaction.
IUPAC definition
The IUPAC Golden Book defines an intermediate as a compound formed directly or indirectly from reactants. Its life span exceeds the time of molecular vibration, but then reacts further to produce the final product of a chemical reaction. This lifetime condition distinguishes true intermediates from transition states, whose lifetimes are closer to the time of molecular vibrations.
Common reaction intermediates
Carbocation
Carbocations are one of the most common reaction intermediates, especially in olefin addition reactions. They serve as intermediates in the synthesis of new compounds in many types of reactions.
Carbocation intermediates in olefin addition
In the HX addition reaction, the π bond of the alkene acts as a nucleophile and combines with the proton of the HX molecule to form a carbocation intermediate, and then X combines with the available positive carbon atom in the cation to form the final product.
Carbocation in nucleophilic substitution reaction
In nucleophilic substitution reactions, carbocations participate in the reaction as intermediates. In the SN1 mechanism, a leaving group is interrupted to form a carbocation, which is then attacked by a nucleophile to form a new compound.
Carbanions
Carbanions are organic molecules with an overall negative charge. Their strong nucleophilicity makes them play an important role in the synthesis reaction of extending the carbon skeleton of alkenes.
Free radicals
Free genes are extremely reactive due to their unpaired electrons. They often react with carbon molecules bearing hydrogen, sometimes producing more stable compounds.
Methane chlorination
Methane chlorination reaction is a chain reaction. During this process, reactive species are produced in a series of reactions that lead to the formation of the final product. The reaction is initiated by pressed chlorine molecules, forming a sustainable chain reaction.
Application
Biological intermediates
In biology, reaction intermediates contribute to a variety of important biological processes. For example, for some bacteria, metallo-β-lactamase can catalyze β-lactam antibiotics, making the bacteria acquire resistance. This is precisely because of the presence of specific intermediates in the reaction process.
Chemical Processing Industry
In the chemical industry, the term "intermediate" can also refer to a stable product of a reaction, whose value itself is often only as a precursor chemical substance for other industries. Alkenes synthesized from benzene and propylene are one example and have important applications in chemistry.
Although these reaction intermediates are not shown in the reaction equation, their existence makes a key contribution. This makes people think, how do these small intermediates affect the dynamics of the entire reaction process?
