An intermediate is a molecule that is formed from two or more reactants and then reacts further to give products. Most chemical reactions require more than one step, and an intermediate is the product of each step, except for the last one, after which the final products are produced. Intermediates very rarely remain in the product mixture due to the short time that they exist. They are seldom isolated, and so usually end up reacting with other chemicals in the reaction to eventually produce the final products. An example of a chemical reaction would be A+B = C+D. In reality, the reaction is more likely to be something like this; A+B = X*, X* = C+D, in which X* is the intermediate. There can be a high number of intermediates in every reaction, and they’re sometimes difficult to identify due to how short-lived they are.
Intermediates are distinguishable from molecular vibrations, which are merely transitions, though they have similar lifetimes. Intermediates are short-lived and highly reactive, which is why they are usually in very low quantities in reaction mixtures. Of course, when describing intermediates, we need to take into account the reaction in which they are present. A short-lived species in one reaction might actually be considered stable in another. Intermediates are relatively short-lived compare to the other chemicals in the reactions. They often come in the forms of free radicals or unstable ions and sometimes must be produced at a very high temperature or pressure due to their high reactivity. In certain reactions, multiple steps are performed in one batch. Sometimes, it is unnecessary to isolate an intermediate for further reactions – other times, it’s impossible due to their high reactivity with other chemicals in the reaction mixture. It’s hard to isolate intermediates when their lifespan is so comparatively short. An example is the esterification of a diol, where a monoester is first produced, which then reacts to from a dioester.
An example of an intermediate in the chemical industry is cumene. The term intermediate in the chemical industry usually means a product of a reaction that is only beneficial when used as a precursor chemical for another industry. Cumene is made from benzene and propylene, and is then used to produce acetone. Cumene, without additional reactions, has very little value and no real use, which makes it an intermediate instead of a useful chemical product.