Which statement about reaction intermediates is true?

• A. Always highly stable.
• B. Never isolable.
• C. Usually unstable relative to the reactants and products.
• D. Are identical to products.

Answer: (C)

Reaction intermediates are short‑lived species formed as the reaction moves from reactants toward products. They appear along a reaction pathway as temporary stepping stones between steps, not as final products. Because they occupy a higher energy position in the energy landscape and exist only briefly, they don’t accumulate and are typically too unstable to be isolated under ordinary conditions. That’s why the statement describing intermediates as usually unstable relative to both the reactants and the products is the correct way to characterize them.

For example, in an SN1 reaction, the carbocation formed after the leaving group departs is a transient high‑energy species that quickly reacts with the nucleophile, illustrating the fleeting, intermediate nature. Intermediates are not identical to products, and while some can be stabilized or trapped under special conditions, saying they are never isolable is too absolute.