In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation. When their existence is indicated, reactive intermediates can help explain how a chemical reaction takes place.
Most chemical reactions take more than one elementary step to complete, and a reactive intermediate is a high-energy, hence unstable, product that exists only in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product or a simple reaction intermediate only in that it cannot usually be isolated but is sometimes observable only through fast spectroscopic methods. It is stable in the sense that an elementary reaction forms the reactive intermediate and the elementary reaction in the next step is needed to destroy it.
When a reactive intermediate is not observable, its existence must be inferred through experimentation. This usually involves changing reaction conditions such as temperature or concentration and applying the techniques of chemical kinetics, chemical thermodynamics, or spectroscopy. Reactive intermediates based on carbon are radicals, carbenes, carbocations, carbanions, arynes, and carbynes.
Common features
Reactive intermediates have several features in common:
- low concentration with respect to reaction substrate and final reaction product
- with the exception of carbanions, these intermediates do not obey the lewis octet rule, hence the high reactivity
- often generated on chemical decomposition of a chemical compound
- it is often possible to prove the existence of this species by spectroscopic means
- cage effects have to be taken into account
- often stabilisation by conjugation or resonance
- often difficult to distinguish from a transition state
- prove existence by means of chemical trapping
Carbon
- Radical
- Carbene
- Carbocation
- Carbanion
- Carbyne
-
Benzyne (an aryne)
Other reactive intermediates
- Carbenoid
- Ion-neutral complex
- Keto anions
- Nitrenes
- Oxocarbenium ions
- Phosphinidenes
- Phosphoryl nitride
- Tetrahedral intermediates in carbonyl addition reactions
See also
- Activated complex
- Transition state
References
- Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. ISBN 0-306-41198-9.
- March Jerry; (1992). Advanced Organic Chemistry reactions, mechanisms and structure (4th ed.). New York: John Wiley & Sons ISBN 0-471-60180-2
- Gilchrist, T. L. (1966). Carbenes nitrenes and arynes. Springer US. ISBN 9780306500268.
- Moss, Robert A.; Platz, Matthew S.; Jones, Jr., Maitland (2004). Reactive intermediate chemistry. Hoboken, N.J.: Wiley-Interscience. ISBN 9780471721499.
Extranol links
Media related to Reactive intermediates at Wikimedia Commons
In chemistry a reactive intermediate or an intermediate is a short lived high energy highly reactive molecule When generated in a chemical reaction it will quickly convert into a more stable molecule Only in exceptional cases can these compounds be isolated and stored e g low temperatures matrix isolation When their existence is indicated reactive intermediates can help explain how a chemical reaction takes place Most chemical reactions take more than one elementary step to complete and a reactive intermediate is a high energy hence unstable product that exists only in one of the intermediate steps The series of steps together make a reaction mechanism A reactive intermediate differs from a reactant or product or a simple reaction intermediate only in that it cannot usually be isolated but is sometimes observable only through fast spectroscopic methods It is stable in the sense that an elementary reaction forms the reactive intermediate and the elementary reaction in the next step is needed to destroy it When a reactive intermediate is not observable its existence must be inferred through experimentation This usually involves changing reaction conditions such as temperature or concentration and applying the techniques of chemical kinetics chemical thermodynamics or spectroscopy Reactive intermediates based on carbon are radicals carbenes carbocations carbanions arynes and carbynes Common featuresReactive intermediates have several features in common low concentration with respect to reaction substrate and final reaction product with the exception of carbanions these intermediates do not obey the lewis octet rule hence the high reactivity often generated on chemical decomposition of a chemical compound it is often possible to prove the existence of this species by spectroscopic means cage effects have to be taken into account often stabilisation by conjugation or resonance often difficult to distinguish from a transition state prove existence by means of chemical trappingCarbonRadical Carbene Carbocation Carbanion Carbyne Benzyne an aryne Other reactive intermediatesCarbenoid Ion neutral complex Keto anions Nitrenes Oxocarbenium ions Phosphinidenes Phosphoryl nitride Tetrahedral intermediates in carbonyl addition reactionsSee alsoActivated complex Transition stateReferencesCarey Francis A Sundberg Richard J 1984 Advanced Organic Chemistry Part A Structure and Mechanisms 2nd ed New York N Y Plenum Press ISBN 0 306 41198 9 March Jerry 1992 Advanced Organic Chemistry reactions mechanisms and structure 4th ed New York John Wiley amp Sons ISBN 0 471 60180 2 Gilchrist T L 1966 Carbenes nitrenes and arynes Springer US ISBN 9780306500268 Moss Robert A Platz Matthew S Jones Jr Maitland 2004 Reactive intermediate chemistry Hoboken N J Wiley Interscience ISBN 9780471721499 Extranol linksMedia related to Reactive intermediates at Wikimedia Commons
