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Concept
Intramolecular reaction
Summary
In chemistry, intramolecular describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.
intramolecular hydride transfer (transfer of a hydride ion from one part to another within the same molecule)
intramolecular hydrogen bond (a hydrogen bond formed between two functional groups of the same molecule)
cyclization of ω-haloalkylamines and alcohols to form the corresponding saturated nitrogen and oxygen heterocycles, respectively (an SN2 reaction within the same molecule)
In intramolecular organic reactions, two reaction sites are contained within a single molecule. This creates a very high effective concentration (resulting in high reaction rates), and, therefore, many intramolecular reactions that would not occur as an intermolecular reaction between two compounds take place.
Examples of intramolecular reactions are the Smiles rearrangement, the Dieckmann condensation and the Madelung synthesis.
Intramolecular reactions, especially ones leading to the formation of 5- and 6-membered rings, are rapid compared to an analogous intermolecular process. This is largely a consequence of the reduced entropic cost for reaching the transition state of ring formation and the absence of significant strain associated with formation of rings of these sizes. For the formation of different ring sizes via cyclization of substrates of varying tether length, the order of reaction rates (rate constants kn for the formation of an n-membered ring) is usually k5 > k6 > k3 > k7 > k4 as shown below for a series of ω-bromoalkylamines. This somewhat complicated rate trend reflects the interplay of these entropic and strain factors:
For the 'small rings' (3- and 4- membered), the slow rates is a consequence of angle strain experienced at the transition state. Although three-membered rings are more strained, formation of aziridine is faster than formation of azetidine due to the proximity of the leaving group and nucleophile in the former, which increases the probability that they would meet in a reactive conformation.
Official source
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