Direct Effects of Orbital Overlap on Reactivity

We will start with the simplest case - interaction of two non-bonding orbitals with an overall population of two electrons. This case corresponds to the classic formation of a two-center/two-electron (2c,2e) chemical bond. However, even this familiar situation allows for a number of interesting modifications. For example. 

Stereoelectronic Effects A Bridge Between Structure and Reactivity, First Edition. Igor V. Alabugin. 2016 John Wiley Sons, Ltd. Published 2016 by John Wiley Sons, Ltd. 

PhgC-CPhg 2 CPhg BDE = 17 kcal/mol AG298 = -9.0 kcal/mol 

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Direct Effects of Orbital Overlap on Reactivity 25 2 CH2 BDE = 174.1 kcal/mol... 

In addition to steric effects, there are other important substituent effects that influence both the rate and mechanism of nucleophilic substitution reactions. As we discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in these systems than in alkyl systems. Direct displacement reactions also take place particularly rapidly in benzylic and allylic systems for example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone." These enhanced rates reflect stabilization of the Sjv2 TS through overlap of the /2-type orbital that develops at carbon." The tt systems of the allylic and benzylic groups provide extended conjugation. This conjugation can stabilize the TS, whether the substitution site has carbocation character and is electron poor or is electron rich as a result of a concerted Sjv2 mechanism. 

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