Stabilized Carbocations Generated by Photoheterolysis

Authored by: Hans-Werner Abraham

CRC Handbook of Organic Photochemistry and Photobiology

Print publication date:  March  2012
Online publication date:  March  2012

Print ISBN: 9781439899335
eBook ISBN: 9781466561250
Adobe ISBN:

10.1201/b12252-19

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Abstract

Carbocations play important roles as intermediates in organic chemistry. Stable and metastable ions are formed in acidic media [1] and since the lifetime of these intermediate species is often very short, time-resolved spectroscopy on the order of fs–ns is used to detect them. For this purpose, carbocations must be generated by laser impulses or lamp flashes of shorter duration than the time resolution of the spectroscopic equipment. A common method of photochemical carbocation generation is heterolytic bond fission of C–X bonds, whereby X is an electron-withdrawing group with proper nucleofuge properties [2]. Heterolytic photocleavage is favored in solvents of high polarity and with compounds yielding cations with low pK R + values and having leaving groups X with low pK a values for the conjugate acid. While the photoreaction may start from either the excited singlet or triplet state, singlet states are most often involved. The pathways of the photoreactions differentiate between adiabatic or diabatic bond fissions. The diabatic photoreaction is the dominant process, but in some cases, indications of the generation of excited carbenium ions were found [3,4]. In general, the efficiency of the photoheterolysis is high, resulting in high quantum yields [2].

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