Reactive intermediates ions versus radicals

There are two ways of breaking a covalent bond. The unsymmetrical cleavage is called heterolytic cleavage (or heterolysis), and this leads to the formation of ions (cations and anions). The symmetrical cleavage is called homolytic cleavage (or homolysis), and this leads to the formation of radicals. 

Curly arrows can be used to represent bond cleavage. A double-headed arrow represents the movement of two electrons (and is used in ionic mechanisms). A single-headed arrow (or fishhook) is used to represent the movement of a single electron (and is used in radical mechanisms). Curly arrows therefore always depict the movement of electrons. Heterolysis (use one double-headed arrow) 

Radicals contain an odd number of electrons, and a dot is used to represent the unpaired electron. 

Processes that involve unsymmetrical bond cleavage or bond formation are known as ionic (or polar) reactions. Processes that involve symmetrical bond cleavage or bond formation are known as radical reactions. 

A two-electron arrow goes from the anion/lone pair to the cation. The arrow points from the electron-rich centre to the electron-deficient centre 

Keynotes in Organic Chemistry, Second Edition. Andrew F. Parsons. 

The stability of carbenium ions (R3C ), carbanions (R3C ) and carbon radicals (R3C ) is discussed in Section 4.3 

Both electrons in the two-electron bond move to only 

The two-electron bond is split evenly and one electron moves to each of the atoms 

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Exploitation of time-resolved spectroscopy allows the direct observation of the reactive intermediates (i.e., ion-radical pair) involved in the oxidation of enol silyl ether (ESE) by photoactivated chloranil (3CA ), and their temporal evolution to the enone and adduct in the following way.41c Photoexcitation of chloranil (at lexc = 355 nm) produces excited chloranil triplet (3CA ) which is a powerful electron acceptor (EKelectron-rich enol silyl ethers (Em = 1.0-1.5 V versus SCE) to the ion-radical pair with unit quantum yield, both in dichloromethane and in acetonitrile (equation 20). 

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