Carbocations, carbanions and carbon radicals

This lowers the nucleophilicity because a solvent shell surrounds the anion and this hinders attack at the electrophile. Large anions are less solvated, and so are stronger nucleophiles than small anions in protic solvents. For example, the smaller F ion is more heavily solvated than 1 and so is a weaker nucleophile. 

An electrophile with a positive charge is always a more powerful electrophile than its conjugate base (which is neutral). 

The relative electrophihc strength (or electrophilicity) of a cation depends on the stability of the positive charge. Inductive (+1), mesomeric (+M) and/or steric effects (see Section 4.4) can all lower the reactivity of the cation. 

The relative electrophilic strength of an electrophilic site within a neutral molecule depends on the size of the partial positive charge (8+). Carbon atoms are electrophilic when attached to electronegative atoms (—1 groups). The more electronegative the atom(s) bonded to carbon, the more electrophilic the carbon atom. 

Carbocations, which include carbenium and carbonium ions, contain a positive charge on carbon. Carbenium ions have three bonds to the positively charged carbon (e.g. MeaC ), while carbonium ions contain five bonds (e.g. HsC ). (Carbenium ions are the most important.) 

Carbon radicals are similar to carbenium ions because both contain an electron-deficient carbon atom. For carbenium ions, the carbon atom is deficient of two electrons, and for carbon radicals, the carbon atom is deficient of one electron. 

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The order of stability of carbocations, carbanions and carbon radicals bearing electron donating (+1) alkyl groups, R, is as follows. 

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