Reactions of Triphenylmethyl Carbocation, Carbanion, and Radical

Prelab Exercise Write all of the resonance structures of the triphenylmethyl carbocation. 

Triphenylmethanol, prepared in the experiment in Chapter 31, has played an interesting part in the history of organic chemistry. It was converted to the first stable carbocation and the first stable free radical. In this experiment triphenylmethanol is easily converted to the triphenylmethyl (trityl) carbocation, carbanion, and radical. Each of these is stabilized by ten contributing resonance forms and consequently is unusually stable. Because of their long conjugated systems, these forms absorb radiation in the visible region of the spectrum and thus can be detected visually. 

The reactions of triphenylmethanol are dominated by the ease with which it dissociates to form the relatively stable triphenylmethyl carbocation. When colorless triphenylmethanol is dissolved in concentrated sulfuric acid, an orange-yellow solution results that gives a fourfold depression of the melting point of sulfuric acid, meaning that four moles of ions are produced. If the triphenylmethanol simply were protonated only two moles of ions would result. 

The central carbon atom in the carbocation is sp hybridized and thus the three carbons attached to it are coplanar and disposed at angles of 120°  

However, the three phenyl groups, because of steric hindrance, cannot lie in one plane. Therefore, the carbocation is propeller-shaped  

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Chapter 32 Reactions of Triphenylmethyl Carbocation, Carbanion, and Radical... 

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