Carbocations triphenylmethanol

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

When triphenylmethanol is dissolved in concentrated sulfuric acid, a solution with an intense yellow color is formed. The yellow species is the triphenylmethyl carbocation, formed by the following reaction ... 

As described above, although the p/ R+ values for symmetrically trisubstituted triarylmethanols [3(X = Y = Z)] give a linear Y-T correlation against a d-scale with an r value of 0.76 (Yukawa et ai, 1966), clearly non-linear Y-T correlations are obtained for the p/Cr+ values for monosubstituted triphenylmethanols [3(X,H,H)j or any non-equivalently substituted series [3C (X Y Z)]. In any polyarylmethyl carbocation the molecule adopts a... 

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. 

Triphenylmethanol is a tertiary alcohol and undergoes, as expected, SnI reactions. The intermediate cation, however, is stable enough to be seen in sulfuric acid solution as a red-brown to yellow solution. Upon dissolution in concentrated sulfuric acid, the hydroxyl is protonated then the portion is lost as HjO (which is itself protonated) leaving the carbocation. The bisulfate ion is a very weak nucleophile and does not compete with methanol in the formation of the product, trityl methyl ether. 

In this experiment the triphenylmethanol is dissolved in a good ionizing solvent, acetic acid, and allowed to react with a strong acid and nucleophile, hydrobromic acid. The intermediate carbocation reacts immediately with bromide ion. 

Concentrated sulfuric acid "dehydrates" the alcohol, producing a highly conjugated, colored carbocation, and protonates the water to prevent the reverse reaction. Upon adding more water, however, there are too many water molecules for the acid to protonate, and triphenylmethanol is regenerated. 

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