Carbocations tropylium bromide

In certain cases, the carbocations are so stable that their solid salts have been isolated. For example, triphenylmethyl perchlorate (2.1) exists as a red crystalline solid and tropylium bromide (2.2) has been isolated as a yellow solid. Tropylium bromide (2.2) is stabilized by aromatization as the tropylium cation is planar and has 6tt electrons like benzene. 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

The history of carbocations dates back to 1891 when G. Merhng reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water-soluble material, C H Br. He did not suggest a structure for it however. Doering and Knox convincingly showed that it was tropylium (cycloheptatrienyhum) bromide (Figure 2.2). This ion is predicted to be aromatic by the Htickel rule. 

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See also in sourсe #XX -- [ ]

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