Terf-Butyl carbocation

The rearrangements of both the 1-butyl and 2-butyl carbocations to the tert-butyl carbocation occur rapidly in superacid solution. Both of these rearrangements proceed through several steps and must involve an unfavorable secondary carbocation to primary carbocation rearrangement. Show the steps in the rearrangement of the 1-butyl carbocation to the terf-butyl carbocation. 

These common features suggest that carbocations are key intermediates in alcohol dehydrations, just as they are in the reaction of alcohols with hydrogen halides. Figure 5.6 portrays a three-step mechanism for the acid-catalyzed dehydration of tert-butyl alcohol. Steps 1 and 2 describe the generation of terf-butyl cation by a process similar to that which led to its formation as an intermediate in the reaction of ferf-butyl alcohol with hydrogen chloride. 

The ratio of substitution and elimination remains constant throughout the reaction, which means that each process has the same kinetic order with respect to the concentration of terf-butyl halide. The SN1 and El reactions have a common rate-determining step, namely, slow ionization of the halide. The solvent then has the choice of attacking the intermediate carbocation at the positive carbon to effect substitution, or at a /3 hydrogen to effect elimination ... 

Alkyl cations can also be generated by decarbonylation of tertiary acylium ions, like the pivaloyl cation 26 [Eq. (3.20)].91 This reaction corresponds to the reverse of Koch-Haaf acid synthesis, which is known to involve carbocation intermediates. Indeed the reaction of the terf-butyl cation with carbon monoxide gives the pivaloyl cation.91 135... 

The Steglich Esterification is a mild reaction, which allows the conversion of sterically demanding and acid labile substrates. It s one of the convenient methods for the formation of terf-butyl esters because r-BuOH tends to form carbocations and isobutene after a subsequent elimination under the conditions employed in the Fischer Esterification. 

In view of these results, it is surprising that no rearrangement occurs during the aluminium chloride-catalyzed reaction of terf-butyl chloride with ethylene [97]. The intermediacy of a primary carbocation can, therefore, be ruled out and the selective formation of 18 may be rationalized by assuming the intermediacy of a bridged cation (tert-butyl-bridged ethylene). Alternatively, the attack of the /er/-butyl cation at ethylene may be nucleophilically assisted by the AIC14 ion because of the low stability of the 3,3-dimethyl-l-butyl cation (Scheme 22) [98],... 

We discussed the planar shape of the methyl cation in Chapter 4 (p. 103), and the terf-butyl cation is similar in structure the electron-deficient central carbon atom has only six electrons, which it uses to form three a bonds, and therefore also carries an empty p orbital. Any carbocation will have a planar carbon atom with an empty p orbital. Think of it this way only filled orbitals contribute to the energy of a molecule, so if you have to have an unfilled orbital (which a carbocation always does) it is best to make that unfilled orbital as high in energy as possible to keep the filled orbitals low in energy, p orbitals are higher in energy than s orbitals (or hybrid sp, sp2, or sp3 orbitals for that matter) so the carbocation always keeps the p orbital empty. 

For example, hydrolysis of a solution of 2-chloro-2-methylpropane gives the expected 2-methyl-2-propanol (ferf-butyl alcohol), with a rate constant Quite a different result is obtained when the same experiment is carried out in the presence of the soluble salt calcium formate 1,1-Dimethylethyl formate (terf-butyl formate) replaces the alcohol as the product, but the reaction still proceeds at exactly the same rate ki. In this case, formate ion, a better nucleophile than water, wins out in competition for bonding to the intermediate carbocation. The rate of disappearance of the starting material is determined by ki (regardless of the product eventually formed), but the relative yields of the products depend on the relative reactivities and concentrations of the competing nucleophiles. 

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