Tert-Butyl cation intermediate

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m 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 tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

Figure 12 7 illustrates attack on the benzene ring by tert butyl cation (step 1) and subsequent formation of tert butylbenzene by loss of a proton from the cyclohexadienyl cation intermediate (step 2)... 

Carbinolamines are formed by nucleophilic addition of an amine to a carbonyl group and are intermediates in the for mation of imines and enamines Carbocation (Section 4 8) Positive ion in which the charge re sides on carbon An example is tert butyl cation (CH3)3C Carbocations are unstable species that though they cannot normally be isolated are believed to be intermediates in certain reactions... 

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. 

In the early days of stable ion chemistry, the experimental measurements of parameters such as NMR chemical shifts and IR frequencies were mainly descriptive, with the structures of the carbocations being inferred from such measurements. While in cases such as the tert-butyl cation there could be no doubt of the namre of the intermediate, in many cases, such as the 2-butyl cation and the nonclassical ions, ambiguity existed. A major advance in reliably resolving such uncertainties... 

But the addition product is 99+% tert-butyl bromide so the reaction clearly is kinetically controlled, tert-butyl bromide being formed considerably faster than isobutyl bromide. The slow, or rate-determining, step in this reaction is the formation of the intermediate cation rather than the reaction of the cation with bromide ion. So to account for the formation of tert-butyl bromide we have to consider why the tert-butyl cation is formed more rapidly than the isobutyl cation ... 

The reaction scheme can be represented conveniently in the form of an energy diagram (Figure 10-10). The activation energy, AH ert for the formation of the tert-butyl cation is less than AHlrim for the formation of the isobutyl cation because the tertiary ion is much more stable (relative to the reactants) than the primary ion, and therefore is formed at the faster rate. The second step, to form the product from the intermediate cation, is very rapid... 

The reaction proceeds via a pentacoordinate hydroxycarbonium ion transition state, which cleaves to either fert-butyl alcohol or the tert-butyl cation. Since 1 mol of isobutane requires 2 mol of hydrogen peroxide to complete the reaction, one can conclude that the intermediate alcohol or carbocation reacts with excess hydrogen peroxide, giving fcrt-butyl hydroperoxide. The superacid-induced rearrangement and cleavage of the hydroperoxide results in very rapid formation of the dimethylmethyl-carboxonium ion, which, upon hydrolysis, gives acetone and methyl alcohol. 

Why are intermediates (8) and (9) more stable than intermediates (8 ) and (9/) This can be explained by the inductive effect (I effect) and the hyperconjugation effect. The methyl group has an electron donation ability through the a bond. So, the tert-butyl cation and the terf-butyl radical can be stabilized by the inductive effect of the methyl group (Figure 1.4). Normally, the inductive effect is increased in the following order ... 

The tert-butyl cation (50) reforms isobutene, according to Bartlett et al. Nenitzescu et al., ° and Schmerling, by hydride abstraction from isobutane involving the tertiary C-H bond only, through intermediate structure 52, and thus no methine hydrogens would exchange with the deuterosulfuric acid. 

In contrast, the hydrolysis of tm-butyl bromide (2-bromo-2-methylpropane) occurs in a stepwise manner (reaction 1.1b). In the first slow step, the C-Br bond breaks, with the bromine atom taking both electrons from the bond and leaving as a negatively charged bromide ion. The remainder of the molecule is the positively charged tert-butyl cation (2-methylprop-2-yl cation). This is a highly reactive intermediate, which reacts rapidly with the hydroxide ion to form the corresponding alcohol. 

Figure 22.10 Energy profile for possible products of proton transfer from a zeolitic Bronsted site to isobutene, see Fig. 22.1. Standard heats of formation for intermediates 2, 4 and 5 obtained by hybrid MP2/DFTcalculations [50], barriers between them are tentative, after Ref [49]. Structures of (a) adsorbed isobutene (2) and (b) tert-butyl cation (4) [48] are also shown.

In the third step, the carbocation intermediate is captured by a chloride ion, and the energy barrier for this cation-anion combination is relatively low. The transition state is characterized by partial bond formation between the nucleophile (chloride anion) and the electrophile (tert-butyl cation). 

Great question First, let s review what led you to this question. In the unimolecular substitution reaction we looked at, an intermediate tert-butyl cation was formed, but in the bimolecular substitution reaction, the cation (this time a methyl cation, CH3+) was too high in energy. In this case the hydroxide ion directly displaced the chloride ion. 

In the second step the intermediate tert-butyl cation reacts rapidly with water to produce a t rt-butyloxonium ion, (CH3)3COH2, which in the third step, rapidly transfers a proton to a molecule of water producing t rt-butyl alcohol. 

The mechanism for the reaction of tert-butyl chloride with water (Section 6.9) can be described in three steps. See the box Mechanism for the SnI Reaction below, with a schematic free-energy diagram highlighted for each step. Two distinct intermediates are formed. The first step is the slow step—it is the rate-determining step. In it a molecule of tert-butyl chloride ionizes and becomes a tert-butyl cation and a chloride ion. In the transition state for this step the carbon-chlorine bond of tert-butyl chloride is largely broken and ions are beginning to develop ... 

The mechanism of the deprotection of Boc-amino acids is different from that of the normal ester hydrolysis (Section 20-4) It proceeds through the intermediate 1,1-dimethylethyl (tert-butyl) cation. Formulate this mechanism... 

Stepanov AG, Zamaraev KI, Thomas JM. CP/MAS and H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-bntyl silyl ether intermediates. Catal Lett 1992 13 407-22. 

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