Carbocations tert-butyl cation

The transition state is closer m energy to the carbocation (tert butyl cation) so Its structure more closely resembles the carbocation than it resembles tert butyloxonium ion The transition state has considerable carbocation character meaning that a significant degree of positive charge has developed at carbon... 

The main observations can be accounted for by a carbocationic mechanism incorporating intermolecular hydride transfer (77). The multistep transformation illustrated by the reaction between isobutane and 1-alkenes is initiated by the carbocation formed by protonation of the alkene (by the protic acid or the promoted metal halide catalysts) (eq. 48). Intermolecular hydride transfer between isobutane and the cation then generates a new carbocation (tert-butyl cation 13 eq. 49). The cation 13 adds to the alkene to form cation 14 (eq. 50), which then, through intermolecular hydride transfer, forms the product (eq. 51). In this final product-forming step, 13 is regenerated from the isoalkane and then starts a new cycle. Alkane-alkene alkylation, therefore, can be considered a chain reaction with 13 as the chain carrier. 

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

The major difference between the two mechanisms is the second step The second step m the reaction of tert butyl alcohol with hydrogen chloride is the ummolecular dis sociation of tert butyloxonium ion to tert butyl cation and water Heptyloxonium ion however instead of dissociating to an unstable primary carbocation reacts differently It IS attacked by bromide ion which acts as a nucleophile We can represent the transition state for this step as... 

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... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

Both compounds react by an S l mechanism and their relative rates reflect their acti vation energies for carbocation formation Because the allylic chloride is more reactive we reason that it ionizes more rapidly because it forms a more stable carbocation Struc turally the two carbocations differ m that the allylic carbocation has a vinyl substituent on Its positively charged carbon m place of one of the methyl groups of tert butyl cation... 

The tert butyl group is cleaved as the corresponding carbocation Loss of a proton from tert butyl cation converts it to 2 methylpropene Because of the ease with which a tert butyl group is cleaved as a carbocation other acidic reagents such as trifluoroacetic acid may also be used... 

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... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

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... 

Finally, two studies have reported on the reactions of carbocations with Mg atoms using mass spectrometry The types of products formed depend on the nature of the carbocation. The labeled methanium ion, CH4D+, reacts via proton transfer (equation 11), deuteron transfer (equation 12) and charge transfer (equation 13). The ethyl cation reacts via charge transfer (equation 14) while the tert-butyl cation reacts via proton transfer (equation 15). In all cases there was no evidence for formation of an organomagnesium species. 

Protonated secondary thiols are stable even at higher temperatures. Protonated isopropyl thiol cleaves slowly at 0°C in HS03F-SbF5 (1 1 M) solution. No well-identified carbocations were found in the NMR spectra due to the instability of the isopropyl cation under these conditions. Protonated. sec-butyl thiol 55 cleaves to tert-butyl cation at this temperature [Eq. (4.33)]. 

As is apparent in the last step, isobutane is not alkylated but transfers a hydride to the Cg+ carbocation before being used up in the middle step as the electrophilic reagent (tert-butyl cation 4). The direct alkylation of isobutane by an incipient tert-butyl cation would yield 2,2,3,3-tetramethylbutane,142 which indeed was observed in small amounts in the reaction of ferf-butyl cation with isobutane under stable ion conditions at low temperatures (vide infra). 

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. 

Superelectrophilic activation has also been proposed to be involved, based upon the reactivity of carbocations with molecular hydrogen (a a-donor).16 This chemistry is probably even involved in an enzymatic system that converts CO2 to methane. It was found that A. A -menthyl tetrahy-dromethanopterin (11) undergoes an enzyme-catalyzed reaction with H2 by hydride transfer to the pro-R position and releases a proton to give the reduced product 12 (eq 15). Despite the low nucleophilicity of H2, cations like the tert-butyl cation (13) are sufficiently electrophilic to react with H2 via 2 electron-3 center bond interaction (eq 16). However, due to stabilization (and thus delocalization) by adjacent nitrogen atoms, cations like the guanidinium ion system (14) do not react with H2 (eq 17). 

In the polar reaction, a proton in HBr first adds to the terminal sp2 carbon in isobutene to produce a stable tert-butyl cation (8), and then it reacts with the counter bromide anion to form tert-butyl bromide. Thus, the proton in HBr adds to the less substituted sp2 carbon in alkene to produce a more stable carbocation. This is based on the Markovnikov rule. In radical reactions, the hydrogen atom of HBr is abstracted first by the initiator, PhCO (or Ph ) derived from (PhC02)2, and the formed bromine atom then adds to the terminal sp2 carbon in isobutene to form the stable (3-bromo tert-butyl radical (9), and then it reacts with HBr to produce /so-butyl bromide and a bromine atom. This bromine atom again... 

Arnett and Hofelich measured heats of reaction of a variety of alcohols with SbF5/FS03H in sulfuryl chloride fluoride to form their respective carbocations at constant temperature (-40 °C). In this superacid medium there were no ion-pair complications and hence reliable calorimetric data were obtained for various cyclopropyl and phenyl substituted cations. The heats of reaction for the formation of tricyclopropylcarbinyl cation (-59.2 kcalmoT ), trityl cation ( 9.0 kcalmoT ) and tert-butyl cation (-35.5 kcalmol ) show that the relative order of the stabilization of the cationic center is cyclopropyl >... 

Radicals can be generated by cathodic reduction from carbocations, protonated C=X bonds, and the reduction of halides or onium salts. The reduction potentials of carbocations range from 1.87 V (vs nhe) for NCCH(4-CN-C6H4)+, 0.97 V for the benzylcation, 0.33 V for the tert-butyl cation, and 0.0 V for the methoxymethyl-cation to -0.88 V for Et2N=CHCH3+ [143],... 

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). 

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