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Transition state carbocation rearrangement

This rearrangement is shown in orbital terms in Figure 5.8. The relevant orbitals of the secondary carbocation are shown in structure (a), those of the transition state for rearrangement in (b), and those of the tertiary carbocation in (c). Delocalization of the electrons of the C—CH3 a bond into the vacant p orbital of the positively charged carbon by hyperconjugation is present in both (a) and (c), requires no activation energy, and... [Pg.216]

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... [Pg.222]

Even though the rearrangements suggest that discrete carbocation intermediates are involved, these reactions frequently show kinetics consistent with the presence of at least two hydrogen chloride molecules in the rate-determining transition state. A termolecular mechanism in which the second Itydrogen chloride molecule assists in the ionization of the electrophile has been suggested. ... [Pg.356]

Examine the transition state for the hydride shift. Calculate the barrier from the more stable initial carbocation. Is the process more facile than typical thermal rearrangements of neutral molecules (.05 to. 08 au or approximately 30-50 kcal/mol) Is the barrier so small (<.02 au or approximately 12 kcal/mol) that it would be impossible to stop the rearrangement even at very low temperature Where is the positive charge in the transition state Examine atomic charges and the electrostatic potential map to tell. Is the name hydride shift appropriate If not, propose a more appropriate name. [Pg.110]

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). [Pg.1383]

We summarize here a procedure to predict the feasibility and the stereochemistry of thermally concerted reactions involving cyclic transition states. The 1,2 rearrangement of carbocations will be used to illustrate the approach. This is a very important reaction of carbocations which we have discussed in other chapters. We use it here as an example to illustrate how qualitative MO theory can give insight into how and why reactions occur ... [Pg.1010]

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. [Pg.661]

Propagation proceeds by the electrophilic addition of carbenium ions to double bonds with the regeneration of carbocations. The transition state is relatively late, and it was estimated that approximately half of the charge is transferred into the developing carbocation (Chapter 2). This may explain the fact that dormant species (covalent esters and onium ions) do not react directly with alkenes. The charge on the a-C atoms in the dormant species is not sufficient for the formation of the transition state. A multicenter rearrangement process is additionally disfavored by entropy. In contrast, a two-step process in which carbocations are formed and then very rapidly add to alkenes is free of this difficulty. [Pg.357]

Solvation stabilizes carbocations in solution a great deal but the interaction forces can be long range and non-specific. Thus many isomeric ions and transition states may be stabilized to a comparable extent differential solvation effects on carbocation rearrangements are probably not of large magnitude. Solvation of less stable ions on the other hand may be structure specific... [Pg.227]

The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). [Pg.359]

Because syn addition to the double bond occurs and no carbocation rearrangements are observed, carbocations are not formed during hydroboration, as shown in Mechanism 10.5. The proposed mechanism involves a concerted addition of H and BH2 from the same side of the planar double bond the it bond and H-BH2 bond are broken as two new o bonds are formed. Because four atoms are involved, the transition state is said to be four-centered. [Pg.388]


See other pages where Transition state carbocation rearrangement is mentioned: [Pg.188]    [Pg.188]    [Pg.206]    [Pg.195]    [Pg.210]    [Pg.323]    [Pg.334]    [Pg.1381]    [Pg.12]    [Pg.15]    [Pg.290]    [Pg.739]    [Pg.12]    [Pg.14]    [Pg.108]    [Pg.216]    [Pg.217]    [Pg.108]    [Pg.566]    [Pg.658]    [Pg.291]    [Pg.225]    [Pg.291]    [Pg.419]    [Pg.115]    [Pg.454]    [Pg.672]    [Pg.330]    [Pg.12]    [Pg.15]    [Pg.277]    [Pg.211]    [Pg.1920]    [Pg.211]    [Pg.291]   
See also in sourсe #XX -- [ Pg.209 ]

See also in sourсe #XX -- [ Pg.209 ]

See also in sourсe #XX -- [ Pg.209 ]

See also in sourсe #XX -- [ Pg.188 ]

See also in sourсe #XX -- [ Pg.205 ]

See also in sourсe #XX -- [ Pg.194 ]




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Carbocations rearrangements

Carbocations transition states

Transition 2,3]-rearrangement

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