Carbocations, from

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

The mechanism for formation of the 3 methyl glycoside is shown The mechanism for for mation of the a isomer is the same except that methanol approaches the carbocation from the axial direction... 

The use of antimony pentafluoride as a component of superacids for the generation of carbocations from various organic compounds was reviewed recently [16],... 

The formed carbocation from the last step may abstract a hydride ion from an isohutane molecule and produce 2,2-dimethylpentane, or it may rearrange to another carbocation through a hydride shift. 

The isomerization of isopentenyl diphosphate to dimethylally diphos phate is catalyzed by JPP isomerase and occurs through a carbocation pathway Protonation of the IPP double bond by a hydrogen-bonded cysteine residue ir the enzyme gives a tertiary carbocation intermediate, which is deprotonated b a glutamate residue as base to yield DMAPP. X-ray structural studies on the enzyme show that it holds the substrate in an unusually deep, well-protectec pocket to shield the highly reactive carbocation from reaction with solvent 01 other external substances. 

Aliphatic ketones can be reduced to hydrocarbons by triethylsilane and gaseous BF3.178 The BF3 is a sufficiently strong Lewis acid to promote formation of a carbocation from the intermediate alcohol. 

Figure 7.7 Free-energy diagrams for the formation of carbocations from protonated tertiary, secondary, and primary alcohols. The relative free energies of activation are tertiary < secondary primary.

Quantum Chemical Studies of Carbocations from Oxidized Metabolites of Aza-Polycyclic Aromatic Flydrocarbons... 

Figure 2. Studied carbocations from PAHs and Aza-PAHs. Figure adapted from reference 26.

Figure 16. Computed NPA heavy atom charges (Acharges related to the neutral compound in parentheses) for carbocations from BaP and azaBaP. [Dark circles are roughly proportional to the magnitude of C Acharges, and white circles to N Acharges threshold was set to 0.030].

Fig. 1 for stepwise solvolysis of R-X is due to the increase in ks (s ), with decreasing stability of the carbocation intermediate, relative to the constant value of az (M s ) for the diffusion-limited addition of azide anion. The lifetime for the carbocation intermediate eventually becomes so short that essentially no azide ion adduct forms by diffusion-controlled trapping, because addition of solvent to R occurs faster than escape of the carbocation from the solvent cage followed by addition of azide ion (k Now, the nucleophile adduct must form through a... 

Although we have shown the hydroxide ion attacking the carbocation from the left-hand side, it could equally well have attacked from the right-hand side (see below). This is a consequence of the carbocation having a planar arrangement of bonds around the central carbon atom. 

In the presence of orthoester and perchloric acid, 3-acetyltropolone (74) with benzaldehydes affords pyranones 76 directly (90JHC891). The reaction is considered to pass through a carbocation (from 74 and orthoester) which is stabilized by electron-donating substituents in the benzaldehyde (Ar). Therefore, the one-step synthesis might be limited to methoxy- and hydro-xybenzaldehydes. These synthons are also used to prepare corresponding... 

The diazotization reaction generates the same type of /i-hydroxy carbocation that is involved in the pinacol rearrangement. (See Section 5.6 in Part A for a discussion of the formation of carbocations from diazo compounds.)... 

Generation and NMR studies of the carbocations from various classes of PAHs under stable ion conditions, in combination with computational studies, provide a powerful means to model their biological electrophiles. These approaches allow the determination of their structures, relative stabilities, charge delocalization modes, and substituent effects, as a way to understand structure/reactivity relationships. 

Fig. 4 Simplified carbocations generated as model for carbocations from the BaP and BeP skeletons.

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