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Sec-Butyl carbocation

Attack of Br" ion on the sec-butyl carbocation. Attack from the top leads to S product and is the mirror image of attack from the bottom, which leads to It product. Since both are equally likely, racemic product is formed. The dotted C- -Sr bond in the transition state indicates partial bond formation. [Pg.337]

Yet a final limitation to the Friedel-Crafts reaction is that a skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used. Treatment of benzene with 1-chlorobutane at 0 °C, for instance, gives an approximately 2 1 ratio of rearranged (sec-butyl) to unrearranged (butyl) products. [Pg.556]

This can be tested by drawing on extensive information on carbocation stabilities in the gas phase. Heats of formation of ethyl, isopropyl, sec-butyl and /-butyl cations2 are shown below. From these values it is evident that the /-butyl cation is more stable than the sec-butyl cation by 13kcalmol 1. This corresponds to the direct comparison of (isomeric) ion stabilities noted above by Arnett and Mayr. [Pg.25]

HIAs of the carbocations are listed in Table 1 as differences in values from the /-butyl cation (AHIA in free energies mol-1). Returning to the comparison of isopropyl and sec-butyl cations it can be seen that the difference in their... [Pg.26]

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)]. [Pg.333]

Primary cations can never be observed by NMR—they are too unstable. But secondary cations can, provided the temperature is kept low enough, sec-Butyl chloride in SO2CIF at -78 °C gives a stable, observable cation. But, as the cation is warmed up, it rearranges to the f-butyl cation. Now this rearrangement truly is a carbocation rearrangement the starting material is an observable car-bocation, and so is the product, and we should just look at the mechanism in a little more detail. [Pg.980]

For both tertiary cations and secondary benzylic carbocation, the ratio of substitution to elimination is quite high. For example, for l-(/i-tolyl)ethyl cation in 50 50 TFE-water, the ratio is 1400."° For the fert-butyl cation, the ratio is about 30 in water" and 60 in 50 50 TFE-water. " These ratios are on the order of 10 if account is taken of the need for solvent reorganization in the substitution process." The generalization is that under solvolysis conditions, tert-alkyl and sec-benzyl carbocations prefer substitution to elimination. [Pg.439]

Among 2° carbocations, only the isopropyl, sec-butyl, and cyclopentyl carbocations have been observed in solution. These three structures nevertheless illustrate the kinds of delocalization effects that may be expected in other 2° carbocations. The cyclic structure of the cyclopentyl cation prevents significant C-C-C bridging. The calculated geometry (Figure 5.43) shows a twisted C2 structure with the C -H bonds aligned parallel with the empty p orbital for maximum hyperconjugative overlap. ... [Pg.297]

Theoretical calculations indicate ttiat the barriers for 1,2-hydride shifts in carbocations are ordered as follows cyclopropyl > cyclobutyl > cyclopentyl > sec-butyl. Propose an explanation for this trend. [Pg.325]

The result of this proton-transfer reaction is formation of a carbocation, a species in which one of its carbons has only six electrons in its valence shell and carries a charge of -r 1. Because the carbon bearing the positive charge in the sec-butyl cation has only two other carbons bonded to it, it is classified as a secondary (2°) carbocation. We will study the formation, structure, and reactions of carbocations in detail in Chapter 6. [Pg.195]

An example of this type of a Lewis acid-base reaction is that of a carbocation (a Lewis acid) with bromide ion (a Lewis base). The sec-butyl cation, for example, reacts with bromide ion to form 2-bromobutane. [Pg.211]

The alkylation of isobutane with C3-C5 olefins involves a series of consecutive and simultaneous reactions with carbocation species as the key intermediates. Scheme 6.10.2 shows the reaction of 2-butene and isobutane as a typical example. In the initial step, proton addition to 2-butene affords a sec-butyl cation. This sec-butyl cation can either isomerize or accept a hydride from a molecule of isobutane, giving n-butane and the thermodynamically more stable fert-butyl cation. These initiation reactions are required to generate a high level of ions in the start-up phase of alkylation but become less important under steady state conditions. [Pg.653]


See other pages where Sec-Butyl carbocation is mentioned: [Pg.43]    [Pg.312]    [Pg.312]    [Pg.320]    [Pg.698]    [Pg.199]    [Pg.43]    [Pg.312]    [Pg.312]    [Pg.320]    [Pg.698]    [Pg.199]    [Pg.223]    [Pg.25]    [Pg.66]    [Pg.486]    [Pg.86]    [Pg.39]    [Pg.40]    [Pg.43]    [Pg.10]    [Pg.304]    [Pg.137]    [Pg.170]    [Pg.125]    [Pg.141]    [Pg.308]    [Pg.1057]   
See also in sourсe #XX -- [ Pg.297 , Pg.298 ]

See also in sourсe #XX -- [ Pg.698 ]




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Carbocations butyl

Sec-Butyl

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