Sec-Butyl cation

There is good reason to believe that the potential of NMR studies of carbenium ions on solid metal halitks exceeds that of corresponding studies in superacid solutions. Of course the advantages of working in solids include Ae possibility of very low temperatures and the mass transport restrictions of frozen media. Thus, Mehre and Yannoni were able to characterize the sec-butyl cation in frozen SbF5 by NMR [20] and Schleyer and coworkers have obtained infrared evidence of the allyl cation in the same medium [21]. So far, we have been successful in every case in which we have tried to duplicate known solution carbenium ion chemistry on... 

The sec-butyl cation has been prepared by slow addition of jec-butyl chloride to SbFj-SOjCIF solution at - 110°C (Saunders Hagen Rosenfeld J. Am. Chem. Soc. 1968, 90, 6882] and by allowing molecular beams of the reagents to impinge on a very cold surface (Saunders Cox Ohlmslead J. Am. Chem. Soc. 1973, 95. 3018 Saunders Cox Lloyd J. Am. Chem. Soc. 1979,101, 6636 Myhre Yannoni J. Am. Chem. Soc. 1981, 103, 230). 

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. 

On the other hand, the difference between the /-butyl and sec-butyl cations shows a much smaller reduction, from 13 to 10.2kcalmol Moreover, instead of the energy of the isopropyl cation being lOkcalmol-1 greater than the sec-butyl cation it is now 2.3 kcalmol-1 less. In the gas phase the extra CH2... 

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... 

Alternatively, if the reaction involved trivalent w-butyl cation 468 (from ethylation of ethylene) the ion would inevitably rearrange via 1,2-hydrogen shift to sec-butyl cation 19, which in turn would isomerize into the ferf-butyl cation (1) and thus give isobutane (461) [Eq. (3.125)]. 

It was found possible to measure the kinetics of cleavage of protonated sec-butyl methyl ether 24 by following the disappearance of the methoxy doublet in the NMR spectrum with simultaneous formation of protonated methanol and ferf-butyl cation. The cleavage shows pseudo-first-order kinetics. Presumably, the rate-determining step is the formation of sec-butyl cation followed by rapid rearrangement to the more stable ferf-butyl cation... 

Tetramethylbutane 36 was not formed when n-butane and. sec-butyl cation were reacted. The isomer distribution of the octane isomers for typical butyl cation-butane alkylations is shown in Table 5.7. 

The relative Importance of Reactions B and C depend to at least some extent on the concentrations of R and of sec-butyl cations at the reaction site, thought to be the Interface between the two liquid phases (7,15,16). in general, there will always be a fairly high concentration of R after the Induction period, but the concentration of sec-butyl Ions will presumably always be low. It also seems quite probable that R s are more reactive than sec-butyl cations relative to Isobutane. 

The proton spectrum of the sec-butyl cation at —120 0 consists of two resonances at 6 3.2 and 6 6.7 of relative area 2 119S This is a result of a degenerate 1,2 hydride shift which at this temperature is fast with respect to the NMR time-scale. 

Initiation (or olefin protonation) In this step, a f-butyl cation is formed from isobutene. A sec-butyl cation is formed from 1-C4= or 2-C4=. The sec-butyl cation can form a f-butyl cation by methyl shift, or it can undergo hydride transfer from isobutane, forming n-C and a t-butyl cation. 

Fragmentation occurs at the indicated bond to form mainly sec-butyl cation and. to a lesser extent, ethyl cation (m/z = 29). Spectrum A fits best. 

STEP 2 Reaction of an electrophile and a nucleophile to form a new covalent bond. The reaction of the sec-butyl cation (an electrophile and a Lewis acid) with chloride ion (a nucleophile and a Lewis base) completes the valence shell of carbon and gives 2-chlorobutane ... 

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. 

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. 

The sec-butyl cation can react as both a Bronsted-Lowry acid (a proton donor) and a Lewis acid (an electron pair acceptor) in the presence of a water-sulfuric acid mixture. In each case, however, the product is different. The two reactions are as follows ... 

Reaction of the sec-butyl cation (an electrophile) with bromide ion (a nucleophile) completes the valence shell of carbon and gives 2-bromobutane. 

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. 

---