Protonation of propene

Shown in Table 1 are free energies of formation of the same ions in aqueous solution at 25°C. The measurements of pfrom which they are derived are described later in the chapter (p. 47). Suffice to say here that the relative values for isopropyl and secondary butyl cations are based on the inference from measurements of equal rate constants for protonation of propene and 1-butene42 that the pifas of the conjugate acids of these alkenes are the same. It can be seen that the differences in energies of formation between the cations are significantly less than in the gas phase. Thus the difference between the /-butyl and ethyl cations is reduced from 45kcalmol 1 to less than 20kcalmol 1. 

The value of pAR = -20.0 for the isopropyl cation is 2.1 log units more positive than the value derived from protonation of propene directly. In its favor, protonation of 2-butene occurs at a secondary rather than (as for 2-propene) a primary vinylic carbon atom, as is also true of formation of the cations correlated in Fig. 1. However, that correlation refers to benzylic rather than alkyl cations and there is no good reason to suppose their behavior is strictly comparable. 

The major product is isopropylbenzene. Protonation of propene gives the 2-propyl cation, a secondary carbocation, rather than the less stable 1-propyl cation, a primary carbocation. 

Referring to the discussions presented in Chapter 5 regarding the relative stabilities of carbocations (and hyperconjugation), we are reminded that tertiary carbocations are more stable than secondary carbocations, which, in turn, are more stable than primary carbocations. Since, as shown in Scheme 7.9, protonation of propene results in cationic character at both a secondary carbon and a primary carbon, a greater presence of cationic character on the secondary site is expected compared to the primary. This allows... 

Scheme 7.9 Protonation of propene introduces cationic character to both primary and secondary centers.

Carbenium ions are sp2 hybridized with the empty orbital perpendicular to the plane containing the three substituents. Calculations confirm both the flat structure of carbenium ions and the shorter linkage with aryl substituents due to the partial double-bond character. As discussed previously, the positive charge is not localized just on the sp2-hybridized carbon of the carbenium ion, but is dispersed onto neighboring substituents. Table 1 presents the charge distribution in model carbenium ions formed by protonation of propene, isobutene, styrene, a-methylstyrene, methyl vinyl ether, and methyl propenyl ether. 

Protonation of the cyclopropane ring can easily be achieved in the gas phase, and the collisional activation mass spectra of the so-obtained C3H7 ions were found to be perfectly identical with the one obtained from protonation of propene. This has been interpreted as evidence that the isomerization C-C3H7 - S-C3H7 is completed in t < 10 s. 

The acidity of the PW12O40 is such that it can stabilize the adsorption of an alkene. Janik et alJ l have shown that the barriers for the protonation of ethylene, propene, 2-butene and isobutene are 69, 50.5, 51 and 14 kJ/mol, respectively. The results indicate that the formation of the carbenium ion from the n adsorbed state is favored on the Keggin structure over the similar corresponding state in the chabazite (see Chapter 4). For example, the barrier for the protonation of propene in chabazite was calculated to be -1-56 kJ/mol in comparison with the value of 50.5 kJ/mol calculated on the phosphotungstic (HPW) Keggin structure. The barriers, however, for the reaction of a surface alkoxy to the carbenium ion state are all higher on phosphotungstic acid than on chabazite. 

Scheme 6.86. A representation of a pathway for the formation of 2-phenylpropane by the reaction of benzene (CgHg) with propene (CH3CH=CH2) in the presence of phosphoric acid (H3PO4). The formation of a secondary cation or its eqnivalent by phosphoric acid protonation of propene (CH3CH=CH2) presumably proceeds the attack by benzene (C He).

Protonation of Propene at C2—More Substituted Carbon (Does Not Occur)... 

While the diazonium salt route is probably the most commonly used laboratory synthesis of phenols, phenol itself is manufactured from wo-propylbenzene, which has the common name cumene. Cumene is prepared by a Friedel-Crafts reaction between propene and benzene, in the presence of a strong acid such as H3PO4 (Figure 13.28). The key intermediate is the 2-propyl cation, formed by protonation of propene. 

The proton affinity is defined as the negative of the energy of protonation. For example, the proton affinity of propene is given by -AH for the reaction ... 

Is the stable cation that formed as a result of protonation of the more electron-rich end of the alkene Examine electrostatic potential maps for propene, 2-methylpropene and 2-methyl-2-butene. For each, can you tell whether one end of the 7t bond is more electron rich than the other end If so, does protonation on the more electron-rich end lead to the more stable carbocation ... 

An isopropyl carbocation cannot experience a beta fission (no C-C bond beta to the carbon with the positive charge).It may either abstract a hydride ion from another hydrocarbon, yielding propane, or revert back to propene by eliminating a proton. This could explain the relatively higher yield of propene from catalytic cracking units than from thermal cracking units. 

Further protonation of the trimer produces a C9 carhocation which may further react with another propene molecule and eventually produce propylene tetramer. 

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

The theoretical study of the structure of propene was then used as a model to calculate the effect of the structure on the proton affinity, and later to predict the acidity of similar systems such as cycloalkenes46. Deformation of the CCC angle as a function of the stability of the anion was probed, and the results were in agreement with the acidities of the hydrogens of propene. The allylic protons were found to be more acidic than the vinylic ones, which is in contrast to the results of Grundler47. 

H2 activation by protonation of the coordinated allene in 8 with formation of the palladium hydride intermediate 9 and propene. 

Protonation of tetrakis(trimethylsilyl)allene 33 with HSO3F/ SbF5 (1 1) gives the 1,1,3,3-tetrakis(trimethylsilyl)-l-propen-2-yl cation 34. The isomeric allyl cation 35 is not formed (12, 44). 

Block et al. (1979) have recently shown that the lachrymatory factor of the onion is a sulfine, namely, CH3CH2CH=S=0. This sulfine is also thought to be formed from a sulfenic acid precursor, in this case by the intramolecular shift (14) of the sulfenic acid proton of trans- 1-propene-l -sulfenic acid. 

The protonation of a a-allyltungsten complex to the cationic w-propene complex and the NaBH4 reduction of this to the isopropyl complex in complete analogy to the corresponding iron compounds has been demonstrated by Green and Stear (4S) ... 

Electrophilic addition of HBr to propene gives predominantly the so-called Markovnikov orientation Markovnikov s rule states that addition of HX across a carbon-carbon multiple bond proceeds in such a way that the proton adds to the less-substituted carbon atom, i.e. that already bearing the greater number of hydrogen atoms (see Section 8.1.1). We rationalized this in terms of formation of the more favourable carbocation, which in the case of propene is the secondary carbocation rather than the alternative primary carbocation. 

The process involves reacting butenes and mixtures of propenes and butenes with either a phosphoric acid type catalyst (UOP Process) or a nickel complex-alkyl aluminum type catalyst (IFP Dimersol Process) to produce primarily hexene, heptene, and octene olefins. The reaction first proceeds through the formation of a carbocation which then combines with an olefin to form a new carbocation species. The acid proton donated to the olefin initially is then released and the new olefin forms. Hydrotreatment of the newly formed olefin species results in stable, high-octane blending components. 

It can be seen that the HOMO energy of cyclopropane is higher than that of cyclobutane or cyclohexane, and that the much more reactive bicyclo[1.1.0]butane has a much higher HOMO energy, which is close to that of propene. Another important factor is the polarizability, which reflects how easily the electron density may be shifted in the presence of an electric field (such as that developed by a proton). Here again, cyclopropanes have significantly higher polarizability than other cycloalkanes.52... 

The oxyhydration of propene to acetone occurs at a much lower temperature than the allylic oxidation and demands, in principle, the presence of excess steam. The reaction is initiated by addition of a proton from the catalyst surface and the acetone formation involves oxygen originating from water. 

Balaban and his coworkers have developed the diacylation of propene derivatives to provide a useful route to pyrylium salts (61MI22401). It is proposed that the reaction proceeds through acylation of the alkene to a keto carbocation. Elimination of the a-proton involving a cyclic six-membered transition state then leads to the enol (657) and hence the non-conjugated enone. A second acylation and subsequent dehydration yield the pyrylium salt (Scheme 261). 

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