Isobutene structure

Equation (21) depicts the pathway of this co-dimerization of IB2 and isobutene. Structural analysis showed the obtained IB3 to consist of 23 and 24 only, in the absence of the other possible isomers 25 and 26 (Eq. (22)). Thus, IB3 is formed by the reaction of IB2 with the protonated isobutene (Eq. (21)), and not through addition of protonated IB2 to isobutene (Eq. (22)). Perhaps protonation of isobutene may occur more readily than in the sterically more hindered IB2. The selective formation of IB3 requires the IB3 cation 22 to release its P-proton without propagating any further, and the CF3SO3 anion plays a critical role to meet this requirement. Future investigations will have to clarify the reason for the observed complete suppression of dimerization of IB2 in the presence of isobutene in the mixture. 

Presumably, jS-chloro ketones could also react similarly with methyl(ene) ketones. Another logical extension is the possibility of synthesizing pyrylium salts by dehydrogenative condensation of -chlorovinyl ketones with oleflns like styrene, in the presence of stannic chloride (the olefins so far tested, like isobutene, are not suitable structurally). 

The number (nj) of the cross ct conjugations of the trios of a C-C bond and two antiperiplanar C-H bonds is important for the stabilities of alkanes. The cross conjugation number (nj) of an alkane is defined as that of the conformer where the longest C-C chain has trans a zigzag structure. For example, there are three cross conjugations (n = 3) in isobutene and none in n-butane (n = 0) (Scheme 27). Isobutane is more stable than n-butane [34, 35]. 

Fig. 41. Proposed mechanisms for the reactions (a) Y + propene, (b) Y + cis-2-butene, (c) Y + 1-butene, (d) Y + isobutene. Note that the mechanism for Y + trans-2-butene is similar to that for Y + cis-2-butene and so is not shown. Double-sided arrows indicate resonance structures. See text for details.

The nature of the products formed from isobutene in the presence of conventional acids depends on the concentration of the acid, the temperature and the solvent. The products are always of relatively low molecular weight, they may contain fragments of catalyst, and, due to methyl shifts, branching and fragmentation, their structure may not be that of a simple linear oligomer formed by addition of monomers one to another (conjunct polymerisation [21]). There is no record of high polymers having been formed from isobutene under the influence of a conventional acid only, and for this reason these reactions will not be discussed here in detail. 

We shall consider the system isobutene - BF3 - HzO in a non-polar solvent, involving the anion BF3OH (discussed in detail by Skinner [7]), since we are still too ignorant of the nature and structure of the anions derived from the aluminium or Group IV halides. The reaction in question can be represented thus ... 

Am (PIB=). Of the four isomeric olefinic groups which can be formed by removal of CH3 from the polyisobutyl ion and subsequent isomerisation, the structure -CMe2 CMe = CH2 will have the largest AMe+ which is taken as the same as that of isobutene this can be estimated as -70 kcal less than the proton affinity, i.e., -130 kcal. 

The sequential removal of H and H+ from isobutene-type structural units (so-called H2+ abstraction ) was also used to generate the radical anion of non-Kekule benzene , i.e. l,3-dimethylenecyclobutane-l,3-diyl (39) (Scheme 11). As shown by Hill and Squires161, this highly unusual, distonic C(,II(, isomer can be produced in pure form by reaction of O with 1,3-dimethylenecyclobutane (38). Working in a flowing afterglow mass spectrometer, subsequent reactions were again used to characterize this radical anion and differentiate it from other ( VdL, isomers. 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

The reaction of CH2 with cyclopropane1617 46177 gives excited methyl-cyclopropane which is deactivated or undergoes structural isomerization to butene-1, butene-2 (m and trans), and isobutene. The lifetime of methylcyclopropane wa6 found to depend on the CH2 source. 

Infrared spectra of propene and isobutene on different catalysts were measured by Gorokhovatskii [143]. Copper oxide, which converts olefins to butadiene and aldehydes, shows adsorption complexes different from structures on a V2Os—P2Os catalyst which produces maleic acid anhydride. Differences also exist between selective oxidation catalysts and total oxidation catalysts. The latter show carbonate and formate bands, in contrast to selective oxides for which 7r-allylic species are indicated. A difficulty in this type of work is that only a few data are available under catalytic conditions most of them refer to a pre-catalysis situation. Therefore it is not certain that complexes observed are relevant for the catalytic action. 

Friedel-Crafts alkylation of benzo[6]thiophene has received little attention. The published results, which deserve reexamination, indicate that exclusive 3-substitution occurs in some cases, whereas in others, 2-substitution predominates. Benzo[6]thiophene is alkylated with isopropyl chloride, isopropanol, or propene in the presence of various acid catalysts under a variety of reaction conditions to give a mixture of 2- and 3-isopropylbenzo[6]thiophene in which the 2-isomer predominates (78-92%).358 410 In contrast, alkylation with isobutene in the presence of either 80% sulfuric acid415 or 100% phosphoric acid416 is said to afford exclusively 3-/er<-butylbenzo[6]thiophene in yields of 100 and 75%, respectively. In neither case was the structure of the product rigorously confirmed. Likewise, 3-Jeri-amylbenzo [63-thiophene is the exclusive product of alkylation with tert-amyl alcohol in the presence of stannic chloride414 alkylation with pent-l-ene, hex-l-ene, and a Ci8 propylene polymer is also claimed to give... 

The values of the relative rates of reactions of oxygen atoms with olefins are listed in Table III, with the rate of the isobutene reaction taken arbitrarily as unity. The trend in the rates with olefin structure is most interesting and will be discussed in the next section. 

To get a better insight into the chlorination reaction, we wanted to avoid a heterogeneous process. Instead of polyethylene or polypropylene, we used polyisobutene, which is soluble in carbon tetrachloride, as are its chlorination products. In addition, we were interested in the structure and properties of the chlorinated products, especially in comparison with polyvinyl chloride (PVC) and vinyl chloride/isobutene (VC/IB) copolymers. 

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