Isobutyl cation

Isobutyl group (Section 2 13) The group (CH3)2CHCH2— Isoelectric point (Section 27 3) pH at which the concentration of the zwittenonic form of an amino acid is a maximum At a pH below the isoelectric point the dominant species is a cation At higher pH an anion predominates At the isoelec tnc point the ammo acid has no net charge Isolated diene (Section 10 5) Diene of the type... 

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

The isobutyl cation spontaneously rearranges to the tart-butyl cation by a hydride shift. Is the rearrangement exergonic or endergonic Draw what you think the transition state for the hydride shift might look like according to the Hammond postulate. 

The more stable tert-butyl cation is formed rather than the much less stable isobutyl cation in step 1 => the reaction produces tert-butyl alcohol. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

The earlier paper (Subira et al., 1976) contains results on isobutyl vinyl ether (IBVE) which are republished together with new measurements on methyl (MVE)-, ethyl (EVE)-, and isopropyl (IPVE)-vinyl ethers in CH2C12 over the temperature range 0 °C to -40 °C with m = (2-15) x 10"2, [Ph3CSbCl6] = (5.5-8) x 10"5 mold 1 (Subira et al., 1988). The rate of disappearance of the Ph3C+ ion in the presence of the monomer is considerably smaller than the rate of consumption of the monomer under all conditions, and as mentioned above, the authors identify the rate of disappearance of the trityl cation with the rate of initiation, in our notation... 

Recently, the radical cation of PBN has been characterized by matrix spectroscopy and its reactivity has been studied by fast spectroscopic methods (Zubarev and Brede, 1994), and found to conform to the behaviour deduced from the OsCU and TBPA + studies. y-Radiolysis of PBN in a glassy matrix of isobutyl chloride or Freon-113 (CF2C1CFC12) at 77 K produced an intensely green glass containing PBN +, the epr spectrum of which had an anisotropic nitrogen coupling constant Ay = 2.75 mT and gy = 2.0037. Tlie mechanism of the radiolysis reaction is well established (Neta, 1976) and involves the formation of solvated electrons (e ), which add to the matrix species and produce chloride ion, and positive holes (h+) which eventually come to rest at the matrix component of lowest Ip (Symons, 1997), in this case PBN (see reactions (30) and (31)). 

The behaviour of the butyl system provides important information on the nature of the intermediate formed during the rearrangement of the isobutyl to the 2-butyl cation. Thus, from the observation that isobutyl chloride yields n-butane which has exchanged one proton with the acid, while the solvolysis of 2-butyl chloride in the same acid (2% HjO), yields unexchanged n-butane one might deduce that an intermediate was formed during the former s solvolysis which exchanged one proton with the acid before it converted to a secondary butyl ion. A reasonable mechanism is shown in Scheme 1. 

An example of the first type of study is the cationic pol erization of alkenes and heterocyclic monomers in the presence of 2-alWlfurans. As discussed above, electrophilic substitution at C5 is quite facile with these compounds and one can therefore prepare monofunctional oligomers bearing a furanic end-group. By a judicious choice of experimental conditions this transfer reaction will predominate over all other chain-breaking events and virtually all the chains will have the same terminal structure, i.e. a 5-oligomer-2-al lfuran. Structure 32 illustrates this principle with isobutyl vinyl ether oligomers capped by 2-methylfuran ... 

The cationic polymerization of vinyl isobutyl ether at —40°C produces stereoregular polymers (structure 5.21). The carbocations of vinyl alkyl ethers are stabilized by the delocalization of p valence electrons in the oxygen atom, and thus these monomers are readily polymerized by cationic initiators. Poly(vinyl isobutyl ether) has a low Tg because of the steric hindrance offered by the isobutyl group. It is used as an adhesive and an impregnating resin. 

Unwanted branching of many polymers probably occurs through such isomerizations. PP, formed using cationic polymerization, has methyl, ethyl, w-propyl, w-butyl, isopropyl, gem-dimethyl, isobutyl, and t-butyl groups connected to the main chain. 

Which technique would you choose for producing a polymer of isobutyl vinyl ether (a) cationic or (b) anionic ... 

The first reported instance of stereoselective polymerization was probably the cationic polymerization of isobutyl vinyl ether in 1947 [Schildknecht et al., 1947]. A semicrystalline polymer was obtained when the reaction was carried out at —80 to —60°C using boron tri-fluoride etherate as the initiator with propane as the solvent. The full significance of the polymerization was not realized at the time as the crystallinity was attributed to a syndiotactic structure. X-Ray diffraction in 1956 indicated that the polymer was isotactic [Natta et al., 1956a,b], (NMR would have easily detected the isotactic structure, but NMR was not a routine tool in 1947.)... 

Cationic Polymerization of Isobutyl Vinyl Ether with BFj-Etherate at Low Temperatures... 

R.F. Storey, D.W. Baugh, and K.R. Choate, Poly(styrene-b-isobutyl-ene-b-styrene) block copolymers produced by living cationic polymer-... 

Apart from some experiments with methyl and /i-chloroethyl vinyl ethers the initiator concentrations employed were such that the initiating cations, and presumably the propagating species, were essentially dissociated from the corresponding counterion. Once again therefore this data is a measure of the reactivity of the free polymeric cations derived from the various monomers. Isobutyl vinyl ether is the monomer most widely studied, and as would be anticipated for free cationic reactivities, the data varies little with the counterion employed (SbClg or BF4), or indeed with the carbocation used as initiator (C7H7 or Ph3C+) under similar experimental conditions. 

Isobutyl chloride- rerr-Butyl cation Tetrachloroaluminate... 

Subsequent reactions of the cations with bromide ion give fert-butyl bromide and isobutyl bromide. In the usual way of running these additions, the product is very pure tert-butyl bromide. 

But the addition product is 99+% tert-butyl bromide so the reaction clearly is kinetically controlled, tert-butyl bromide being formed considerably faster than isobutyl bromide. The slow, or rate-determining, step in this reaction is the formation of the intermediate cation rather than the reaction of the cation with bromide ion. So to account for the formation of tert-butyl bromide we have to consider why the tert-butyl cation is formed more rapidly than the isobutyl cation ... 

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