Proton alkenes

Identify which protonation reaction alkene A protonated alkene A, alkene B protonated alkene B)... 

Electrostatic potential map for protonated alkene B shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

Besides the rearrangement of carbocations resulting in the formation of isomeric alkylated products, alkylation is accompanied by numerous other side reactions. Often the alkene itself undergoes isomerization prior to participating in alkylation and hence, yields unexpected isomeric alkanes. The similarity of product distributions in the alkylation of isobutane with n-butenes in the presence of either sulfuric acid or hydrogen fluoride is explained by a fast preequilibration of n-butenes. Alkyl esters (or fluorides) may be formed under conditions unfavorable for the hydride transfer between the protonated alkene and the isoalkane. 

Identify which protonation reaction (alkene A —> protonated alkene A, alkene B —>protonated alkene B) is more favorable. The energy of proton is given at right. Compare geometries of the two alkenes. Which is more strained Why How is this likely to affect the proton affinity Compare electrostatic potential maps for the two alkenes. Is the n bond in one more susceptable to protonation than that in the other Compare maps for the two protonated forms. Is the charge in one more delocalized than that in the other Suggest an explanation to account for both the reactivity difference and the structural changes. 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

Transfer of a proton to an unsaturated carbon atom occurs more readily because 7r-electron pairs are available to form a new covalent (sigma) bond. A protonated alkene may undergo addition of a nucleophile in the second step, viz. 

HF has Ho = — 11.0, Addition of 4% SbF5 lowers Hq to —21.0. Explain why this should be true, and why the resulting solution is so strongly acidic that it can protonate alkenes ... 

An alternative to tosyl or trisyl hydrazones is the use of phenylaziridinyl hydra-zones (e.g. 42). If the protonated alkene product is desired, then the base LDA can be used as a catalyst. LDA is regenerated by reaction of the alkenyllithium species with the by-product, diisopropylamine (2.40). ... 

For a discussion and leading references to proposals of a proton-alkene n complex as an intermediate in the dehydration of alcohols and the protonation of alkenes, see reference 120. 

The 1 1 adduct of thiolan with bromine was shown by A"-ray analysis to have a pyramidal configuration at sulphur, whilst rapid pseudorotation in the ring rendered it apparently planar. N.m.r. data were consistent with rapid inversion of pyramidal co-ordination at sulphur, and with the presence of a highly charged sulphur atom. N.m.r. data have also been recorded for protonated thiolan and thian in FSOgH-SbFe at — 78 °C. Thiolan acted as a very efficient trap for carbonium ions with protonated alkenes it gave alkylsulphonium salts, and with the carbonium ions derived from acidified pinacol it gave the sulphonium salts (37) and (38). ... 

The charged carbocations produced after the C-H and C-C bond cleavage can be considered protonated alkenes or carbenium ions, in which the hybridization around the positively charged carbon is sp. The carbenium ions subsequently adsorb to form alkoxy intermediates on the zeolite lattice, as illustrated in Fig. 4.5. 

For reactions in H2O an analogous discussion applies as for reactions with the protonated sulfuric acid form. The reactive intermediate now, however, is H3O+. H3O+ has a much stronger OH bond than H3SO4 +, and hence its reactivity is much less. The species that is formed by reaction with alkene in an aqueous solution can best be compared with protonated alcohols forms typically known as alkyl oxonimn ionsl l Protonated alkene is hydrated in aqueous media. The formation of alkyloxonimn ions instead of the carbenium ions in H2O can be viewed as due to the basicity of water. The major difference between solid acids and acidic solutions arises because the hydrogen atoms in solid acids are part of strong covalent bonds and are not present as protons that are present in in the solution phase. In a solution there is a equilibrium between non-dissociated acid molecules and the... 

Other H—X molecules are not strong enough acids to protonate alkenes. Water (HOH) is an excellent example. However, such molecules wiU add if the reaction is acid-catalyzed. Enough acid catalyst is added to give the protonated alkene, which is attacked by water. The catalyst is regenerated in the last step and recycles to carry the reaction further. 

The following reactions of olefins have also been studied formation of dialkyl adducts (517) from vinyl monomers and dienes with lithium metal and alkyl bromides in THF alkylsulphonium salts (518) from thioethers and protonated alkenes hydroformylation of styrene and a-methylstyrene in the presence of bis-(iV-a-methylbenzylsalicylaldiminato)cobalt(n) to give 2-phenylpropanal (optical purity 1.9%) and 3-phenylbutanal (optical purity 2.9%) various electron-rich olefins of the type (519) with primary amines... 

The propylene oligomerization on HY zeolites at 480—580°C occurs through surface alkoxy derivatives formed in the reaction between protonated alkenes and surface oxygens of zeolites. Carbocations are not intermediates in this case. The mechanism of this reaction is presented in the following [6641 ... 

The oligomerization of propene on zeolite H-Y has been studied [33,37] by variable-temperature MAS NMR. Alkoxy species formed between protonated alkenes and oxygens of the zeolitic framework were found to be important long-lived intermediates in these reactions. Simple secondary or tertiary carbocations are either absent in the zeolite at low temperatures, or are so transient as to be undetectable by NMR even at temperatures as low as 163 K. There was, however, evidence for long-lived alkyl-substituted cyclopentenyl carbocations, which are formed as free ions in the zeolite at room temperature. At 503 K the oligomers crack to form branched butanes, pentanes and other alkanes. The final product was highly aromatic coke. The structure, dynamics and reactivity of an alkoxy intermediate formed from acetylene on zeolite catalysts have been investigated by Lazo et. al. [32]. 

The choice of potential alkylating agents is quite wide. Complexes of alkyl halides with Lewis acids, especially AICI3, protonated alcohols, and protonated alkenes can all provide carbonium ions or other reactive alkylating agents ... 

Reaction of cyclo-octa-1,3-diene with ethylene in the presence of a catalyst prepared from n-allylnickel chloride, ethylaluminium sesquichloride, and a chiral phosphane can give 3-vinylcyclo-octane of up to 70 % optical purity. Alkylsulphonium salts are obtained from the reaction of thioethers with protonated alkenes thus cyclo-octa-1,3-diene gives (315) and cyclododeca-... 

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