Carbocations reaction pathways

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Rearrangements when they do occur are taken as evidence for carbocation inter mediates and point to the S l mechanism as the reaction pathway Rearrangements are never observed m 8 2 reactions... 

The tertiary carbocation formed m this step can react according to any of the various reaction pathways available to carbocations One of these is loss of a proton to give a double bond... 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25... 

Normally, only a small stoichiometric excess (2-30 mol%) of silane is necessary to obtain good preparative yields of hydrocarbon products. However, because the capture of carbocation intermediates by silanes is a bimolecular occurrence, in cases where the intermediate may rearrange or undergo other unwanted side reactions such as cationic polymerization, it is sometimes necessary to use a large excess of silane in order to force the reduction to be competitive with alternative reaction pathways. An extreme case that illustrates this is the need for eight equivalents of triethylsilane in the reduction of benzyl alcohol to produce only a 40% yield of toluene the mass of the remainder of the starting alcohol is found to be consumed in the formation of oligomers by bimolecular Friedel-Crafts-type side reactions that compete with the capture of the carbocations by the silane.129... 

As shown in Table 4.38, three major reaction pathways are available to hypova-lent metals in the presence of an alkene (A) and (C) dative and synergistic coordination (B) carbocation formation and (D) and (E) metallacyclic and migratory insertions. The latter types are of particular importance in metal-catalyzed alkene polymerizations and will be given primary attention in the discussion that follows. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Figure 4 shows the reaction pathway for the isobutene oligomerization. After the dimer formation, the addition of another one butene molecule will depend mainly of Brbnsted acidity to stabilize the formation of the carbocation. The oligomerization to heavier olefins will be favored on catalyst showing a low L/B acids sites ratio [12]. 

Figure 2.3 may be used to illustrate the following progression in pathways for nucleophilic substimtion at X-l-Y and X-2-Y with changes in the lifetime of the respective carbocation reaction intermediates. 

An alternative mechanism for the acid-catalyzed aromatization of oxepins involves solvent capture of the intermediate carbocation. This reaction pathway has been proposed for aromatization of oxepin (99) and is shown in Scheme 18 (72JA7876). 

The regioselectivity of this latter reaction pathway may be diminished owing to the tendency of carbocations to rearrange, particularly when branching of the carbon chain occurs in the -position. Hence the method is preparatively useful only with secondary alcohols (e.g. butan-2-ol) where one unique secondary carbocation is involved (see also Section 5.5.2, p. 560). 

RAIN is a computer program that finds the reaction pathways for interconverting EM(B) and EM(E). These pathways may correspond to the mechanistic pathways of chemical reactions, or to multistep sequences of chemical reactions, depending on the nature of the valence schemes that are considered. If the valence schemes are confined to those of stable compounds, a program like RAIN will generate sequences of chemical reactions, such as bilaterally generated synthetic pathways (ref. 24), networks of reaction mechanisms are obtained, when the valence schemes of transient intermediates (e.g. carbenes, radicals, carbocations, carbanions) are also included. 

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