Alkane-Alkene Alkylation

The acidity dependence of the isobutane-isobutylene alkylation was studied using triflic acid modified with trifluoroacetic acid (TFA) and water in the range of acidity 

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359 

Studies with sulfated zirconia also show similar fast catalyst deactivation in the alkylation of isobutane with butenes. It was found, however, that original activities were easily restored by thermal treatment under air without the loss of selectivity to trimethylpentanes. Promoting metals such as Fe, Mn, and Pt did not have a marked effect on the reaction.362,363 Heteropoly acids supported on various oxides have the same characteristics as sulfated zirconia.364 Wells-Dawson heteropoly acids supported on silica show high selectivity for the formation of trimethylpentanes and can be regenerated with 03 at low temperature (125°C).365 

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Alkylation combines lower-molecular-weight saturated and unsaturated hydrocarbons (alkanes and alkenes) to produce high-octane gasoline and other hydrocarbon products. Conventional paraffin-olefin (alkane-alkene) alkylation is an acid-catalyzed reaction, such as combining isobutylene and isobutane to isooctane. 

In alkane-alkene alkylation systems it is always the Jt-donor alkene that is alkylated by carbocations formed in the system. In the absence of excess alkenes (i.e., under superacidic conditions), however, the cr-donor alkanes themselves are alkylated. Even methane or ethane, when used in excess, are alkylated by ethylene to give propane and n-butane, respectively ... 

Friedel-Crafts catalysts are more easy handling, fewer side reactions, and longer catalyst lifetime. Over the years numerous technologies applying different reactors have been developed.7 277 284-289 Because of their rapidly declining activity, zeolites have not reached commercial application in alkane-alkene alkylation.7... 

Attempts have recently been made to prepare solid acids by loading triflic acid into various inert oxides including silica,184 titania,185,186 and zirconia.187,188 Silica functionalized with anchored aminopropyl groups was also used to immobilize triflic acid.189 These new catalysts have been tested in a variety of organic transformations, such as alkane-alkene alkylation, Friedel-Crafts acylation, alkene dimerization, and acetalization. Silica nanoboxes prepared by dealumination of Na-X- and Ca-A-type zeolites were also loaded with triflic acid up to 32 wt%.190 The materials were thoroughly characterized but have not been tested as catalysts. 

The yield of the alkane-alkene alkylation in homogeneous HF-TaF5 depending on the alkene-alkane ratio has been investigated by Sommer et al.149 in a batch system with short reaction times. The results support direct alkylation of methane, ethane, and propane by the ethyl cation and the product distribution depends on the alkene-alkane ratio (Figure 5.14). 

Conventional alkane-alkene alkylation is an acid catalyzed reaction which involves the addition of a tertiary carbenium ion generated from an alkane to an alkene to yield (after hydride addition) a saturated hydrocarbon of higher molecular weight. 

Mechanistically, as elucidated by Schmerling (19) and as illustrated for isobutane-ethylene (alkane-alkene) alkylation (Scheme 1), the reaction is initiated by protonation of the alkene (ethylene) to form a very acidic primary ethyl cation (step 1) which rapidly abstracts a hydride ion from an isobutane molecule to generate the chain carrying t-butyl cation (step 2). This can then alkylate another molecule of ethylene to form the secondary-2-methyl-t-butyl carbenium ion (step 3). This cation rapidly undergoes a... 

The conditions required to carry out conventional alkane-alkene alkylation reactions selectively depends upon (a) the strength of the acid catalyst, (b) the acidity (reactivity) of the carbenium ion generated upon protonation of the alkene, e.g., (the ethyl cation is so much more reactive than the -butyl cation that it can be consumed in ways other than tertiary -hydride abstraction), and (c) the alkane reactant should have a tertiary-hydrogen because the hydride abstraction steps are easier. 

Problem 6.20 Assuming the choice to be limited to alkane, alkene, alkyl halide, secondary alcohol, and tertiary alcohol, characterize compounds A, B, C, D, and E on the basis of the following information ... 

In principle, any functionality capable of producing a carbenium ion under strongly acidic conditions will be able to participate in a Ritter-type reaction. Such classes of compounds include alcohols, aldehydes, alkanes, alkenes, alkyl halides, carboxylic acids, dienes, epoxides, esters, ethers, glycols, ketones, IV-methylolamides and oximes. Consequently, an enormous number of examples is reported and only a representative selection can be presented here. A comprehensive listing of examples reported up to 1966 is provided in the review by Krimen and Cota. ... 

Alkane cracking, alkane-alkene alkylation, alkane disproportionation Alkane isomerization Alkene partial oxidation Oxidation of CO, SOj, and hydrocarbons... 

The main observations can be accounted for by a carbocationic mechanism incorporating intermolecular hydride transfer (77). The multistep transformation illustrated by the reaction between isobutane and 1-alkenes is initiated by the carbocation formed by protonation of the alkene (by the protic acid or the promoted metal halide catalysts) (eq. 48). Intermolecular hydride transfer between isobutane and the cation then generates a new carbocation (tert-butyl cation 13 eq. 49). The cation 13 adds to the alkene to form cation 14 (eq. 50), which then, through intermolecular hydride transfer, forms the product (eq. 51). In this final product-forming step, 13 is regenerated from the isoalkane and then starts a new cycle. Alkane-alkene alkylation, therefore, can be considered a chain reaction with 13 as the chain carrier. 

Alkane-alkene alkylations are frequently referred to as alkylation of alkanes by alkenes. From a mechanistic point of view, it is obvious, however, that during the reaction the alkene is alkylated by a carbocation, and the isoalkane serves as the reservoir for the alkylating carbocation. 

In addition to the primary alkylated product formed according to the above mechanism (eqs. 48-51), various other products may also be formed during alkane-alkene alkylation. These may be isomeric alkylated products resulting from the rearrangements of carbocations. Carbocation 13 may participate in a series of hydride and alkyde shifts, and each carbocation thus formed may react via hydride ion transfer to form isomeric alkanes. In addition, alkenes often undergo isomerization prior to participating in alkylation. It was observed, for example, that in the alkylation of isobutane with n-butenes in the presence of protic acids product distributions are very similar. This may be explained by a fast equilibration of n-butenes prior to participating in the alkylation step. 

Amine, Alcohol, Aldehyde, Alkane, Alkene, Alkyl Halide, Alkyne, Carboxylic Acid, Epoxide, Ether, Ketone, Nitrile (cyano), Nitro, Phenyl Group (benzene ring) and Thiol. 

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