Acid strength alkenes

The catalyst acidity is determined by the number of acid sites and their acid strength. The total concentration of acid sites, C<, can be obtained from independent TPD measurements. The average acid strength of the sites is characterized by the alkene standard protonation enthalpy,, and is typically determined by regression using reference... 

The esters differ from each other in stability. To decompose the isopropyl ester, higher temperatures and higher acid strengths are needed than for decomposition of the s-butyl ester. It is claimed that the resulting carbenium ions are stabilized by solvation through the acid (25-27). Branched alkenes do not form esters. It is believed that they are easily protonated and polymerized (28). 

The spent acid strength is maintained at about 90 wt% H2SO4. The molar isobutane/alkene feed ratio ranges from 7 1 to 10 1. Typical operating alkene space velocities (LHSV) range from 0.2 to 0.6 h-1 (corresponding to WHSVs from 0.06 to 0.19 h 1). The optimum reaction temperatures range from 279 to 283 K, but some units are operated at temperatures up to 291 K. 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

The pioneer work in this field was carried out on polystyrene-supported acid catalysts [161]. Thereafter, several works on the use of sulfonic, strong acidic cation exchangers as acid catalysts were reported for alkylation, hydration, etherification, esterification, cleavage of ether bonds, dehydration, and aldol condensation [162,168-171], Besides, industrial applications of these materials were evaluated with reactions related to the chemistry of alkenes, that is, alkylation, isomerization, oligomerization, and acylation. [163,169], Also, Nation, an acid resin which has an acid strength equivalent to concentrated sulfuric acid, can be applied as an acid catalyst. It is used for the alkylation of aromatics with olefins in the liquid or gas phases and other reactions however, due to its low surface area, the Nation resin has relatively low catalytic activity in gas-phase reactions or liquid-phase processes where a nonpolar reactant or solvent is employed [166],... 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

There are a number of factors that determine whether a protonic acid can initiate polymerization of alkenes. Their acidity (pKa), and therefore the basicity of the resulting counteranion, determines the efficiency of initiation. Although reliable pKa values of acids stronger than sulfuric or hy-droiodic (pKa < -9) are difficult to obtain in aqueous solutions due to their nearly complete dissociation, the pKa values of acetic acid (4.75) and trichloroacetic acid (0.7) in water provide useful references. Conductometric and potentiometric estimates of the pK values of selected protonic acids in various organic solvents are summarized in Table 11 in descending acid strength. These values are not very precise, however, because the amount of moisture in each system was not monitored precisely. 

The requirement for zeolites of high acid strength for the alkylation of arrunonia by alcohols contrasts with the need for weakly acidic zeolites for the addition reactions between alkenes and ammonia [79]. In these reactions the alkene has to be activated by the Bronsted acid site of the zeolite and that is only possible when the acid sites are not fully blocked by... 

Strong Lewis acids are often chosen for use with epoxides when the goal is intramolecular addition to an alkene or aromatic system. In an informative study, Sutherland et al. compared the effect of different catalysts on the ratio of cyclization (325) to ring contraction (326) for the system shown in equation (136). A positive correlation between Lewis acid strength and cyclization was noted. Thus AlCb was reasonably effective in this regard (ratio (325) (326) = 3 1) but it was inferior to TiCU which gave only cyclized product (55%). Other catalysts (ratio) are FeCb (1.1) SnCU (0.6) BF3-Et20 (0.8) and ZnCb (0.3). 

Oxymercuration reactions are usually carried out in a three-component system involving an alkene, alkyne, or cyclopropane, a mercury compound HgX2, and a nucleophile HY, where HY may be HOH, ROH, HOOH, HOOR, or HOCOR and is frequently the solvent . Reactivity follows the sequence of Lewis acid strength in HgX2, with Hg(OCOMe)2 < Hg(OCOCF3)2, Hg(C104)2, and a steric sequence in alkene, with CH2=CH2 > RCH=CH2 > cis-RCH=CHR > trans-RCH=CHR > R2C=CHR > R2C=CR2. 

On solid acids, such as zeolites in their H-form or sulfated zirconia, Brensted sites of high acid strength exist. These are 0-H" groups, adjacent to an 0 ion. Although the polarity of the O-H dipole is very small, they can react with weak bases such as alkenes. The adsorption complex is best described by an alkoxy group, i.e. a new C-H bond and a C-0 bond is formed with the Bronsted acid/Lewis base pair at the surface 3, 4]. This is illustrated in scheme 6 ... 

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