Basic catalysts, strength

Adolph Baeyer is credited with the first recognition of the general nature of the reaction between phenols and aldehydes in 1872 ([2,5-7] [18], Table 5.1). He reported formation of colorless resins when acidic solutions of pyrogallic acid or resorcinol were mixed with oil of bitter almonds, which consists primarily benzaldehyde. Baeyer also saw resin formation with acidic and basic solutions of phenol and acetaldehyde or chloral. Michael and Comey furthered Baeyer s work with additional studies on the behavior of benzaldehyde and phenols [2,19]. They studied a variety of acidic and basic catalysts and noted that reaction vigor followed the acid or base strength of the catalyst. Michael et al. also reported rapid oxidation and darkening of phenolic resins when catalyzed by alkaline materials. 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

The most general methodology followed to prepare alkaline earth metal oxides as basic catalysts consists of the thermal decomposition of the corresponding hydroxides or carbonates in air or under vacuum. BaO and SrO are prepared from the corresponding carbonates as precursor salts, whereas decomposition of hydroxides is frequently used to prepare MgO and CaO. Preparation of alkaline earth metal oxides with high surface areas is especially important when the oxide will be used as a basic catalyst, because the catalytic activity will depend on the number and strength of the basic sites accessible to the reactant molecules, which is dependent on the accessible surface area. 

The initial work of Baker and Holdsworth [19], discussed in Section 7.1, outlined the role of a basic catalyst and demonstrated that an increase in base strength was accompanied by increased catalytic strength except when steric hindrance interfered. Evidence for. association of the tertiary airiine base with the isocyanate was given. Later workers have confirmed... 

In alkylation of phenol derivatives, the 0/C ratio usually decreases when the catalyst acid or basic strength increases [11,14-16]. The situation becomes more complex when the strong basic sites deactivate under reaction conditions, as usually happens in liquid-phase reactions. The o/p ratio is the highest (up to values higher than 30) for basic catalysts and the lowest (==2) for acid catalysts, but is also affected by the softness or hardness of the cation bound to the basic oxygen [17]. A soft cation may fevour the interaction with the... 

The product distribution in the t-butylation of phenol with isobutanol may be explained based on the nature of acid-base strengths and the mode of adsorption of phenol. It has been reported in the literature that phenol is adsorbed horizontally on acid-catalysts like AI2O3 and a vertical mode of adsorption is proved on basic catalysts like MgO [16,17], The horizontal adsorption of phenol results in O-alkylation and also C-alkylation at ortho and para positions which are close to the surface of the catalyst whereas in the vertical adosrption mode, only the ortho selectivity is observed. The extent of C-alkylation depends on the strength of the acid site. However, a combined participation of acid-base properties is also reported in the methylation of phenol over hydrotalcites [18]. Hence, a scheme depicting the correlation between the acid-base properties of the catalysts with the product distribution in the t-butylation of phenol is shown below as. 

Quantitative study of nucleophilicity can be said to have begun with the work in 1924 of Brpnsted and Pederson (16), who noted a correlation of rates of nitramide decomposition by bases (B ) and strength (pKHB) of the basic catalysts ... 

A patent (19) assigned to Phillips Petroleum Co. covers the use of di-, tri-, or tetra-thiols which react with sulfur in the presence of basic catalysts such as amines. Typical materials are prepared by heating, at 135 °C, a mixture of up to 20 parts by weight of a poly thiol containing 0.1 parts tributylamine with 100 parts of sulfur. The resultant mixture was compression-moulded at 24,000 psi and 100 °C. Typical results for tensile strength and Shore D hardness are given in Table I. No mention is made of the possible variation in these properties with storage time. 

Since acid-base catalysis always involves the transfer of a proton from the acid catalyst or to the basic catalyst, it is natural to seek a correlation between the effectiveness of a catalyst and its strength as an acid or base, since this strength is a measure of the ease with which the catalyst transfers a proton to or from a water molecule. The most satisfactory.relationship between the rate constant fcj,..and the add di oelation, constant of a inonoWic acid is the equation. 

On the other hand, when fluoride ion acts as a basic catalyst for the removal of a proton from an uncharged substrate, its catalytic activity is found to be very similar to that of a carboxylate anion of the same basic strength.In this situation the effect of charge delocalization is much less marked, since the effect of the proton upon the charge distribution in the catalyst anion is largely counterbalanced by the effect of the negative charge borne by the deprotonated substrate. ... 

The large sensitivity of the rate constants to base strength for weakly basic catalysts indicates that the catalyst resembles its conjugate acid in the transition state, i.e. there is a large amount of, or complete, proton transfer to the catalyst in the transition state. For strongly basic catalysts the small sensitivity of the rate constants upon base strength suggests that the catalyst resembles its free unprotonated basic form in the transition state. 

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