Acid strengths of Lewis acids

Lewis acidity is also affected by periodic table considerations. In comparing acid strengths of Lewis acids of the form MX ... 

Kuhl (84) reported a decrease in the number of acid sites in calcined or steamed H-mordenite, due to thermal dealumination of the framework. Barthomeuf et al. (85), using data obtained from ammonia and pyridine sorption, concluded that the total acidity and the number and strength of Lewis sites decreases upon acid-dealumination of H-mordenite to a SiO /Al O ratio of 18, while the Bronsted acidity remained unchanged. 

B.2.2. Dialkyl Ether Cleavage. Varying the mole fraction of AICI3 in an ionic liquid can optimize reactions that are sensitive to the strength of Lewis acidity. When the ionic liquids [EMIM]C1/A1C13 were applied for the acylative cleavage of dialkyl ethers (Scheme 6), the reaction was found to be sensitive to the bulk Lewis acidity of the ionic liquid solvent 138). 

The evaluation and correlation of strengths of Lewis acids and bases have attracted the interest of many inorganic chemists. Recently gas-phase data have become available. but before that many systems were studied in aprotic, nonpolar solvents. Even today, such solvents allow the collection of large amounts of data by various methods. Solvation effects will be small and, it is hoped, approximately equal for reactants and products such that their neglect will not cause serious errors. 

A quantitative determination of the strength of Lewis acids to establish similar scales (Ho) as discussed in the case of protic (Br0nsted-type) superacids would be most useful. However, to establish such a scale is extremely difficult. Whereas the Brpnsted acid-base interaction invariably involves a proton transfer reaction that allows meaningful comparison, in the Lewis acid-base interaction, involving for example Lewis acids with widely different electronic and steric donating substituents, there is no such common denominator.25,26 Hence despite various attempts, the term strength of Lewis acid has no well-defined meaning. 

Recently, highly functionalized allyl boronates such as 150 were synthesized from allylic acetates 148 and pinacolatodiboron 149. These boronates were employed for the allylation of aldehydes, furnishing the borate esters 151 which undergo lactonization to furnish the a-methylene-7-butyrolactones in high yield and selectivity. The strength of Lewis acids as well as the substituents on the aromatic aldehydes have a profound influence on the stereochemical outcome of the reaction (Scheme 25) <20040L481>. 

M. Misono, E. Ochiai, Y. Saito, and Y. Yoneda, A new dual parameter scale for the strength of Lewis acids and bases with evaluation of their softness, J. Inorg. Nucl. Chem. 29 2685 (1967). 

With simple probe molecules, such as H2, information about the number of surface metal atoms is readily obtained by using adsorption measurements. However, even with such simple probe molecules further information about the heterogeneity of a surface may be obtained by performing temperature-programmed desorption measurements. With probe molecules which are chemically more specific (e.g., NH3 and organic amines, H2S and organic sulfides) it may be possible to obtain information about the number and nature of specific types of surface sites, for example, the number and strength of Lewis or Bronsted acid sites on oxides, zeolites or sulfides. 

Laurence, C., Gal, J. (2010). Lewis Basicity and Affinity Scales Data and Measurement. New York John Wiley Sons. Includes a discussion of the various acid-base theories and quantitative scales for judging the strengths of Lewis acids and bases. 

Recently, the C-NMR method was also applied to these studies and the following order of the relative strengths of Lewis acids and bases, rdated to equilibrium (47) was given ... 

This review has detailed the utility of copper salts as Lewis acids. Their ready availability, or ease of preparation, low toxicity, and counter ion-dependent tuning of the strength of Lewis acidity makes them ideal for use in a variety of organic transformations. The last decade has witnessed dramatic progress in catalytic methods for the preparation of enantiomerically pure compounds. Copper Lewis acids have played a key role in their development. The future holds a lot of promise for the identification of other processes for which copper will be the Lewis acid of choice. 

Though the means of comparing the strengths of Lewis acids is not entirely satisfactory, SO3 is clearly one of the strongest. 

Zhang, Y. (1982b). Electronegativities of Elements in Valence States and their Applications. 2. A Scale of Strengths of Lewis Acids. Inorg.Chem., 21,3889-3893. 

Friedel and Crafts themselves observed that aluminum chloride is by no means the only specific catalyst in the Friedel-Crafts reaction. A number of other acidic metal halides could also be employed however, these were less reactive. The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Hence it is extremely difficult to establish a scale of strength of Lewis acids in a manner analogous to that used for Brpnsted acids. Despite the difficulties, a number of qualitative orders of reactivity have been proposed. A comparative study of the activity of various Friedel-Crafts catalysts was performed by Olah and coworkers. Thus the activity index (the lowest temperature at which reaction occurs) of a large number of Lewis acid halides was measured using the benzylation reaction as the probe. 

Deka, R.Ch., Roy, R.K. and Hirao, K. (2004) Local reactivity descriptors to predict the strength of Lewis acid sites in alkali cation-exchanged zeolites. Chem. Phys. Lett., 389, 186-190. 

Zhang, Y. (1982b) Electronegativities of elements in valence states and their applications. 2. A scale of strengths of Lewis acids. Inorg. Chem., 21, 3889-3893. 

The acid-base concepts described in this section focus on bases that react with protons, the classical Br0nsted-Lowry definition of a base. There are other types of electron deficient molecules that function as acids, called Lewis acids. The following section will discuss the relative strength of Lewis acids. 

These three statements can be used to estimate tbe strength of Lewis acids according their Group in the periodic table. ... 

The Edward s equation - (Eqn. 2.2) can he of value for determining the relative strength of Lewis acids and is written as... 

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