Superacids and superbases

Proc. 13th Intern. Conf. Catal. ( New Trends in Solid Superacids and Superbases ,... 

The acidic and basic properties of aqueous solutions are changed within the pH range from 0 to 14, while for solid acids and bases, they can be changed within the range -30 < Ho, H. < +40. A boundary point between Ho and H. is Ho = 7. One can see that these limits are much broader than in the case of aqueous solutions. Acids with Hq value within 7 + -12 correspond to usual acids, while above -12, they are superacids. Similarly, the bases with H. from 7 to 26 correspond to usual bases while above 26 they are superbases. The concept of superacids and superbases is widely used to explain the processes that take place in acid-base catalysis [21]. 

T4.5 Start from Section 4.14 Superacids and superbases which covers classical examples of superbases, nitrides, and hydrides of s-block elements. However, there are several groups of superbases that you can look at. For example, amides (diisopropylamide related) are commonly used superbases. More modem are phosphazene bases which are neutral rather bulky compounds containing phosphorus and nitrogen. An interesting topic is a proton sponge (or Alder s base) and its derivatives as well as several theoretical and practical approaches used in rational design of superbases. 

F rca iu, D. Presented at the EUCHEM Conference on Superacidic and Superbasic Media (Liquid and Solid), Cirencester, England, Sept 13, 1984. 

Oxidation of unfunctionalized alkanes is notoriously difficult to perform selectively, because breaking of a C-H bond is required. Although oxidation is thermodynamically favourable, there are limited kinetic pathways for reaction to occur. For most alkanes, the hydrogens are not labile, and, as the carbon atom cannot expand its valence electron shell beyond eight electrons, there is no mechanism for electrophilic or nucleophilic substitution short of using extreme (superacid or superbase) conditions. Alkane oxidations are therefore normally radical processes, and thus difficult to control in terms of selectivity. Nonetheless, some oxidations of alkanes have been performed under supercritical conditions, although it is probable that these actually proceed via radical mechanisms. 

Many studies have been made in the last few years in superacid or superbasic solution, because one has available non-aqueous media having acidities (expressed as acidity function) varying between about —13 and +30. 

Superbase — This term is used for reagents which possess an extremely strong - proton affinity. Alkali-organyls, e.g., butyl-potassium, and methyl-lithium are examples for that group. Superbases have been known since the mid-19th century however, the name has been only created following the discovery of - superacids. See also - acid-base theories. 

In comparison to intense activities in the field of the solid superacids, not much work has been done on solid superbase catalysis[l]. Here we report first the preparation of a solid superbase which exhibits remarkably strong basicity, and then its synthetic application to the olefin isomerizations[2] and side-chain alkylations of alkylbenzenes with olefins[3]. The reactions proceed smoothly under mild reaction conditions to give the products quantitatively. 

In contrast to solid acid catalysts (including solid superacids) described earlier, solid base catalysts (including superbases) have received much less attention. A common basic catalyst used in organic synthesis is Ba(OH)2. The use of alumina-supported sodium in the double bond isomerization of olefins, first studied by Pines et al. (1955), is also widely known. More recent developments include the use of K, Na/K, MgO, Zr02, Th02, and K/KOH/AI2O3 zeolites containing different metal ions to control their acid-base properties, solid superbases, and... 

Addition of alkali metals to certain types of oxides resulted in the formation of very strong base sites. Materials which possess base sites stronger than H- = —26 are caUed superbases. The H- value 26 proposed to be set in conformity vrith the definition of superacid. The critical Ho value for superacid is ca. —12, whidi differs by 19 Ho units from Ho=7, the neutral acid-base strength. The H- value 26 differs from H- = 7 for neutral acid-base strength by 19 H- units. Althou alkali and alkaline earth oxides show superbasicity without the addition of alkali metals, as described in Sections 3.1.1 and 3.1.2, this section deals only with alkali metal-added materials showing strong basicity. The materials which show superbasicity by the addition of alkali metals are limited to alkaline earth oxides and alumina. 

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