Hard acid or base

See also Retro-Hantzsch Hard acid or base, see HSAB theory Heating curves, in solid-state reactions,... 

This approach describes a hard acid or base as a species that has a large difference between its ionization energy and its electron affinity. Ionization energy is assumed to measure the energy of the HOMO and electron affinity is assumed to measure the... 

Tables 1.4 and 1.5 contain acids or bases with a wide range of acceptor or donor atoms. In such cases, use of different references will not give identical orderings. Usually the variations are not very great, being shifts of two soft, or two hard, acids or bases with respect to each other.

Hard acids or bases have large relative values of or Ji/jv res sshile soft acids or bases have large relative values of Hard... 

One of the most important factors influencing the corrosion rate is the formation and protective ability of the corrosion products formed. The specific corrosion products formed are dependent on the peuticipating dissolved metal ions and the access to anions solved in the aqueous layer. Formation of the film of corrosion products take place in a sequence of consecutive steps— dissolution, coordination, reprecipitation. When the dissolution step is acid-dependent, coordination is based on the hard and soft acid base principle (i.e., hard acids are preferably coordinated with hard bases and soft acids are preferably coordinated with soft bases). Acids or bases with tightly held valence electrons that are not easily distorted are hard acids or bases. Acids or bases having valence electrons that are easily polarized or removed are considered to be soft acids or bases. 

Acylglycerols can be hydrolyzed by heating with acid or base or by treatment with lipases. Hydrolysis with alkali is called saponification and yields salts of free fatty acids and glycerol. This is how soap (a metal salt of an acid derived from fat) was made by our ancestors. One method used potassium hydroxide potash) leached from wood ashes to hydrolyze animal fat (mostly triacylglycerols). (The tendency of such soaps to be precipitated by Mg and Ca ions in hard water makes them less useful than modern detergents.) When the fatty acids esterified at the first and third carbons of glycerol are different, the sec-... 

The facility with which an acid-base reaction takes place depends of course on the strengths of the acid and the base. But it also depends on quite another quality, called the hardness or softness of the acid or base. Hard and soft acids and bases have these characteristics ... 

Once acids and bases have been classified as hard or soft, a simple rule can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases (the HSAB principle)P The rule has nothing to do with acid or base strength but merely says that the product A—B will have extra stability if both A and B are hard or if both are soft. Another rule is that a soft Lewis acid and a soft Lewis base... 

The first task is to decide whether the members of a given group are Lewis acids or bases. Then evaluate the relative softness and hardness based on polarizability, taking into account correlations with electronegativity, size, and charge. Refer to the periodic table in assessing the trends. 

The EA/CA ratio was proposed as a measure of hardness of the Lewis acid, and EB/CB as hardness of the Lewis base in aqueous solution (17). It now seems that the E/C ratio is not a measure of hardness in the sense in which Pearson (5,5a) defined hardness. Rather, the E/C ratio for a Lewis acid or base is a measure of the tendency to ionicity in the M-L bonds formed. The EAICA ratio should rather be called IA, and the EbICb ratio IB, the tendency to ionic bonding in forming the M-L bond. Acids and bases in Tables I and II are placed in order of increasing tendency towards ionicity in the M-L bond, according to the E/C ratios IA and 7b. A justification for this interpretation is that the order of IA values for metal ions in aqueous solution strongly resembles the order of hardness derived by Pearson (19) from enthalpies of complex forma-... 

Incorporation of lsO into the ketone occurs hardly at all under these conditions, i.e. at pH 7, but in the presence of a trace of acid or base it occurs [via the hydrate (13)] very rapidly indeed. The fact that a carbonyl compound is hydrated will not influence nucleophilic additions that are irreversible it may, however, influence the position of equilibrium in reversible addition reactions, and also the reaction rate, as... 

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