Acids, acid strength hardness

Acid—Base Strength Hardness and softness refer to special stability of hard-hard and soft-soft interactions... 

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 ... 

Lewis acid strength and hardness of the lithium cation. Both LiBH4 and Ca(BH4)2 are more reactive than sodium borohydride. This enhanced reactivity is due to the greater Lewis acid strength of Li+ and Ca2+, compared with Na+. Both of these reagents can reduce esters and lactones efficiently. 

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

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. 

Using these properties, a number of species have been placed in the hard, soft, or borderhne categories in Table 3.2. This table can be used to predict, at least qualitatively, the strength of complexation as measured by the stability constants. For example, Pu is a hard acid, F, a hard base, and T, a soft base. This leads to the prediction that log PdPuF O would be larger than log PdPuFOi the experimental log (3i values are 6.8 and <1.0, respectively. By contrast, since Cd is a soft acid, log Pi(CdF ) could be expected to be less than log (3i(CdT) the respective values are 0.46 and 1.89. However, many metals of interest such as... 

An inner-sphere complex is formed between Lewis acids and bases, while an outer-sphere complex involves a water molecule interposed between the acid and the base. A hard Lewis acid is a molecular unit of small size, high oxidation state, high electronegativity, and low polarizability whereas a soft Lewis acid is a molecular unit of relatively large size, characterized by low oxidation state, low electronegativity, and high polarizability. Based on this characterization, hard bases prefer to complex hard acids, and soft bases prefer to complex soft acids, under similar conditions of acid-base strength. 

The thermodynamic tendency of a substance to act as a Lewis acid. The strength of a Lewis acid depends on the nature of the base with which the Lewis acid forms a Lewis adduct. Hence, comparative measures of Lewis acidities are given by equilibrium constants for the formation of the adducts by a common reference base. See Lewis Acid Electrophilicity Hard Acids Soft Acids Acceptor Number... 

In conclusion, the C/E ratios for donors (acids) indicate whether hardness or softness is most important in interactions of a particular donor (acid), but softness or hardness so defined does not enable one to predict even the relative strength of interaction towards a soft or hard acid (base) because the magnitude of the C and E numbers are lost when the ratio is taken. 

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).112a 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 tend to form a covalent bond, while a hard acid and a hard base tend to bond ionically. 

Chapter 9 Acid-Bose Chemistry 318 Acid-Base Concepts 318 Measures of Acid-Base Strength 330 Hard and Soft Acids and Bases 344... 

We have discussed the influence of substituents on acid strengths of simple carboxylic acids as though the full electrostatic effect of the substituent were exerted solely on the A// of ionization. However, careful thermodynamic analysis of acidities in aqueous solution show that entropy effects (Section 4-4B) are very important. This may seem surprising because entropy effects ought to be small for relative acid strengths, which can be assessed by the constants for simple equilibria such as Equation 18-3, in which (1) there are the same number of molecules on each side of the equation, and (2) the constraints on the species involved hardly seem different from one side of the equation to the other ... 

However it is now clear that acid strength is not a singlevalued function, that different kinds of bases will tend to rank acid strengths differently, depending on their "hardness" or "softness". This is illustrated by the work of Kobayashi (13), who used a spectroscopic method of determining [B]/[BH+] on a sample of silica-alumina titrated with n-butylamine and one titrated with pyridine (Figure 2). Evidently the acidity of indicators and of individual acid sites on solids needs at least two constants for complete characterization of acid strength. 

Corrosivity of used oils. The classical determination of TBN and TAN involves a titrimetric procedure, whereby the oil sample is dissolved in a particular solvent system and neutralized by strong acid or strong base (ASTM D664 or 2896), equivalent to (IP 171 or 276). TBN and TAN values do not correlate with corrosivity and the titrimetric analysis has a very limited ability to differentiate between acids of varying strengths. A quantitative differential infrared spectroscopy technique used to monitor the neutralization reaction is more meaningful, since the technique applies to reactions in hydrocarbon solvents. The classical reaction between corrosive acids and hard-core RMs results in formation of the metal salt of the acid and carbonic acid ... 

Hard bases prefer to complex hard acids and soft bases prefer to complex soft acids, under comparable conditions of acid-base strength. 

The solubilities of salts in water (clearly of importance in aquatic, analytical, and geochemistry) can be fairly well predicted and explained using two principles later we shall return to the more familiar of these, the hard soft acid base (HSAB) principle (see Hard Soft Acids and Bases). To control this principle, we must first consider oifly salts of anions that are hard bases, that is, salts of 0x0 and fluoro anions, and oxides, hydroxides, and fluorides. The solubilities of salts of these anions can be fairly well predicted and explained on the basis of the acidity classification of the cation and the basicity classification of the anion, that is, on a principle of acid base strength. The numerous solubility rules taught in General Chemistry could be replaced with four solubility principles (Table 4), two of which are quite reliable and two of which are less reliable, for known reasons. 

More detailed comparisons are possible, but another factor, called the inherent acid-base strength, must also be kept in mind in these comparisons. An acid or a base may be either hard or soft and at the same time be either strong or weak. The strength of the acid or base may be more important than the hard-soft characteristics both must be considered at the same time. If two soft bases are in competition for the same acid, the one with more inherent base strength may be favored unless there is considerable difference in softness. As an example, consider the following reaction. Two hard-soft... 

Si and Ge yield enamine complexes with special salen type ligands, but Sn does not Salen ligands are able to adapt to the sterical requirements of the other substituents in their environment, but the coordinated acid anions hardly disturb the chelating effect of the salen ligand. Regarding the variety of anions, their coordination strength toward the (salen )PhSi-unit differs in each case... 

It is demonstrated that besides ionic interactions, covalent interactions can be important in zeolitic systems. Then, besides acid strength, the concept of acid softness-hardness in zeolites needs to be considered and its importance to explain selectivity effects in orbital controlled reactions is discussed. The influence of zeolite composition and pore structure (electronic confinement) on acid softness-hardness, and orbital control, and their effects on para/ortho selectivity during alkylation of toluene by methanol in large pore zeolites is presented. 

---