Acid-base strength inherent

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... 

Drago s system emphasized the two factors involved in acid-base strength (electrostatic and covalent) in the two terms of his equation for enthalpy of reaction. Pearson s system put more obvious emphasis on the covalent factor. Pearson proposed the equation log K = + o- ag, with the inherent strength S modified by a soft-... 

You might have noticed the numbers for ammonia and acetic acid are the same, but don t be misled to believe these numbers are universal for all weak acids and bases. It is entirely coincidental that the numbers for acetic acid and ammonia are the same. The degree of ionization is dependent on the inherent acid or base strength of the species. This is a topic we will consider shortly. 

Hardness and softness refer to special stability of hard-hard and soft-soft interactions and should be carefully distinguished from inherent acid or base strength. For example. both OH and F are hard bases yet the basicity of the hydroxide ion is about lO times that of the fluoride ion. Similariy, both S( and EtjP may be considered soft bases however, the latter is 10 times as strong (toward CHjHg ). It is possible for a strong acid or base to displace a weaker one, even though this appears to violate the principle of hard and soft acids and bases. For example, the stronger, softer base, the sulfite ion, can displace the weak, hard base, fluoride ion, from the hard add. the proton, H -... 

Polyetherimides are suitable for applications that require high temperature stability, high mechanical strength, inherent flame resistance with extremely low smoke evolution, outstanding electrical properties over a wide frequency and temperature range, chemical resistance to aliphatic hydrocarbons, acids and dilute bases, UV stability, and ready processability on conventional equipment. 

The H30 /H20/0H reference for quantifying relative acid and base strengths is only useful when the examined acid is inherently weaker than H30 or the examined base is weaker than OH. Hydronium ion and hydroxide ion are the strongest acid and base. 

Flagrant examples which violate the HSAB principle are known. Most of these can be attributed to the inherent strength of the acid-base complex which overrides the match in softness. That the extremely soft Lewis base combines with the very hard acid H exothermically to give the stable hydrogen molecule is a case in point. 

Based on the flash photolysis results, it appears that, at room temperature, escape of O2 from the distal pocket to the solvent plus entry of water (i.e., C to A in Figure 8) has about the same activation barrier as does geminate recombination of O2 in the distal pocket with the heme (i.e., from C to E in Figure 4). Studies of a large number of engineered distal pocket amino acid variants show that the activation barrier to O2 dissociation involves not only the inherent strength of the Fe—O2 bond, but also the steric and electrostatic interactions of the Fe—O2 unit within the distal pocket shown in Figure 8. These interactions are discussed in more detail in the next subsection. 

---