Basicity

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Basicity is a term from chemistry and metallurgy .

Basicity in chemistry

In chemistry , the term basicity , also alkalinity,

  1. the measure of the ability of a chemical compound to take up protons , i.e. its base behavior, expressed by the base constant or the pK B value.
  2. the base strength ( hydroxide ion concentration) of a solution, see pOH value .

Basicity plays a major role in connection with nucleophilicity in organic chemistry, for example in nucleophilic substitution . The more basic a compound is, the more likely it is to share its free electron pair with other compounds, e.g. B. with a proton. A stronger base easily shares its electron pair with a proton and therefore easily bonds with and absorbs it. A weaker base dissolves the bond to a proton more easily (the bond is weaker) and is therefore a stronger acid at the same time. The basicity can be read off from the base constant K b or acid constant K s . The base constant is an equilibrium constant that indicates the ratio between protonated and deprotonated particles. The nucleophilicity indicates the tendency of a particle to attack an electrophile (an electron-poor particle). Basicity and nucleophilicity are directly related to each other: strong bases are good nucleophiles and weak bases are bad nucleophiles. Exceptions to this rule exist if the basic center is sterically hindered by sterically demanding groups, for example in the case of lithium diisopropylamide . For example, hydroxide ions (OH - ) are a better base and a better nucleophile than water (H 2 O), while water is the better leaving group. A crucial difference between basicity and nucleophilicity, however, is that basicity is a thermodynamic quantity, whereas nucleophilicity is a kinetic one . The basicity describes an equilibrium between a base and its conjugate acid, for example in water:

with the equilibrium constant K.

Nucleophilicity, on the other hand, describes the speed of a process, for example the attack of a nucleophile on an electrophile:

with the rate constant k.

Polarizability, solvents and steric effects can also have an influence on basicity and nucleophilicity.

Basicity in metallurgy

This is an empirical quantity which, in its simplest form, indicates the mass ratio of CaO and SiO 2 in metallurgical slags . The concept of slag basicity B has nothing to do with chemical basicity, but is based only on the fact that, unlike the second component of the slag, CaO forms the basic substance calcium hydroxide when it reacts with water . Correspondingly, if the slag basicity is greater than one, one speaks of basic slags and a basicity of less than one of acidic slags.

Since in practice this does not come very close to the real conditions for slag, possible further components of slag (e.g. MgO , Al 2 O 3 ) are assigned to basic or acidic components. However, these must be weighted with correction factors (k), since, for example, MgO in the slag does not have the same basic effect as CaO when it reacts with water.

See also

Web links

Individual evidence

  1. Paula, Yurkanis, Bruice: Organic Chemistry. 4th edition, Prentice-Hall, 2003, ISBN 0-131-41010-5 , pp. 410-419.
  2. ^ KPC Vollhardt, Neil E. Schore: Organische Chemie , Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2005, 4th edition, H. Butenschön, pp. 259-267, ISBN 3-527-31380-X .