what is the role of Pka value& how it is useful in deciding the acidic nature of a molecule explain with proper example ?
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Acids include strong acids, which completely dissociate in water, and weak acids, which only partially dissociate. When an acid dissociates, it releases a proton to make the solution acidic, but weak acids have both a dissociated state (A-) and undissociated state (AH) that coexist according to the following dissociation equilibrium equation.
AH = H+ + A-
The concentration ratio of both sides is constant given fixed analytical conditions and is referred to as the acid dissociation constant (Ka). Ka is defined by the following equation.
Ka = [H+][A-] / [HA]
The square brackets indicate the concentration of respective components. Based on this equation, Ka expresses how easily the acid releases a proton (in other words, its strength as an acid). In addition, the equation shows how the dissociation state of weak acids vary according to the [H+] level in the solution.
Carboxylic acids (containing -COOH), such as acetic and lactic acids, normally have a Ka constant of about 10-3 to 10-6. Consequently, expressing acidity in terms of the Ka constant alone can be inconvenient and not very intuitive.
Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows.
pKa = -log10 Ka
For example, the Ka constant for acetic acid (CH3C00H) is 0.0000158 (= 10-4.8), but the pKa constant is 4.8, which is a simpler expression. In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3.8, so that means lactic acid is a stronger acid than acetic acid.
The effect of pKa and pH on a reaction mechanism:
Some reactions are catalyzed by acid and others by base, therefore the effect of the pH is a significant in determining the reaction rate
For example, hydrolysis of esters can be catalyzed by an acid or a base. This means that the rate of the ester hydrlysis in presence of an acid or base is higher than that without their presence.
Another example is the intramolecular acid-catalyzed hydrolysis on N-alkyl maleamic acids. These amides undergo fast hydrolysis since they are catalyzed by intarmolecular proton transfer and their rate is more than million time than their intermolecular counter parts.
Regarding the pKa: if the pH of a solution is above the pKa the compound will predominantly exist as an anion and if the pH is below the compound's pKa the predominant form is the free acid. This has a vast effect on the separation of compounds via HPLC.
For example, if the pH of the mobie phase is equal to the pKa of the organic acid to be analyzed by HPLC, you will see two peaks: one for the anion and the second for the free acid form. Therefore, the pH of the mobile phase should be at least 3 log units above the pKa or three log units below the pKa in order to obtain only one peak (the anion or the free acid form).
In addition, pH and pKa has great effect on the binding of compounds to receptor or enzyme's active sites. The strength of the binding of the anion or free acid form will be dependent on the chemical nature of the active site.
Finally, the optimum activity of all enzymes is strongly dependent on the pH of the medium in which the enzymes reside.
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AH = H+ + A-
The concentration ratio of both sides is constant given fixed analytical conditions and is referred to as the acid dissociation constant (Ka). Ka is defined by the following equation.
Ka = [H+][A-] / [HA]
The square brackets indicate the concentration of respective components. Based on this equation, Ka expresses how easily the acid releases a proton (in other words, its strength as an acid). In addition, the equation shows how the dissociation state of weak acids vary according to the [H+] level in the solution.
Carboxylic acids (containing -COOH), such as acetic and lactic acids, normally have a Ka constant of about 10-3 to 10-6. Consequently, expressing acidity in terms of the Ka constant alone can be inconvenient and not very intuitive.
Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows.
pKa = -log10 Ka
For example, the Ka constant for acetic acid (CH3C00H) is 0.0000158 (= 10-4.8), but the pKa constant is 4.8, which is a simpler expression. In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3.8, so that means lactic acid is a stronger acid than acetic acid.
The effect of pKa and pH on a reaction mechanism:
Some reactions are catalyzed by acid and others by base, therefore the effect of the pH is a significant in determining the reaction rate
For example, hydrolysis of esters can be catalyzed by an acid or a base. This means that the rate of the ester hydrlysis in presence of an acid or base is higher than that without their presence.
Another example is the intramolecular acid-catalyzed hydrolysis on N-alkyl maleamic acids. These amides undergo fast hydrolysis since they are catalyzed by intarmolecular proton transfer and their rate is more than million time than their intermolecular counter parts.
Regarding the pKa: if the pH of a solution is above the pKa the compound will predominantly exist as an anion and if the pH is below the compound's pKa the predominant form is the free acid. This has a vast effect on the separation of compounds via HPLC.
For example, if the pH of the mobie phase is equal to the pKa of the organic acid to be analyzed by HPLC, you will see two peaks: one for the anion and the second for the free acid form. Therefore, the pH of the mobile phase should be at least 3 log units above the pKa or three log units below the pKa in order to obtain only one peak (the anion or the free acid form).
In addition, pH and pKa has great effect on the binding of compounds to receptor or enzyme's active sites. The strength of the binding of the anion or free acid form will be dependent on the chemical nature of the active site.
Finally, the optimum activity of all enzymes is strongly dependent on the pH of the medium in which the enzymes reside.
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