The acidic nature of the phenolic hydroxyl group

Since phenol is benzene with a hydroxyl group, the reactivity of phenol and phenolic compounds is in many ways dictated by the chemical properties of the benzene ring. The first property to consider is acidity. A compound is considered an acid when it can release a proton (H+) while in solution. The acid constant Ka of a compound defines to what extent the proton is released. Strong acids will completely dissociate, whereas weak acids (HA) are at equilibrium with their dissociated state:

The Ka is defined as:

The pKa is the negative 10log of Ka, which tends to result in more convenient numbers.

Phenolic compounds are, in general, weak acids. Compared to the hydroxyl group of unsubstituted aliphatic alcohols, however, the phenolic OH-group is more acidic. The reason for this is that the anion formed after abstracting the proton from the hydroxyl group is relatively stable because of the existence of several mesomeric structures. The anion is referred to as the phenolate anion. Hence, phenol (2.5) is a weak acid, with a pKa value of 10. This places phenol in between carboxylic acids (pKa = 4-5) and aliphatic alcohols (pKa = 16-19).


1.3.1 The effects of substituents on acidity

The actual pKa of phenolic compounds depends upon the overall structure of the molecule and the nature of the substituents on the aromatic ring(s). The compound 2,4,6-trinitrophenol (2.6) has a pKa value 0.71, equivalent to that of a strong acid. This low pKa-value is the result of the electron-pulling NO2 substituents. The presence of these groups stabilizes the anion that results after the proton is removed.


1.3.2 Use of acidity in separations

The pKa is important since it affects the way certain phenolic compounds are extracted. If we consider having a mixture of phenols that range from weak acids to strong acids, the addition of sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3) to the mixture will allow separation of these phenols. The weak base picks up the H+ from the strong acids or the more acidic phenols. This results in formation of phenolate salts of the phenols that are soluble in water.

The less acidic phenols are not neutralized or do not lose a H+ and tend to remain as "free phenols" and will not be as soluble in water. These compounds can then be extracted with organic solvents.

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