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Acidity of Phenols, Effect of Substituents on Acidity

Alcohols' pKa values are close to 16 while for phenols' pKa value is 10, which is stronger than alcohols'.

Acidity of Phenols

Alcohols' pKa values are close to 16 while for phenols' pKa value is 10, which is stronger than alcohols'. Phenols, which are acidic in nature, cause blue litmus to turn red and react with alkalis in water to produce phenate. Alcohols do not demonstrate either reaction. Phenol is a weak acid in comparison to carboxylic acids.

Thus, hydroxy groups bonded to benzene rings are much more acidic than hydroxy groups bonded to alcohols. It is due to phenol's resonance. A resonance effect creates a positive charge on the oxygen atom in phenol, weakening the oxygen-hydrogen bond and allowing a proton to be released.

The deprotonation of phenol initiates the formation of phenoxide and phenate ions, which also exist as resonances.

As a result, the resonance stabilizes both phenyl and phenoxy ions. Due to the delocalization of the negative charge on the benzene ring, the phenoxide ion is more stable than phenol. Although phenol's resonating structure entails a separation of negative and positive charges, it is still a simple structure. Due to the proton release nature of phenol, it is more likely to form phenate. Because of the negative inductive effect of the alkyl group, deprotonation of alcohol produces alkoxide ions, which are less acidic than phenol.

Due to the electron density increase between O and H in the hydrocarbon chain of Ethoxide, resonance is prevented, and H + is less likely to leave the alcohol. A chain that is long and branched will have a greater effect, decreasing the acidity of the alcohol.

Effect of Substituents on Acidity of Phenols

As phenate is more stable than phenol, it has a more acidic nature. A stabilized phenoxide ion will cause the acidity of phenol to increase therefore any substituent attached to the aromatic ring. The acidic nature of phenol will be decreased by substituents that increase the negative charge of the phenate ion. The presence of electron withdrawing compounds on benzene rings in phenol will increase the acidity of the molecule, and the presence of electron releasing compounds will decrease it. Nitro groups, for example, will increase the acidity of phenol if they are substituted. The nitro group of nitrophenol imparts mesomere and inductive effects, thereby acting as an electron withdrawing group, causing it to be more acidic than phenol. Phenol's acidity will be affected by the position of its nitro group. In phenol, nitro groups at ortho- and para-positions withdraw electrons from hydroxy groups, respectively, by stronger –M effects, whereas nitro groups at -meta positions withdraw electrons by weaker –I effects, as meta position cannot involve in resonance with hydroxy groups. Thus, m-nitrophenol and o-nitrophenol are more acidic than those. When the number of nitro groups on phenol increases, its acidity also increases. According to the following table, nitrophenols are decreasingly acidic.

Amino (NH2) and alkyl (R) groups donate electrons to phenol, decreasing its acidity. Because of the positive inductive effect and hyperconjugation of the methyl group, cresol and methylphenols are less acidic in comparison to phenol. The electron density of the hydroxyl group is increased due to both effects. This reduces the acidity of the phenol. Inductive effect can only operate at orthogonal and para positions, whereas hyperconjugation can only operate at orthogonal and para positions. This means that o-cresol is the least acidic of the three. Below is a list of acidity in decreasing order.

In terms of their pKa values, the following substituted phenols are shown.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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