Hydroxy Compounds - Acidity of Hydroxy compounds

Phenol are generally insoluble in water. The large phenyl substitutent is hydrophobic, hence despite the -OH functional group is able to form hydrogen bonding with water, the larger substituent restricts the solubility of phenol in water.

In order to dissolve phenol, NaOH is added. This is because there is the formation of the sodium phenoxide, a soluble ionic salt. However, when NaOH is added to an alcohol, a sodium salt is not formed. Hence, the difference in the reaction with NaOH, suggests that the acidity of the two hydroxy compound differs.

When we obtain separate aqueous solutons of phenol and an alcohol, the pH of the former is less than 7 while the pH of the latter is 7. Hence, this implies that alcohol does not like to dissociate in water, despite the polar OH present. Whereas, phenol likes to dissociate in water.

Acidity of the hydroxy compound can be explained from two fronts. (1) The polarity of the O-H bond which results in the ease of losing of the proton. (2) The stability of the conjugate base, thus affecting the equilibrium position.

(1) The polarity of the O-H bond.
The phenyl substitutent is electron-withdrawing. This results in the O to be even more electron-withdrawing and this in turn causes the O-H bond of phenol to be even more polarised. The H is now even more electron deficient and becomes more susceptible to be lost as a H₊. While the alkyl groups are electron-donating and the converse effect is observed.

(2) The stability of the conjugate base.
From the earlier illustration, the conjugate base of phenol is the phenoxide ion. This ion is stabilised by resonance (due to alignment of the p-orbtial, similar to aryl halides), hence the negative charge on O can be delocalised throughout the ring and not found on O only. (Note: pi electron pair does not go to O with the negative charge as it will cause the structure to violate octet rule.)

However, the conjugate base of alcohol, a alkoxide ion, is being destabilised. The alkyl group is electron-donating and hence, it intensifies the negative charge on O. This effect is a destabilising effect.

As a result, the stability of the phenoxide ion, results in the equilibrium position to favour the right hand side. Hence, phenol is a weak acid. (Note: Stability in chemistry is due to lower in the energy content of the substance (aka potential energy).)

In conclusion, I prefer to use reason (2) to account for the acidity of phenol. Although, reason (1) is find but I think it is actually more abstract than how I have explained. In addition, despite phenol is a weak acid, it is actually too weak to react with Na₂CO₃.
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Article written by Kwok YL 2009.}

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