The Acidity of Phenols

Phenols are converted into salts with strong alkalis such as sodium 

Dili The acidity of phenols arises from the greater resonance stabilization 

On the other hand, a methyl group exerts a weak +1 effect and thus the methylphenols are slightly less acidic than phenol (e.g. 4-methylphe-nol, ptf 10.14). 

---

It IS necessary to keep the acidity of phenols in mind when we discuss prepara tion and reactions Reactions that produce phenols when earned out in basic solution require an acidification step to convert the phenoxide ion to the neutral form of the phenol... 

Multiple substitution by strongly electron withdrawing groups greatly increases the acidity of phenols as the values for 2 4 dimtrophenol (4 0) and 2 4 6 trimtrophenol (0 4) m Table 24 2 attest... 

Electron releasing substituents attached to the ring have a negligible effect on the acidity of phenols Strongly electron withdrawing groups increase the acidity The compound 4 nitro 3 (tnfluoromethyl)phenol for example is 10 000 times more acidic than phenol... 

To place the acidity of phenol in perspective, note that although phenol is more than a million times more acidic than ethanol, it is over a hundred thousand times weaker than acetic acid. Thus, phenols can be separated from alcohols because they are more acidic, and from carboxylic acids because they are less acidic. On shaking an ether solution containing both an alcohol and a phenol with dilute sodium hydroxide, the phenol is converted quantitatively to its sodium salt, which is extracted into the aqueous phase. The alcohol remains in the ether phase. 

The pa s of cyclohexanol is 18, whereas the of phenol is 10. This means that phenol is eight orders of magnitude more acidic than cyclohexanol. In other words, phenol is 100 million times more acidic than cyclohexanol. Why is there such a huge difference We can understand why cyclohexanol has a of 18, because that is within the expected range of an alcohol (15-18). So the real question is why is the x>K of phenol so low Why is phenol so much more acidic than a regular alcohol To answer this question, we will use the R of ARIO Resonance) to explain the acidity of phenol ... 

The stabilisation that can result by delocalisation of a positive or negative charge in an ion, via its n orbitals, can be a potent feature in making the formation of the ion possible in the first place (cf. p. 55). It is, for instance, the stabilisation of the phenoxide anion (23), by delocalisation of its charge via the delocalised n orbitals of the nucleus, that is largely responsible for the acidity of phenol (cf. p. 56) ... 

The effect of 4-NC>2 on the acidity of phenol is somewhat enhanced by the introduction of methyl groups in the 2,6 positions134. Thus, ApKa values for 4-nitro- and... 

The acidity of phenols is mainly due to an electrical charge distribution in phenols that causes the —OH oxygen to be more positive. As a result, the... 

This section describes typical reactions that occur as a result of the acidity of phenols. 

Electron-donating substituents (e.g. alkyl-groups) have the opposite effect and decrease the acidity of phenols. 

Two approximations usually applied in plotting Hammett relations for phenols are the assumed equality of ortho and para positions and an expected additivity of single substituent effects for polysubstituted cases. It is clear that neither of these can be exactly valid, although recent evidence shows that both approximations are fairly good for the acidity of phenols (Bolton et al., 1970 Sucha et al., 1970). 

The titration method is based on the acidity of phenolic hydroxyl groups and, under proper conditions, can be used to determine this group together with carboxyl or sulfonate groups that may be present coincidentally in lignin. An improved nonaqueous titration method in DMF developed by Pobiner (1983),... 

The acidity of phenols arises from the greater resonance stabilization of the phenoxide anion compared with phenol itself (Scheme 4.6). There is no energy-demanding separation of charge in the resonance structures... 

The Br0nsted acidity of a molecule is its capacity to give up a proton. It can be expressed either by the equilibrium constant, the pfsTa value, the change of standard free energy (AGj) or simply the energy of the deprotonation reaction AH A - - H+. The acidities of phenols were measured experimentally " , including a series of 38 meta-... 

The acidities of phenols were found to be greatly increased upon electronic excitation. Due to a change of about 40 kJ moD in the free energy of deprotonation, phenol is intrinsically 7 pTsTa units more acidic in the S than in the Sq state in the gas phase. Similarly, intermolecular proton transfer in solution from an S excited phenol to, e.g., a solvent base is typically characterized by a pAia value of some 6-7 units less than that of the corresponding ground state. In aqueous solution, the pK of phenol amounts to 10.0 in the ground state and 3.6 in the state. 

Phenols are more acidic than alcohols because one of the non-bonding electron pairs on oxygen is drawn into the benzene ring by resonance. This stabilizes the phenoxide ion that is formed upon ionization and thus the acidity of phenol is enhanced by the phenomenon. This same withdrawal of electrons by the benzene ring stabilizes aniline and decreases the availability of the nonbonding electron pair on nitrogen. Both effects decrease the basicity of aniline relative to alkyl amines. 

This means that substituents that can withdraw electrons conjugatively will have a greater effect on the acidity of phenols (reaction 3.9) than on the acidity of benzoic acids. The difference will only be appreciable at the para position for the meta position, no through resonance is possible. If we make a plot of logATA for the acidities of mcta-substituted phenols against Hammett cr constants, we find that the points lie on a straight line, the slope of which gives the p value for the reaction as +2.23... 

---