Acidities of phenols

TABLE 22.1 Comparison of Physi cal Properties of an Arene, a Phenol, and an Aryl Halide  

One of the hydroxybenzoic acids is known by the common name salicylic acid. Its methyl ester, methyl salicylate, occurs in oil of wintergreen. Methyl salicylate boils over 50°C lower than either of the other two methyl hydroxybenzoates. What is the structure of methyl salicylate Why is its boiling point so much lower than that of either of its regioisomers  

The most characteristic property of phenols is their acidity. Phenols are more acidic than alcohols but less acidic than carboxylic acids. Recall that carboxylic acids have pA. s of approximately 5, whereas the pAT s of alcohols are in the 16-20 range. The pAfj, for most phenols is about 10. 

To help us understand why phenols are more acidic than alcohols, compare the ionization equilibria for phenol and ethanol. In particular, consider the differences in charge delocalization in ethoxide ion and in phenoxide ion. The negative charge in ethoxide ion is localized on oxygen and is stabilized only by solvation forces. 

Because of Its acidity, phenol was known as carbolic ac/d when Joseph Lister Introduced It as an antiseptic In 1865 to prevent postoperative bacterial infections that were then a life-threatening hazard in even minor surgical procedures. 

The negative charge in phenoxide ion is stabilized both by solvation and by electron delocalization into the ring. 

Electron delocalization in phenoxide is represented by resonance among the structures  

Because of its acidity, phenoi was known as carbolic acid when Joseph Lister introduced it as an antiseptic in 1865 to prevent postoperative bacteriai infections that were then a iife-threatening hazard in even minor surgi-cai procedures. 

The most characteristic property of phenols is their acidity. Phenols are more acidic than alcohols bnt less acidic than carboxylic acids. Recall that carboxylic acids have ioniza- 

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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. 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

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 major activity in gas-phase studies now depends on the use of modem techniques such as ion cyclotron resonance (ICR). Thus, as already mentioned (Section ELD). Fujio, Mclver and Taft131 measured the gas-phase acidities, relative to phenol, of 38 meta- or para-substituted phenols by the ICR equilibrium constant method, and their results for +R substituents led them to suggest that such substituents in aqueous solution exerted solvation-assisted resonance effects. It was later163 shown by comparison of gas-phase acidities of phenols with acidities of phenols in solution in DMSO that solvation-assisted resonance effects could also occur even when the solvent did not have hydrogen-bond donor properties. Indeed for p-NC>2 and certain other substituents these effects appeared to be larger than in aqueous solution. 

In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions (Unit 13, Class XI). The -OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become eiectron rich due to the resonance effect caused by -OH group. The resonance structures are shown under acidity of phenols. 

Give two reaetlons that show the aeldlc nature of phenol. Compare acidity of phenol with that of ethanol. 

Acidity of phenols Phenols are fairly acidic compounds. Aqueous hydroxides, e.g. NaOH, convert phenols into their salts (not by aqueous bicarbonates). 

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... 

A)J.P.Gray,USP 1438759(1922) St CA 17,882 (1923)(Rosin 2-12% is added to BkPdr as retarder) B)Lignosa S.Akcyjna.FrP 641442 (1927) CA 23,1272(1929)(Charcoal in BkPdr is replaced by hydrocarbons such as C10Hg sulfo acids of C10Hg or phenols, nitrosulfo acids of phenols, nitrosulfo acids of phenols. Carbohydrates, such as cellulose, starch or sawdust also may be used) C)Dynamit A—G, PhrNaoum... 

Two carbonyl groups greatly increase the acidity. For example, 2,4-pentanedione (acetylacetone, 2) has a pATa s 9, which is comparable to the O—H acidity of phenols (see Table 17-1). The reason is that the enolate anion 3 has the charge largely shared by the two oxygen atoms (cf. 3b and 3c). As a result, the enolate anion 3 is stabilized more with respect to the... 

This method is particularly useful for the synthesis of phenoxides and tends to work well due to both the increased acidity of phenols and their lower volatility relative to alcohols. Hence phenoxides of a large number of elements have been prepared by this route including main group,85 d-block,86 lanthanides87 and actinides88 (equations 18, 19 and 20). 

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

Alkoxides, the conjugate bases of alcohols, are prepared from alcohols by reaction with reactive metals or metal hydrides. They are used as organic bases. Because of the greater acidity of phenols, phenoxides can be obtained from phenols and aqueous base. 

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