Phenols

Phenols are aromatic compounds containing hydroxyl group directly attached to the nucleus. Phenols are compounds of the general formula ArOH, where Ar is phenyl, substituted phenyl, or some other aryl group (e.g., naphthyl). Phenols differ from alcohols in having the OH group attached directly to an aromatic ring.

Both phenols and alcohols contain the –OH group, and as a result the two families resemble each other to a limited extent. In most of their properties, however, and in their preparations, the two kinds of compounds differ so greatly that they well deserve to be classified as different families.

• Physical Properties of Phenols

The simplest phenols are liquids or low-melting solids; because of hydrogen bonding, they have quite high boiling points. Phenol itself is somewhat soluble in water, most other phenols are essentially insoluble in water.

When the physical properties of the isomeric nitrophenols are compared, o-nitrophenol is found to have lowb.p. and much lower solubility in water than its isomers. It is the only one of the three that is readily steam – distillable. Steam distillation depends upon a substance having an appreciable vapour pressure at the boiling point of water. In the ortho isomer, the intramolecular hydrogen bonding takes the place of intermolecular hydrogen bonding with other phenol molecules and with water molecules; therefore o-nitrophenol does not have the low volatility of an associated liquid, nor does it have the solubility characteristic of a compound that forms hydrogen bonds with water.

• Nomenclature of Phenols and Phenyl Ethers

Compound having a hydroxyl group directly attached to a benzene ring are called phenols. The term phenol is also used for the parent compound, hydroxybenzene. Hydroxybenzene, may be regarded as an enol, as implied by the name phenol, from phenyl + enol. However unlike simple ketones, which are far more stable than their corresponding enols, the analogous equilibrium for phenol lies far on the side of the enol form. The reason for this difference is the resonance energy of the aromatic ring, which provides an important stabilization of the enol form.

Since the functional group occurs as suffix in phenol, many compounds containing hydroxyl group are named as derivatives of the parent compound phenol, as illustrated by the IUPAC names.

Suffix  groups such as sulfonic acid and carboxylic acid take priority, and when these groups are present the hydroxyl group is used as a modifying prefix.

Phenyl ethers are named in the IUPAC system as alkoxyarenes, although the ether nomenclature is used for some compounds.

Phenols and their ethers are widespread in nature, and, as is usual for such compounds, trivial names abound.

• Salts of Phenols

Phenols are fairly acidic compounds, and in this respect, markedly differ from alcohols, which are even more weakly acidic than water. Aqueous hydroxides converts phenols into their salts, aqueous mineral acid convert the salts back into the free phenols.

Most phenols have K_a values in the neighbourhood of 10^–10 and are thus weaker acids than the carboxylic acids (K_a values about 10^–5). Most phenols are weaker than carbonic acid and hence unlike carboxylic acids do not dissolve in aqueous bicarbonate solution.

• Preparation of Phenols

Industrial Methods for Preparation of Phenols

1. Dow process (by Benzyne Mechanism)?

2. From Cumene Hydroperoxide?

Laboratory Methods for Preparation of Phenols

3. Alkali Fusion of Aryl Sulphonate Salts

Phenols may be prepared by fusion of sodium arylsulphonates with sodium hydroxide

4.  Aromatic Nucleophilic Substitution of Nitro Aryl Halides

Phenols are formed when compounds containing an activated halogen atom are heated with aqueous sodium hydroxide, e.g. p-nitrophenol from
p-chloronitrobenzene.

5. Hydrolysis of diazonium salts
           When a diazonium sulphate solution is steam distilled, a phenol is produced

6. Distillation of phenolic acids with soda-lime produces phenols, e.g. sodium salicylate 

• Chemical Properties of Phenols

1.   Reactions of H of the OH group

Acidity of Phenols

Phenols are weak acids (pK_a = 10). They form salts with aqueous NaOH but not with aqueous NaHCO₃.The considerably greater acid strength of PhOH (pK_a = 10) than that of ROH (pK_a = 18) can be accounted for as the negative charge on the alkoxide anion, RO^- , cannot be delocalized, but on PhO^– the negative charge is delocalized to the ortho and para ring positions as indicated by the starred sites in the resonance hybrid.

PhO^– is therefore a weaker base than RO^–, and PhOH is a stronger acid. the effect of

1. electron – attracting and
2. electron – releasing substituents on the acid strength of phenols

Electron – attracting substituents disperse negative charges and therefore stabilize ArO^– and increase acidity of ArOH. Electron – releasing substituents concentrate the negative charge on O destabilizes ArO^– and decreases acidity of ArOH

In terms of resonance and inductive effects we can account for the following relative acidities.

a)         p-O₂NC₆H₄OH > m – O₂NC₆H₄OH > C₆H₅OH

b)         m – ClC₆H₄OH > p-ClC₆H₄OH  > C₆H₅OH

The –NO₂ is electron – withdrawing and acid – strengthening. Its resonance effect, which occurs only from para and ortho positions, predominates over its inductive effect, which occurs also from the meta position. Other substituents in this category are 

b)   Cl is electron – withdrawing by induction. This effect diminishes with increasing distance between Cl and OH. The meta is closer than the para positions and m-Cl is more acid – strengthening than the p-Cl. Other substituents in this category are F,Br, I, ⁺NR₃.

We can assign numbers from 1 for LEAST 2 for MOST to indicate the relative acid strengths in the following groups:

a) phenol, m-chlorophenol, m-nitrophenol, m-cresol;

 2,3,4,1. Because

Has + on N, it has a greater electron – withdrawing inductive effect than Cl.

The decreasing order of relative acid strengths

Benzoic acid > carbonic acid > p-nitrophenol > phenol

b)  phenol, benzoic acid, p-nitrophenol, carbonic acid

1,4,2,3

The decreasing order of relative  acid strengths

Benzoic acid > carbonic acid > p-nitrophenol > phenol

c)  phenol, p-chlorophenol, p-nitrophenol, p-cresol

2,3,4,1. The resonance effect of  p-NO₂ exceeds the inductive effect of p-Cl p-CH₃ is electron releasing.

The decreasing order of relative acid strengths

p-nitrophenol > p-chlorophenol > phenol > p-cresol

d)  phenol, o-nitrophenol, m-nitrophenol, p-nitrophenol

1,3,2,4 Intramolecular H-bonding makes the o-isomer weaker than the p-isomer.

The increasing order of relative acids strengths

p-nitrophenol > o – nitrophenol > m – nitrophenol > phenol

e)  phenol, p-chlorophenol, 2,4,6 – trichlorophenol, 2,4 – dichlorophenol

The decreasing order of relative acids strengths

2,4,6 – trichlorophenol > 2,4, - dichlorophenol > p-chlorophenol > phenol

f) phenol, benzyl alcohol, benzenesulfonic acid, benzoic acid 

 2,1,4,3

The decreasing order of relative acid strengths

Acid > benzoic acid > phenol > benzyl alcohol

Formation of ethers from Phenols: 

a)  Williamson Synthesis 

b)  Aromatic nucleophilic substitution

Formation of Esters From Phenols

Phenyl esters (RCOOAr) are not formed directly from RCOOH. Instead, acid chlorides or anhydrides are reacted with ArOH in the presence of strong base

(CH₃CO)₂O + C₆H₅OH + NaOH → CH₃COOC₆H₅ + CH₃COO^–Na⁺ + H₂O

                                                                        Phenyl acetate

C₆H₅COCl + C₆H₅OH + NaOH → C₆H₅COOC₆H₅ + Na⁺Cl^– + H₂O

                                                                        Phenyl benzoate

OH^– converts ArOH to the more nucleophilic ArO^– and also neutralizes the acids formed.

Phenyl acetate undergoes the Fries rearrangement with AlCl₃ to form ortho and para hydroxyacetophenone. The ortho isomer is separated from the mixture due to its volatility with steam.

The ortho isomer has higher vapour pressure because of chelation, O–H---O =   and is steam volatile. In the para isomer there is intermolecular H— bonding with H₂O. The para isomer (rate controlled product) is the exclusive product at 25°C because it has a lower DH and is formed more rapidly. Its formation is reversible, unlike that of the ortho isomer which is  stabilized by chelation. Although it has a higher DH, the ortho isomer (equilibrium – controlled product) is the chief product at 165°C because it is more stable.

2.  Displacement of OH group

Phenols resemble aryl halides in that the functional group resists displacement. Unlike ROH, phenols do not react with HX, SOCl₂, or phosphorus halides. Phenols are reduced to hydrocarbons but the reaction is used for structure proof and not for synthesis.

ArOH + Zn  ArH + ZnO (poor yields)

Reactions of the benzene ring

1. Hydrogenation of Phenols

2. Oxidation of phenols to Quinones 

3.  Electrophilic Substitution Reactions of Phenols

The —OH and even more so the —Oph are strongly activating and o, p directing

Special mild conditions are needed to achieve electrophilic monosubstituion in phenols because their high reactivity favors both polysubstitution and oxidation.

1.  Halogenation of Phenols?

Monobromination is achieved with nonpolar solvents such as CS₂ to decrease the electrophilicity of Br₂ and also to minimize phenol ionization

 b. Nitrosation of phenols

c. Nitration of phenols

Low yields of  p- nitrophenol are obtained from direct nitration of PhOH because of ring oxidation. A better synthesis method is 

d. Sulfonation of phenols

e. Diazonium salt coupling to form azophenols

 Coupling (G in ArG is an electron – releasing group)

ArN₂⁺ + C₆H₅G → p-G —C₆H₄ — N = N — Ar (G = OH, NR₂,NHR, NH₂)

f. Mercuration of Phenols

Mercuricacetate cation, ⁺HgOAC, is a weak electrophile which substitutes in ortho and para positions of phenols. This reaction is used to introduce an I on the ring.

g. Ring alkylation of phenols

RX and AlCl₃ give poor yields because AlCl₃ coordinates with O.

 h. Ring acylation of phenols

Phenolic ketones are best prepared by the Fries rearrangement (Discussed earlier)

i. Kolbe synthesis of phenolic carboxylic acids 

Phenoxide carbanion adds at the electrophilic carbon of CO₂.

j. Reimer – Tiemann synthesis of phenolic aldehydes 

Phenol can be used to synthesize (A) aspirin (acetylsalicylic acid) (B) oil of wintergreen (methyl salicylate)

k. Condensations with carbonyl compounds; phenol – formaldehyde resin.

Acid or base catalyzes electrophilic substitution of carbonyl compounds in ortho and para positions of phenols to form phenol alcohols (Lederer – Manasse reaction).

Acid catalyzed

Phenols are soluble in NaOH but not in in NaHCO₃. With Fe³⁺ they produce complexes whose characteristics colors are green, red, blue and purple.

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