Phenols are the organic compounds containing a benzene ring bonded to a hydroxyl group. phenols are the simplest hydroxy derivative of the benzene ring or aromatic ring. They are also known as carbolic acids. Thus, a phenol molecule consists of two parts one aryl group part and the other hydroxyl group part.

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On the basis of the number of hydroxyl groups attached to the aryl group, it can be classified into mono-, di-, tri- or polyhydric phenols.

Phenols

Phenols (Phenol case):

Introduction and IUPAC nomenclature,

Preparation: Cumene hydroperoxide method, from diazonium salts. Reactions: Electrophilic substitution: Nitration, halogenation and sulphonation. Reimer-Tiemann Reaction, Gattermann-Koch Reaction, Houben–Hoesch Condensation, Schotten–Baumann Reaction. Ethers (aliphatic and aromatic): Cleavage of ethers with HI.

Learning Outcome:

After studying the Alcohols and Phenols student will able to

1. Identify and draw the structures alcohols / phenols from their names or from structure name can be assigned.
2. Able to differentiate between alcohols and phenols
3. Explain / discuss synthesis of alcohols / phenols.
4. Write / discuss the mechanism of various reactions involved.
5. Explain /Discuss important reactions of alcohols / phenols.
6. To correlate reagent and reactions of alcohols / phenols
7. Give synthesis of expected alcohols / phenols.

Nomenclature of phenols:

1. Simple method: In this method the substituent attached on the aromatic is designated as ortho, meta or para as prefix to the name of phenol.

2. Numbering method: In this method, the carbon atom of the aromatic ring to which -OH group is attached is given number 1. Then the carbon atom of the ring to which the other substituent is attached is given lowest possible number.

3. Polysubstituted derivations: If more than two groups are attached to the aromatic ring. only number method is used to indicate their relative positions w.r.t. - OH group.

Physical properties

1. Acidic nature: Although alcohols and phenols have hydroxyl (-OH) as a functional group, alcohols are neutral compounds but phenols are acidic in nature. This is because phenoxide ion formed after the loss of proton gets stabilized due to resonance effect (-R effect) of the ring, on the other hand the alkoxide ion is destabilized due to inductive effect (+I effect).

Secondly, phenols react with bases and form salt which dissolve in aqueous medium.

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The salt on treatment with acid gives phenol back which are generally insoluble

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Alcohols do not react with acid and base to form salt, hence neutral in nature (Lower members are soluble in all aqueous reagents).

2. Physical constants: Phenols show high boiling points than hydrocarbons of comparable molecular weight due to presence of hydrogen bonding in phenolic compounds.

m-nitrophenol and p-nitrophenol show intermolecular hydrogen bonding. While o-nitrophenol show intra molecular hydrogen bonding. Hence, m and p-nitrophenols have higher physical constant than o-nitro phenol.

Due to hydrogen bonding phenols also dissolve in water to a considerable amount. In fact hydrogen bonding between water and phenol is much stronger than between water and alcohol.

Colour: Simple phenols are colourless but if chromophoric groups (NO₂, -NH₂) are introduced on the aromatic ring then they show colour. e.g. nitrophenols and nitroamines are yellow to brown coloured compounds .

Preparation of Phenols

1. Cumene hydroperoxide method

The starting material cumene, can be obtained from benzene and propene by Friedel Craft alkylation reaction.

2. Oxidation of Cumene: When cumene is treated with oxygen at elevated temperature gives Cumene hydroperoxide which on treatment with 10 % H₂SO₄ undergoes rearrangement and yields phenol and acetone. The acetone byproduct obtained from the reaction is also an important solvent .

3. From Diazonium salts: In this process first the arylamine is treated with nitrous acid (NaNO₂) to obtain the diazonium salt, which on acid catalysed hydrolysis yields the phenol.

Reactions of Phenol

phenol is an aromatic compound it undergoes electrophilic substitution reactions. As the hydroxyl group is electron donating group (activating group), the product obtained is ortho - para substituted phenol.

Some of the ring substituted reactions are

1. Nitration
2. Sulphonation
3. Halogenation
4. Nitrososation
5. Kolbe's reaction (carbonation)
6. Reimer-Tiemann reaction

1. Nitration

A. With dil. HNO₃: Phenol reacts with dil. HNO₃ and produces a mixture of O-nitrophenol and p-nitrophenol. The mixture can be easily separated by steam distillation method. Due to presence of intra molecular hydrogen bonding in o-nitrophenol it is steam volatile and passes over along with steam; while p-nitrophenol remains in the distillation flask.

B. with conc. HNO₃:

Phenol reacts with conc. HNO₃ and give di-substituted and tri- substituted products which need further separation. As phenol is activated compound only conc. HNO₃ is enough to bring about nitration.

2. Sulphonation: Phenol reacts with conc. H₂SO₄ and gives o/p substituted product. At room temperature, o-phenol sulphonic acid is obtained while at 100° C. p-phenol sulphonic acid is obtained. If the o-phenol sulphonic acid is heated to 100° C, it isomerises to the para isomer.

3. Halogenation

A. With Br₂ water: Phenol reacts with Br₂ in water (polar solvent) and gives 2,4,6-tribromophenol. This reaction does not required any Lewis acid catalyst.

2,4,6 - tribromophenol (yellowish solid)

B. With Br₂ / CCI₄: Phenol reacts with Br₂ in carbon tetrachloride (non- polar solvent) and gives o-bromo phenol and p-bromophenol.

4. Nitrososation: Phenol reacts with NaNO₂ + H₂SO₄ at 5° C and reaction takes place at para position gives p-nitrosophenol.

5. Carbonation (Kolbe synthesis): The sodium phenoxide is allowed to absorb CO₂ (electrophile) at high pressure and temperature, giving sodium salicylate, which on acidification gives salicylic acid.

6. Reimer-Tiemann Reaction: Phenol reacts with chloroform in basic medium, the aldehyde group gets attached to the ortho position gives an salicylaldehyde. The reaction proceeds through dichlorocarbene formation.

If ortho positions are blocked, reaction goes to the para position

7. Friedel-Craft Reactions:

Phenols and anilines do not undergo Friedel-Craft reaction because the - OH and –NH₂ group coordinates with Lewis acid, which deactivate the aromatic ring. Hence there is no further alkylation or acylation reaction.

However, if -OH is converted into –OR, then F.C. reactions are possible, giving mainly para products.

8. Gattermann Reaction: Phenol is treated with a mixture of hydrogen cyanide and hydrogen chloride in presence of aluminium chloride as catalyst to give salicylaldehyde.

Mechanism: The reaction proceeds with formation of imidoformyl chloride which is attacked by aromatic ring to give corresponding imine. The imine on acidify with dilute acid give the corresponding aldehyde.

9. Houben-Hoesch condensation: This is acid catalysed acylation of dihydric phenols using alkyl cyanides (nitriles) to form an aryl ketone.

Mechanism: Nucleophilic addition to alkyl cyanide is catalyzed by Lewis acid (AlCl₃ or ZnCl₂) to gives an intermediate imine. The imine hydrolysed by dilute acid to gives an ketone.

10. Schotten-Baumann reaction: Phenols on reaction with aromatic aldehyde gives an benzoyl derivatives.

Mechanism: Phenol reacts with NaOH to form sodium phenoxide which on attacks on carbonyl carbon of benzoyl chloride to gives.

Nomenclature rules of Phenols

1. Locate the position of a hydroxyl group attached to the benzene ring.
2. Benzene rings attached to more than one hydroxyl group are labeled with the Greek numerical prefixes such as di, tri, tetra to denote the number of similar hydroxyl groups attached to the benzene ring. If two hydroxyl groups are attached to the adjacent carbon atoms of the benzene ring, it is named as benzene1,2-diol

3. In the case of substituted phenols, we start locating the positions of the other functional groups with respect to the position where the hydroxyl group is attached. For example, if a methyl group is attached at the fourth carbon atom with respect to the hydroxy group, the compound is named as, 4-Methyl phenol.

4. Depending on the position of substituted functional group with respect to the hydroxyl group, words like ortho (when the functional group is attached to the adjacent carbon atom), para (when the functional group is attached to the third carbon atom from the hydroxyl group), meta (when the functional group is attached to the second carbon atom from the hydroxyl group) are also used for the nomenclature of phenols.

Why is phenol acidic? Compounds like alcohols and phenol which contain an -OH group attached to a hydrocarbon are very weak acids. ... Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring.

• Preparation of Phenols from Cumene
• Cumene is an organic compound obtained by Friedel-Crafts alkylation of benzene with propylene. Upon oxidation of cumene (isopropylbenzene) in the presence of air, cumene hydroperoxide is obtained. Upon further treatment of cumene hydroperoxide with dilute acid, phenols are obtained.

When an aromatic primary amine is treated with nitrous (NaNO2 + HCl) acid at 273 – 278 K, diazonium salts are obtained. These diazonium salts are highly reactive in nature. Upon warming with water, these diazonium salts finally hydrolyze to phenols. Phenols can also be obtained from diazonium salts by treating it with dilute acids.

Preparation of Phenol from disonium salt

Nitration of Phenols

Phenols on reaction with dilute nitric acid undergo nitration at low temperature (298 K) to give a mixture of ortho and para nitrophenols. The mixture formed is further separated into ortho and para nitrophenols by steam distillation on the basis of their volatility.

M.P= 114^oC

M.P= 45^oC

B.P= 216^oC

B.P= 284^oC

Due to a highly activating effect of the hydroxyl group in phenols, they undergo halogenation even in the absence of Lewis acids. When phenols are treated with bromine in the presence of a solvent of low polarity like CHCl3 at low temperatures, monobromophenols are formed.

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When phenol is treated with bromine water, a white precipitate of 2, 4, 6-tribromophenol is formed.