Phenol

When a hydrogen atom is replaced by –OH group in aromatic hydrocarbon phenols are formed.

Depending upon the presence of number of –OH group, Phenol may be classified as monohydric, dihydric, trihydric or polyhydric phenol.

Some examples of Phenol

Preparation

Phenol is known as Carbolic acid; first it was isolated from coal tar, Now it can be preparedfrom benzene derivative by any of bellows methods.

1. From Chloro Benzene (Dow’s Process)

Chloro benzene is heated with NaOH at 623K and 300 atm, to form Sodium Phenoxide which further acidification gives Phenol. This process is also Known as Dow Process.

2. From Benzene or Benzene Sulphonic acid.

Benzene on Sulphonation with Oleum to form BenzeneSulphonic acid which on heating with NaOH and further acidification gives Phenol.

3. From Aniline or Diazonium Salt.

Aniline is heated with nitrous acid and HCl (NaNO₂ + HCl) at 273-278K to convert Benzene Diazonium Chloride, which on warm with water or by treatment with dilute acids hydrolysed in to Phenol.

4.From cumene (isopropyl benzene)

In presence of air oxygen, cumene oxidised into cumene hydroperoxide. When it is treated with dilute acid, it converts into phenol.

physical Properties

· Phenols are colourless liquids or solids; they slightly pink colour due to presence of air oxidised products.

· A liquid solution of phenol containing 5% water is called Carbolic acid.

· Boiling points of phenols are higher than aliphatic alcohols of comparable molecular mass, due to presence of strong hydrogen bonding in phenols than the alcohols.

· Ortho isomer of phenols are volatile in nature due to presence of intramolecular hydrogen bonding but para isomer has higher boiling point than ortho and meta isomer due to intermolecular hydrogen bonding.

· Phenols are soluble in water due to formation of hydrogen bond with water but solubility decreases with increase in size of aryl group (hydrophobic nature).

· It may be a poison if taken orally also produce painful blisters in contact with skin.

Chemical Properties

1. Reaction with Na metal and NaOH

· Phenol reacts with Na metal Or NaOH to yield a salt Sodium phenoxide

· This above reaction indicates the acidic character of phenols. But phenols are Bronsted acids because they donate a proton to a strong base.

Acidity of Phenols

The electron density of oxygen atom of –OH group decreases as it is attached to the higher electronegativity of sp2 hybridised carbon atom of phenol.so there is increase in polarity of O-H bond and easily ionise and release H+.

In phenoxide ion, the negative charge is delocalised in the resonance structure, so it is more stable and favouring the ionisation.

But in phenol resonance structure charge separation occur and phenol molecule is less stable than phenoxide ion.

Phenol is more acidic than Alcohol, Why?

In Alkoxide ion the negative charge is localised on oxygen atom whearas in Phenoxide ion ,the charge is delocalised (resonance structure of Phenoxide ion).So phenoxide ion is more stable than alkoxide ion ,thus easily ionise and gives H+ ion so it is more acidic than alcohol.

Effect of Substituent on Acidity of phenol.

If the electron withdrawing group (-NO2, CN, COOH, COR etc) is substituted on Phenol, the acidity increases due to the effective delocalisation of negative charge on phenol and stabilisation of phenoxide ion. This effect is increased in if the substituent present at ortho and para position.

If electron releasing group (CH3, -NH2, -OH, -OR etc) is substituted on phenol, the acidity decreases due to destabilisation of phenoxide ion.

2. Esterification.

Phenol reacts with carboxylic acids, acid chlorides and acid anhydrides to give an ester.

Acetylsalicylic acid (Aspirin) can be prepared by the Acetylation (introduction of acetyl group) of Phenol. Aspirin is used as an analgesic and anti-inflammatory drugs.

3. Electrophilic Substitution reaction.

-OH group of phenol is a ring activator as it activates the electrophilic substitution reaction.

In electrophilic substitution reaction, Phenol directs the incoming group (electrophile) towards ortho and para position because these position are electron rich (shown in resonance structure below), hence it is an ortho and paradirector.

i) Nitration

Phenol is treated with dilute nitric acid at 298K to form a mixture of ortho and para nitrophenol.

In o-nitro phenol intramolecular H-bonding takes place so its B.P is less than p-nitro phenol (intermolecular hydrogen bonding) due to this reason the two isomer can be easily separated out.

But, when phenol is treated with Concentrated HNO3, it is converted into Picric acid.

ii)Halogenation

Phenol is treated with Bromine in low polar solvent CS₂ or CHCl₃ to gives a mixture of orthobromo phenol(minor) and para bromo phenol(major).

But when Phenol reacts with Bromine water, it forms a white precipitated of 2, 4, 6 tribromophenol. This reaction helpful for detection of phenol.

3.Kolbes reaction

Phenol reacts with NaOH to form Sodium Phenoxide, which undergo electrophilic substitution reaction with CO₂(weak electrophile) and form salicylic acid.

4. Riemer Tiemann reaction

Phenol reacts with chloroform in presence of NaOH, a –CHO group issubstituted at ortho position of Benzene ring.

The intermediate substituted Benzal chloride is hydrolysed in alkali and gives salicylaldehyde. This reaction is known as Riemer Tiemann reaction.

5. Reaction with zinc dust

When phenol is heated with Zn dust it converts in to Benzene

6. Oxidation

Phenol on oxidation with chromic acid forms Benzoquionone (conjugated diketone)

Phenol also in presence of air slowly oxidised to quinone.

7. Reaction with FeCl₃

Phenol react with FeCl_{3 and} form a coloured complex (blue,green or red depending upon the nature of Phenol).This reaction is useful for identification of phenol.