AIEEE Chemistry Unit 23B Phenols

Phenols: Introduction

Phenols are aromatic hydroxy compounds. In phenols, one or more hydroxyl group is directly attached to the aromatic (benzene) nucleus.

Phenols are also classified as monohydric, dihydric and trihydric or polyhydric as their molecules contain one, two, three or more OH groups.

Examples of Phenols

Monohydric

Phenol
2-Bromophenol
m-Cresol
p-Cresol

Dihydric

1,2-Dihydroxy benzene

Trihydric

1,2,3-Trihydroxy benzene


If OH group is not directly attached to be carbon atom in the benzene ring, but present in the molecule as a part of the alkyl side chain group, then the compound is not termed as phenol.

It is called aromatic alcohol because it resembles aliphatic alcohols in its characteristics.

Examples:

benzyl alcohol or Phenylmethanol
2-Phenylethanol

Nomenclature of Phenols

Common system

IUPAC system

All substituted phenols are named as derivatives of phenol.
The position of the substituents w.r.t.-OH group is indicated by Arabic numerals(with the carbon carrying-OH group being numbered 1).

Examples

2-Methyl phenol - Methyl group CH3 is present adjacent to OH group in phenol.
3-Methyl phenol - Methyl group CH3 is present in third position when OH group position is counted as 1 in phenol.
2-Bromophenol - Bromine is present adjacent to OH group in phenol.
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Phenols: Methods of Preparation

1. Alkali fusion of sodium benzene sulphonate

NaOH is fused with sodium benzene sulphonate at 573 - 623 K, sodium phenoxide is formed. This on acidification gives phenol.

2. From diazonium salts
An acqueous solution of benzene diazonium salt on warming gives phenol

3. By decarboxylation of sodium salt of salicyclic acid

Fusion of sodium salicylate with soda lime (NaOH and CaO mixture).
sodium phenoxide is formed. This on acidification gives phenol.


4. From Grignard reagent
when oxygen gas is bubbled through an ethereal solution of phenyl magnesium bromide (Grignard reagent RMgX), if forms an oxy compound which upon hydrolysis with dilute mineral acid gives phenol.

Commercial Preparation of Phenols

1. From chlorobenzene (Dow's Process)
Chlorobenzene is heated with 10% acqueous sodium hydroxide solution at about 623 K under 200 atmospheres andin the presence of copper salt acting as catalyst to form sodium phenoxide. The sodium salt when treated with dilute HCl, gives phenol.
2. From cumene

Air or oxygen is passed through a suspension of cumene in acqueous sodium carbonate solution in presence of cobalt or manganese naphthenate catalyst. The oxidation product is cumene hydroperoxide.

The hydroperoxide is then decomposed by hot dilute sulphuric acid when phenol is formed withliberation of acetone. Acetone is removed from phenol by distillation.

3. From Benzene (Raschig's method)
Vapours of HCl are passed over benzene at 500 K in the presence of copper chloride and excess of air to form chlorobenzene. Steam is then passed through chlorobenzene at 800 K in the presence of silica as catalyst to give phenol.

4. Phenol prepared using benzene and H2SO4

Benzene is heated with excess of concentrated sulphuric acid at about 388 K to give benzene sulphonic acid.
It is neutralized with sodium hydroxide solution, when sodium benzene sulphonate is obtained.
Dry sodium benzene sulphonate is next fused with excess of caustic soda at about 573 K when it yields sodium phenate (or sodium phenoxide).
Sodium phenate is decomposed by dilute sulphuric acid to give phenol.


Physical properties

1. State and smell: Phenols are colourless crystalline solids or liquids. They have characteristic phenolic odours.

2. solubility: Phenols are sparingly soluble in water

3. Boiling points: Higher than the boiling points of the aromatic hydrocarbons of comparable molecular masses.

BP of phenol (mol. mass = 94) is 455 K while that of toluene (mol mass = 92) is 384 K. This higher BP is due to intermolecular hydrogen bonding in phenols.

Chemical properties

Can be classified into three groups

A. Reactions of phenolic group (_OH group)
B. Reactions of benzene ring
C. Special reactions

A. Reactions of phenolic group (_OH group)

1. Action with zinc dust
2. Action with ammonia
3. Action with acid chlorides and acid anhydrides
4. Action with benzyl chloride

B. Reactions of benzene ring

1. Bromination

Action of Bromine water on phenol: When phenol is treated with bromine water, it gets decolourised giving a white precipitate of 2,4,6, tribromophenol.

Action of Bromine in CS-2 on phenol:o-Bromophenol + p-Bromophenol mixture is obtained. p-Bromophenol is the major product.

2. Nitration

Action of dilute nitirc acid on phenol: a mixture of o-nitrophenol and p-nitrophenol is formed.

Action of conc. nitric acid in the presence of conc. sulphuric acid on phenol: 2,4,6-trinitrophenol is formed. This is picric acid.

3. Nitrosation
The reaction which involves the substitution by nitroso grou (-NO) is called nitrosation.

Phenol reacts with nitrous acid (NaNO2 + HCl) at low temperature (280 K) to form p-nitrosophenol. It can be further oxidized with dil HNO3 to give p-nitrophenol.

4. Sulphonation: Covered as a special topic

5. Alkylation
Special reactions of Phenol

1. Kolbe's reaction: special topic
2. Reimer-Tiemann reaction: special topic
3. coupling reaction
4. Reaction with pthalic anhydride
5. Condensation with formaldehyde
6. Hydrogenation
7. Oxidation
8. Reaction with ferric chloride
9. Libermann's test



Acidity of Phenols

Phenols are weakly acidic in nature (Ka = 10^-10).
They turn blue litmus read and react with alkali metals and alkalies to form their salts.
The acidic character of phenol is due to polar OH bond.

Phenol is weaker acid than carboxylic acid.
Like carboxylic, it also does not react with sodium carbonate and sodium bicarbonate.
Phenols are more acidic than alcohols.

Phenol is a resonance hybrid of 5 structures. Three of the structures develop +charge on oxygen and facilitate release of H+.

Phenoxide ion which results when H+ is released from phenol is also a resonance hybrid but it is more stable than phenol. Hence the reaction is in favour of phenoxide ion. Therefore phenol is acidic and more acidic than alcohols.

Halogenation of Phenols (electrophylic substitution reaction)

The reaction does not require a lewis acid catalyst. Benzene requires a lewis acid catalyst for halogenation.

Bromine in CS2 reacts with phenol to give 4-Bromophenol(?)
Chlorine at high temperatures react with phenol to give 4-chlorophenol(?)

See http://clem.mscd.edu/~wiederm/oc2chp/oc2chpphenols/page3.htm

You can download a chapter on phenols from
http://www.diacritech.com/samples/science_and_medical/chemistry.pdf

Phenols are readily brominated in an aqueous solution forming 2, 4, 6-tribromophenol. This is a white precipitate.

Mostly p-bromophenol is obtained along with ortho bromophenol by treatment
of phenol with bromine in CCl4 or CS2.

Treatment of phenols with aqueous solutions of bromine results in replacement
of every hydrogen ortho or para to the -OH group. Bromination may
even cause displacement of certain other groups to yield tribromophenol.

Nitration of phenols

Action of dilute nitirc acid on phenol: a mixture of o-nitrophenol and p-nitrophenol is formed.

Action of conc. nitric acid in the presence of conc. sulphuric acid on phenol: 2,4,6-trinitrophenol is formed. This is picric acid.

Sulphonation

Action of conc. sulphuric acid at different temperatures on phenol:
Pheno reacts with conc. sulphuric acid to form a mixture of o-, and p-phenol sulphonic acid.
At low temperature about 288 to 293 K, o-phenol sulphonic acid is the main product formed.

At high temperature about 373 K, p-phenol sulphonic acid is the main product formed.

o-phenol sulphonic acid: IUPAC name is 2-Hydroxy benzene sulphonic acid

p-phenol sulphonic acid : IUPAC name is 4-Hydroxy benzene sulphonic acid