(a) Acid dehydration of ethanol to ethene: When ethanol is heated with concentrated sulphuric acid at 452K, it undergoes dehydration and ethene is formed.
\({ CH }_{ 3 }{ CH }_{ 2 }OH\overset { { H }_{ 2 }{ SO }_{ 4 },453K }{ \longrightarrow } { CH }_{ 2 }={ CH }_{ 2 }+{ H }_{ 2 }OH\)
Mechanism:
Step 1:Protonation of ethanol to form ethyl oxonium ion:

Step 2:Formation of carbocation (rate determining step):

Step 3: Elimination of a proton to form ethene:

(b) On reaction with FeCl₃ phenol gives violet, blue and red colour. While ethanol give no reaction.

ome reactive alkenes undergo direct hydration in the presence of mineral acids which act as catalysts. The addition of water to the double bond takes place in accordance with Markonikoff’s rule.
The mechanism of reaction involves the following three steps:
(i) Protonation of alkene to form carbocation by electrophilic attack of H₃O⁺.
 
(ii) Nucleophilic attack of water of carbocation.

(iii) Deprotonation to from alcohol.

 Pyridinium chlorochromate in CH₂Cl₂
\(\underset { Ethanol }{ CH_{ 3 }CH_{ 2 }OH } \overset { C_{ 2 }H_{ 5 }NH^{ + }CrO_{ 3 }Cl^{ - } }{ \underset { CH_{ 2 }Cl_{ 2 } }{ \longrightarrow } } \underset { Ethanal }{ CH_{ 3 }CHO } \)
Any strong oxidising agent such as acidified KMnO₄ or K₂Cr₂O_7.
\(\underset { Ethanol }{ CH_{ 3 }CH_{ 2 }OH } \overset { KMnO_{ 4 }/H_{ 2 }SO_{ 4 } }{ \underset { or\quad K_{ 2 }Cr_{ 2 }O_{ 7 }/H_{ 2 }SO_{ 4 } }{ \longrightarrow } } \underset { Ethanoic\quad acid }{ CH_{ 3 }COOH } \)

(i)

(ii)
 
(iii)

To prepare phenol, cumene is first oxidized in the presence of air of cumene hydro-peroxide.
 
Then, cumene hydroxide is treated with dilute acid to prepare phenol and acetone as by-products.