Question

Write Dow process for preparation of Phenol. 
What is the action of bromine water on phenol? 
Give reason: Group 16th elements have lower ionisation enthalpy compared to group 15th elements. 
Write two uses of dioxygen. 
 

Hint:

Dow process uses high temperature/pressure; phenol’s high reactivity with Br\(_2\) is due to ring activation; group 16’s paired electrons lower ionisation energy.

Answer 1

Answer: In the Dow process, phenol is prepared by heating chlorobenzene with aqueous sodium hydroxide at high temperature and pressure.

Explanation:

The reaction involves nucleophilic substitution of the –Cl group by –OH group.

Reaction: \[ C_6H_5Cl + NaOH \xrightarrow[300^\circ C]{300\,atm} C_6H_5ONa + NaCl \] \[ C_6H_5ONa + HCl \rightarrow C_6H_5OH + NaCl \]

Thus, phenol is obtained after acidification of sodium phenoxide.

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Answer:

Phenol reacts readily with bromine water to form 2,4,6-tribromophenol, which is a white precipitate.

Explanation:

Reaction: \[ C_6H_5OH + 3Br_2 \rightarrow C_6H_2Br_3OH + 3HBr \]

The white precipitate formed is 2,4,6-tribromophenol.

Reason: The –OH group activates the benzene ring towards electrophilic substitution, especially at the ortho and para positions, making bromination easy even in mild conditions.

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Give reason: Group 16 elements have lower ionisation enthalpy compared to Group 15 elements.

Answer:

Group 16 elements have lower ionisation enthalpy than Group 15 elements because of extra electron–electron repulsion due to paired electrons in the p-orbitals.

Explanation:

Group 15 elements have half-filled p-orbitals (more stable configuration), whereas in Group 16 elements, one of the p-orbitals contains a pair of electrons.

This extra electron–electron repulsion in Group 16 elements reduces the energy required to remove an electron, thereby lowering ionisation enthalpy.

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Write two uses of dioxygen.

Answer:

(i) Dioxygen is used in oxy-acetylene flames for welding and cutting metals. 
(ii) It is essential for respiration and combustion processes.

Explanation:

- Oxygen supports combustion and is vital for life processes. - It is also used in industries for the manufacture of steel, nitric acid, and as an oxidizing agent in various chemical reactions.