Home / AS & A Level Chemistry 29.4 Isomerism: optical: Exam Style Questions Paper 4

AS & A Level Chemistry 29.4 Isomerism: optical: Exam Style Questions Paper 4

A Level Chemistry Paper 4 - All Chapters
Question

(a) Samples of phenol, C₆H₅OH, are reacted separately with sodium and with dilute nitric acid.

Structure of phenol

(i) Write the equation for the reaction of C₆H₅OH with Na.
(ii) Draw the structures of the two major isomeric organic products formed in the reaction of phenol with dilute HNO₃.

Reaction scheme for phenol with HNO₃

(b) Salicylic acid can be synthesised from phenol.

Synthesis of salicylic acid from phenol

One of the steps in this synthesis is the electrophilic substitution reaction of carbon dioxide with the phenoxide ion, C₆H₅O⁻. Complete the mechanism in Fig. 9.3 for the reaction of C₆H₅O⁻ with \(CO_2\). Include all relevant curly arrows, dipoles and charges. Draw the structure of the organic intermediate.

Fig. 9.3: Reaction mechanism

(c) Some syntheses use Diels–Alder reactions, which normally involve a diene and an alkene reacting together to form a cyclohexene.
(i) Draw three curly arrows in Fig. 9.4 to complete the mechanism for the Diels–Alder reaction between buta-1,3-diene and ethene.

Fig. 9.4: Diels-Alder reaction mechanism

(ii) Another Diels–Alder reaction of buta-1,3-diene is shown in Fig. 9.5. Predict the product formed in this reaction.

Fig. 9.5: Diels-Alder reaction with another dienophile
▶️ Answer/Explanation
Solution

(a)(i) \(2C_6H_5OH + 2Na \rightarrow 2C_6H_5O^-Na^+ + H_2\)

Explanation: Phenol reacts with sodium to form sodium phenoxide (\(C_6H_5O^-Na^+\)) and hydrogen gas, similar to alcohols.

(a)(ii) Major products: 2-nitrophenol and 4-nitrophenol.

Structures of 2-nitrophenol and 4-nitrophenol

Explanation: Nitration of phenol with dilute HNO₃ preferentially substitutes at the ortho (2-) and para (4-) positions due to the \(-OH\) group’s activating and directing effects.

(b) Mechanism for reaction of phenoxide ion (\(C_6H_5O^-\)) with \(CO_2\):

Mechanism of phenoxide with CO₂

Explanation:

  1. Nucleophilic attack by phenoxide on the electrophilic carbon of \(CO_2\).
  2. Formation of a carboxylate intermediate (salicylate ion).
  3. Protonation yields salicylic acid.

(c)(i) Diels-Alder mechanism (buta-1,3-diene + ethene):

Diels-Alder mechanism with arrows

Explanation: Three curly arrows show the concerted formation of two new σ-bonds and one π-bond, resulting in cyclohexene.

(c)(ii) Product of buta-1,3-diene with maleic anhydride:

Diels-Alder product with maleic anhydride

Explanation: The reaction yields a cyclohexene derivative with an anhydride group, retaining stereochemistry of the dienophile (cis).

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