Topic 21: Organic Synthesis

Question 1

Definitions I:
(a) Functional group
(b) Synthetic route
(c) By-product

Answer 1

Definitions I:
(a) Group of atoms dictating chemical properties, e.g., -OH.
(b) Sequence of reactions to prepare a target organic molecule.
(c) Secondary product formed alongside the target in a reaction.

Question 2

Identify the functional groups in CH₃CH(OH)CH₂CHO using syllabus reactions. [2 Marks]

Answer 2

-OH (alcohol): Na → H₂; -CHO (aldehyde): Tollens’ → silver mirror (1 mark)
Both confirmed by syllabus tests (1 mark)

Question 3

Predict the reaction of CH₃COOCH₂CH₃ with dilute NaOH, heat, and its product properties. [2 Marks]

Answer 3

Ester hydrolyses: CH₃COOCH₂CH₃ + NaOH → CH₃COO⁻ + CH₃CH₂OH (1 mark)
Salt and ethanol, then acidify to CH₃COOH (1 mark)

Question 4

In a lab, devise a 2-step synthesis of CH₃CH₂OH from CH₃CH₂Br using syllabus reactions. [2 Marks]

Answer 4

Step 1: CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr (aq, heat) (1 mark)
Step 2: Nucleophilic substitution complete (1 mark)

Question 4

Analyse this route: CH₃CH=CH₂ → CH₃CHBrCH₃ → CH₃CH(OH)CH₃. Name reaction types. [3 Marks]

Answer 4

Step 1: Electrophilic addition (HBr) (1 mark)
Step 2: Nucleophilic substitution (NaOH) (1 mark)
By-product: NaBr (1 mark)

Question 5

What functional groups are in CH₃CH₂COOCH₃, and how do you test for them? [2 Marks]

Answer 5

-COO- (ester): NaOH → CH₃CH₂OH + CH₃COO⁻ (1 mark)
Test: hydrolysis, alcohol smell (1 mark)

Question 6

Predict the product when CH₃CH₂CHO reacts with HCN (KCN, heat). [2 Marks]

Answer 6

CH₃CH₂CHO + HCN → CH₃CH₂CH(OH)CN (KCN, heat) (1 mark)
Nucleophilic addition, hydroxynitrile (1 mark)

Question 7

In a classroom, devise a 3-step synthesis of CH₃COOH from CH₃CH₂OH. [3 Marks]

Answer 7

Step 1: CH₃CH₂OH + [O] → CH₃CHO (K₂Cr₂O₇/H⁺, distil) (1 mark)
Step 2: CH₃CHO + [O] → CH₃COOH (KMnO₄/H⁺, reflux) (1 mark)
Oxidation sequence (1 mark)

Question 8

What by-products form when CH₃CH₂Cl reacts with NH₃ (ethanol, pressure, heat)? [2 Marks]

Answer 8

CH₃CH₂Cl + NH₃ → CH₃CH₂NH₂ + HCl (1 mark)
By-product: HCl (or NH₄Cl with excess NH₃) (1 mark)

Question 9

Identify the functional group in CH₃CH₂CN and predict its hydrolysis product with HCl. [2 Marks]

Answer 9

-CN (nitrile): HCl → CH₃CH₂COOH + NH₃ (1 mark)
Hydrolysis to carboxylic acid (1 mark)

Question 10

Analyse this route: CH₃CH₂OH → CH₂=CH₂ → CH₃CH₂Cl. List reagents per step. [2 Marks]

Answer 10

Step 1: CH₃CH₂OH → CH₂=CH₂ (Al₂O₃, heat) (1 mark)
Step 2: CH₂=CH₂ + HCl → CH₃CH₂Cl (room temp) (1 mark)

Question 11

Predict the reaction of CH₃CH(OH)CH₃ with PCl₅ and its product. [2 Marks]

Answer 11

CH₃CH(OH)CH₃ + PCl₅ → CH₃CHClCH₃ + POCl₃ + HCl (1 mark)
Substitution to halogenoalkane (1 mark)