There are several methods of making naphthalenes, one of which is given in the experimental procedure...

 

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There are several methods of making naphthalenes, one of which is given in the experimental procedure below. Please read it carefully and answer the questions that follow. OCH3 D[+] (a) H₂CO CH₂O H₂CO OH (b) SUBSTITUTED NAPHTHALENE Step (a): To a solution of 2,4-dimethoxybenzaldehyde ( (5.45 g. 32.80 mmol) and diethyl succinate (8. 18 cm 3, 8.57 g, 49.20 mmol) in dry t-butyl alcohol (80 cm³) stirred under nitrogen, was added potassium-t-butoxide (5.52 g, 49.19 mmol). The mixture was heated under reflux for 2 hours then allowed to cool to room temperature, poured into a separatory funnel containing ice and acidified to pH 3 with cone. HCl(aq). The product thus precipitated was extracted into EtOAc (2x 100 crn³), the combined extracts dried with magnesium sulfate, filtered through Celite and the filtrate concentrated on a rotary evaporator. The resultant oil was purified by column chromatography (10% EtOAc-hexane) to give the desired intermediate that was not characterized but immediately carried through step (b) below. Step (b): To the Stobbe condensation product from step (a) above, dissolved in acetic anhydride (100 cm³), was added anhydrous sodium acetate (6.72 g, 81.91 mmol) and the mixture heated at 140 "C under argon for 2 hours then allowed to cool. Acetic anhydride was removed in vacuo, water (100 cm³) was added, and the product extracted with dichloromethane (3 x 100 cm³). Drying of the combined extracts (MgSO4), filtration through Celite and removal of the solvent on a rotary evaporator gave a dark brown semisolid which was purified by column chromatography (30% EtOAc-hexane, containing 2% v/v dichloromethane) to give an orange-white solid which was recrystallised from EtOAc to give the desired SUBSTITUTED NPHTHALENE (8.10 g) as yellow crystals (mp. 157-158 "C) in two crops. Lit. 158-159 °C. ¹H NMR/300 MHz/8 (ppm): 8.79 (1H, d, J 1.1, 1-H), 7.81 (1H, d, J1.1, 3-H), 6.66 (1H, d, J 1.9, 5-H), 6.51 (1H, d, J 1.9, 7-H), 4.41 (2H, q, J 7.1, OCH₂CH3), 3.97 (3H, s, OCH3), 3.91 (3H, s, OCH₂), 2.45 (3H, s, CH₂COO) and 1.42 (3H, t, J 7.1, OCH₂CH₂). ¹³C NMR/75 MHz/ 6 (ppm): 169.2 (CH CO), 166.2 (ArCOOEt), 160.9 (6-C), 157.8 (8-C), 145.3 (4-C), 131.0 (4a-C), 124.4 (2-C), 123.2 (1-C), 122.2 (80-C), 119.1 (3-C), 98.5 (7-C), 91.6 (5-C), 61.0 (OCH₂CH₂), 55.7 (OCH3), 55.3 (OCH3), 20.9 (CH3COO) and 14.4 (OCH₂CH3). 1. Calculate the percentage yield of step (a), step (b) and the overall yield of the substituted naphthalene over the two steps. [3] 2. Condensation of aromatic aldehydes with diethyl succinate [step (a)] is called Stobbe Condensation. Please give the detailed reaction mechanism for this reaction. [7] 3. What is Celite? Please google it and explain in not more than a few sentences. [3] 4. The Product of step b, the substituted Naphthalene, was purified by column chromatography, followed by recrystallization. How does recrystallization work? Please explain in details. [5] 5. The melting point of the product is given as a range within 1 degree Celsius, and not as one value. Why do you think that is the case? A literature value is also given for comparison; what is the significance of this? [2] 6. On the basis of the given ¹H- and ¹C-NMR spectral data, please decipher the structure of the substituted naphthalene [the product of step (b)]. Give a detailed explanation of how you arrived at the structure. [20] TOTAL 40 MARKS There are several methods of making naphthalenes, one of which is given in the experimental procedure below. Please read it carefully and answer the questions that follow. OCH3 D[+] (a) H₂CO CH₂O H₂CO OH (b) SUBSTITUTED NAPHTHALENE Step (a): To a solution of 2,4-dimethoxybenzaldehyde ( (5.45 g. 32.80 mmol) and diethyl succinate (8. 18 cm 3, 8.57 g, 49.20 mmol) in dry t-butyl alcohol (80 cm³) stirred under nitrogen, was added potassium-t-butoxide (5.52 g, 49.19 mmol). The mixture was heated under reflux for 2 hours then allowed to cool to room temperature, poured into a separatory funnel containing ice and acidified to pH 3 with cone. HCl(aq). The product thus precipitated was extracted into EtOAc (2x 100 crn³), the combined extracts dried with magnesium sulfate, filtered through Celite and the filtrate concentrated on a rotary evaporator. The resultant oil was purified by column chromatography (10% EtOAc-hexane) to give the desired intermediate that was not characterized but immediately carried through step (b) below. Step (b): To the Stobbe condensation product from step (a) above, dissolved in acetic anhydride (100 cm³), was added anhydrous sodium acetate (6.72 g, 81.91 mmol) and the mixture heated at 140 "C under argon for 2 hours then allowed to cool. Acetic anhydride was removed in vacuo, water (100 cm³) was added, and the product extracted with dichloromethane (3 x 100 cm³). Drying of the combined extracts (MgSO4), filtration through Celite and removal of the solvent on a rotary evaporator gave a dark brown semisolid which was purified by column chromatography (30% EtOAc-hexane, containing 2% v/v dichloromethane) to give an orange-white solid which was recrystallised from EtOAc to give the desired SUBSTITUTED NPHTHALENE (8.10 g) as yellow crystals (mp. 157-158 "C) in two crops. Lit. 158-159 °C. ¹H NMR/300 MHz/8 (ppm): 8.79 (1H, d, J 1.1, 1-H), 7.81 (1H, d, J1.1, 3-H), 6.66 (1H, d, J 1.9, 5-H), 6.51 (1H, d, J 1.9, 7-H), 4.41 (2H, q, J 7.1, OCH₂CH3), 3.97 (3H, s, OCH3), 3.91 (3H, s, OCH₂), 2.45 (3H, s, CH₂COO) and 1.42 (3H, t, J 7.1, OCH₂CH₂). ¹³C NMR/75 MHz/ 6 (ppm): 169.2 (CH CO), 166.2 (ArCOOEt), 160.9 (6-C), 157.8 (8-C), 145.3 (4-C), 131.0 (4a-C), 124.4 (2-C), 123.2 (1-C), 122.2 (80-C), 119.1 (3-C), 98.5 (7-C), 91.6 (5-C), 61.0 (OCH₂CH₂), 55.7 (OCH3), 55.3 (OCH3), 20.9 (CH3COO) and 14.4 (OCH₂CH3). 1. Calculate the percentage yield of step (a), step (b) and the overall yield of the substituted naphthalene over the two steps. [3] 2. Condensation of aromatic aldehydes with diethyl succinate [step (a)] is called Stobbe Condensation. Please give the detailed reaction mechanism for this reaction. [7] 3. What is Celite? Please google it and explain in not more than a few sentences. [3] 4. The Product of step b, the substituted Naphthalene, was purified by column chromatography, followed by recrystallization. How does recrystallization work? Please explain in details. [5] 5. The melting point of the product is given as a range within 1 degree Celsius, and not as one value. Why do you think that is the case? A literature value is also given for comparison; what is the significance of this? [2] 6. On the basis of the given ¹H- and ¹C-NMR spectral data, please decipher the structure of the substituted naphthalene [the product of step (b)]. Give a detailed explanation of how you arrived at the structure. [20] TOTAL 40 MARKS

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