Here is a moderately interesting preparation of a benzaldehyde from its corresponding benzoic acid, taken from J. Org. Chem. 435-439 (1964). What I find hilarious with this synthetic sequence is the absurdly excessive amounts of solvents and reagents used - In the workup of 3,5-Dimethoxybenzyl Alcohol, a little over 20g of product is taken up in three liters of diethyl ether, and then washed with with two liters of water, and one liter each of aqueous NaOH and brine! Then, to produce 12g of the corresponding benzaldehyde, five liters of hot chloroform is used in the reaction and workup... If someone was caught wasting so much chemicals these days, they would be fired on the spot
3,5-Dimethoxybenzyl Alcohol
3,5-Dimethoxybenzoic acid (25.0g, Rf 0.04) was dissolved in warm, dry tetrahydrofuran (400 mL), and this warm solution was added continuously over 90 min to a gently refluxing suspension of lithium aluminum hydride (10 g) in tetrahydrofuran (100 mL). When the addition was complete the mixture was boiled under gentle reflux for 4h, then cooled (ice-water bath). The excess hydride and the complex were decomposed by cautious addition of ethyl acetate (60 mL), water (60 mL), and 2N aqueous hydrochloric acid (100 mL) . The volume of the resulting suspension was reduced to 150 mL (rotatory evaporator, bath temperature of 40°C), diluted by addition of water (200 mL), and extracted with 6x500mL ether. The ether extract was washed with water (1 L), 2N aqueous sodium hydroxide (1 L), water (1 L), and brine (1 L), then dried over sodium sulfate. The solvent was eliminated from this dried extract, leaving a colorless solid (22.9g), which was crystallized from hexane, giving 3,5-dimethoxybenzyl alcohol as needles (21.8g, 94%), mp 46.5-47°C (lit.[7] mp 47-48°C); TLC Rf 0.14.[6]
3,5-Dimethoxybenzaldehyde
Freshly prepared "active" manganese dioxide [8] (201g) was added to a solution of 3,5-dimethoxybenzyl alcohol (17.8g) in dry, ethanol-free chloroform (2 L), and the mixture was stirred in an atmosphere of nitrogen for 24 h at room temperature. The mixture was filtered, and the residue was washed with 6x500mL portions of boiling chloroform. The solvent was eliminated from the combined filtrate and washings, leaving a colorless crystalline solid (13.9g), mp 43-45°C. This was crystallized from pentane, giving 3,5-dimethoxybenzaldehyde as prisms (12.8g, 72%), mp 45-45.5°C (lit.[9] mp 48°C); TLC Rf 0.57.[6]
References & Notes
[6] Thin layer chromatography (TLC) was conducted using silica as the absorbant. 95:5 CHCl3/EtOAc as the developing solvent. and iodine as the visualizing agent; Rf values reported are only reproducible to ±0.05.
[7] J. Am. Chem. Soc. 70, 664 (1948)
[8] J. Chem. Soc., 1094 (1952)
[9] J. Am. Chem. Soc. 76, 133 (1954)