US3755101A

Process for the preparation of saligenol by salicylic acid electrochemical reduction  Rhone Poulenc SA 1970
1337457 Preparing saligenol RHONEPOULENC SA 18 Nov 1971 [19 Nov 1970] 53627/71 Heading C2C [Also in Division C7]

 Saligenol (2-hydroxy-benzyl alcohol) is prepared by electro-chemical reduction of salicylic acid in a cell having cathode and anode compartments separated by a cation exchange membrane and a catholyte initially comprising salicylic acid, water, a cosolvent and a quarternary ammonium salt. The cosolvent may be an alcohol, ether, a nitrile or an aliphatic amide or ester, preferably ethanol or isopropanol. The quaternary ammonium salt may be a tetraalkylammonium sulphate, phosphate or halide. The catholyte can be one or two liquid phases, the latter preferably being an emulsion. The cathode may be Hg or a solid (e.g. lead) amalgam. The anolyte is preferably an aqueous acid solution, e.g. of H 2 SO 4 , H 3 PO 4 , HNO 3 , HCl, H 3 BO 3 or HBF 4 . The anode may be Pb or alloy thereof, Pt, platinized Ti or graphite. The cell may contain spacers to avoid contact of the membrane with the electrodes. Bells may be grouped in series as filter-press systems. After electrolysis, the cosolvent may be removed from unreduced salicylic acid, e.g. by distillation.

EXAMPLE 1 The electrical cell used is that shown in the FIGURE of the accompanying drawing. The cell comprises a main trough 1 having an outer jacket 2 through which liquid at a controlled predetermined temperature may be circulated. Main trough l is divided up into a cathode compartmentcontainingcatholyte 4 and cathode 3 and an anode compartment containing anolyte 7 and anode 6. Catholyte 4 and anolyte-7 are separated from one another by cation exchange membrane 5 and internal cell divider 9. The cathode compartment is provided with amag'netic stirrer 8, thermometer l0 and a gas outlet 12 to remove gas evolved from the cathode compartment. Outlet 12 leads to a gas flow rate mea suring device (not shown).
The electrolysisconditions are as follows: Cathode lead amalgam discof surface area 0.6 dm' (prepared by immersing a disc of lead in a bath mercury for 10 hours) i Catholyte initially a mixture consisting of 250 cm of isopropanol 80 cm of water 45 g of salicylic acid and 30 g of tetraethylammonium bromide.
- Cathode cation exchange membrane distance 2 cm. Cation exchange membrane heterogeneous-type membrane having a matrix of. polyvinylidene fluoride the cation exchange resin is a sulphonic resin bmd on polystyrene crosslinked with divinylbenaene' the ex change capacity of the membrane is 1.05 meg/g its substitution resistance is 9.6 0 cm! men 0.1 N aqueous solution of NaCl and 4.8 0 cm in a normal aqueous solution of NaCl the permeation selectivity of the mem- I brane, measured between two aqueous NaCl solutions which are respectively normal and half-normal, is 96.2
percent.
Ari'ol'y'te'i 10%5y weight aqueous solution of sulphuric ac d. Anode: lead disc of surface area 0.125 dm' Anode/cation exchange membrane distance l cm Temperature: between 30' and 35' C which ensures that the medium is homogeneous.
Current density at the cathode: 6.66 Aldm' until 61,910 coulombs have been passed thereafter 7.5
The electrolysis is stopped after 80,000 coulombs have passed. 3350 cmof hydrogen (measured at 0 C under a pressure of 760 mm Hg) are collected from outlet (12).
The catholyte is 20 mm of mercury, which allows the isopropanolto be removed and causes a precipitate to form. The precipitate is filtered off and washed with water, and the filtrate and the wash waters are combined and neutralised (pH I 7) with sodium bicarbonate. The mixture is extracted with ether, the ether phase driedby means of.
distilled under a reduced pressure of sodium sulphate and concentrated to dryness l4.8 g of saligenol are obtained.
The yield of saligenol based on the acid converted is 66.6 percent while the electricalyield is 57.6 percent.