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When a mixture of anhydrous glycerol and crystalline oxalic acid, is heated the glycerol undergoes esterification, giving first glyceryl monoxalate, the latter, however, decomposes as the temperature reaches about 100°C, losing carbon dioxide and giving glyceryl monoformate. On further heating, particularly if more oxalic acid is added, the monoformate is hydrolysed (the necessary water being provided both by the oxalic acid and by the first reaction), and consequently a distillate of aqueous formic acid is obtained.
Note that allyl alcohol is produced in the reaction. This can be used for many purposes, see the Allyl Alcohol Preparation document.
Required: Glycerol 70ml; oxalic acid 40g.
Since glycerol is a very hygroscopic substance, it is necessary first to ensure that the sample used is anhydrous. For this purpose, place about 70ml in a porcelain evaporating-basin, and heat it carefully over a gauze (preferably in a fume-cupboard), stirring it steadily with a thermometer until the temperature is 175-180°C then maintain this temperature for a further 5 minutes. Allow the glycerol to cool, but while it is still warm (i.e., before it becomes viscous) pour 50ml (63g) into a 250 ml distilling flask containing 40g of powdered crystalline oxalic acid. Fit a thermometer in the flask so that the bulb is completely immersed in the glycerol mixture, and then fit a water-condenser to the flask. Heat the mixture carefully over a gauze so that the temperature rises to 110-120°C, and then adjust the heating so that the temperature remains within these limits. A vigorous effervescence of carbon dioxide occurs, and the aqueous formic acid begins slowly to distill over. When the effervescence tends to subside, remove the Bunsen flame and allow the temperature to fall to 70-80°C: then add a further 40g. of powdered oxalic acid, and continue the heating as before. Ultimately 25-30ml of distillate is obtained, the total period of heating being about 1 hour.
A good yield of formic acid cannot be obtained by merely heating oxalic acid, as a certain portion of the oxalic acid sublimes unchanged. The oxalic acid is therefore heated with glycerol when carbon dioxide and glyceryl monoformate are obtained, and the latter when boiled with water yields formic acid and glycerol.
30g of glycerol is weighed in an evaporating dish and dehydrated by heating on a sand bath until a thermometer immersed in the liquid indicates a temperature of 176-180°C. The mass is then introduced into a 250ml distilling flask and 30g of oxalic acid dihydrate is added. The side tube of the flask is attached to a condenser, and a thermometer introduced through the neck of the flask dips into the liquid. The mixture is heated gradually on a sand bath, and the temperature maintained at about 110°C until the evolution of carbon dioxide gas slackens. When this occurs, another 30g of oxalic acid is added. The heating is renewed, the reaction recommences, and an aqueous solution of formic acid distills over into the receiver. The reaction is a continuous one, and an abundant supply of the distillate may be obtained by the addition of similar quantities of oxalic acid at intervals. The water of crystallization of the oxalic acid serves to hydrolyze the glyceryl monoformate. The residue in the flask after each distillation contains this ester. The product in this experiment is an aqueous solution of formic acid.
The glycerol is dehydrated by heating it gently in a basin on a sand-bath until a thermometer with the bulb immersed in the liquid indicates 175°. Fifty grams of commercial crystallised oxalic acid and 5o grams of glycerol are heated in a retort (250 c.c.) over wire-gauze, with condenser and receiver. A thermometer is fixed through the tubulus with the bulb in the liquid. The reaction begins at about 8o°, and at 90° proceeds briskly, carbon dioxide being evolved. The temperature is maintained at 105-110° until the evolution of gas has slackened. Some aqueous formic acid has meanwhile collected in the receiver. The contents of the retort are now cooled to about 80° and a further 50 grams of oxalic acid added. The reaction recommences on heating with the formation of aqueous formic acid, which becomes more concentrated with each fresh addition of oxalic acid until the distillate eventually contains 56 per cent. of acid. The other portions of oxalic acid are added in the same way. In order to regain the formic acid which remains as monoformin in the retort, the contents are transferred to a round flask, diluted with about 250 c.c. of water and distilled in steam, until the distillate has only a faintly acid reaction (about 250 mL).
Distillation in Steam: A large flask, or preferably, a 1 gallon tin is closed by a double bored cork. A safety-tube passes through one hole, and a bent tube which terminates below the Cork passes through the second hole, and is attached by rubber tubing to the inlet tube of the distilling flask (1 litre). The flask is sloped to prevent the contents being splashed over into the condenser. It is heated on the sand bath or asbestos board to boiling, and steam passed in. The united distillates are poured into a basin and neutralised by adding lead carbonate until, on heating, no further effervescence occurs. The liquid is now left for a moment to settle, and the clear solution decanted, whilst hot, through a fluted filter. The residue in the basin is boiled up again with a volume of water equal to that decanted, and again a third and fourth time, and filtered hot each time until no more lead formate is dissolved. The lead formate will have now passed into solution and the liquid is then evaporated down on a sand-bath or ringburner, until crystals appear on the surface, when the liquid is put on one side to cool. Lead formate crystallises out in long white needles. Yield about 150 grams. In order to obtain pure formic acid, hydrogen sulphide is passed over the heated lead salt. It is carried out as follows:
The powdered salt, dried on the waterbath, is introduced in a long layer into a sloping wide tube, loosely stopped at the lower end by a plug of glass wool or asbestos. To the lower end of the tube a receiver, in the form of a distilling flask, is attached, which is protected from moisture by a drying-tube. The salt is heated gently by moving a flame along the tube whilst hydrogen sulphide, washed through water, and dried by passing through a U-tube containing calcium chloride, is led over the salt in not too rapid a stream. The lead formate blackens, and is slowly converted into lead sulphide and formic acid, which drops into the receiver. The acid, which retains a strong smell of hydrogen sulphide, is freed from the latter by distillation over a little dry lead formate. Yield is nearly theoretical.
Formic acid data and purification4
Formic acid [64-18-6] mw 46.0 (anhydr.), Fp. 83°C, b 25°C/40mm, 100.7°C/760mm, n 1.37140, n(25) 1.36938, d 1.22. Anhydrous formic acid can be obtained by direct fractional distillation under reduced pressure, the receiver being cooled in ice-water. The use of P2O5 or CaCl2 as dehydrating agents is unsatisfactory. Reagent grade 88% formic acid can be satisfactorily dried by refluxing with phthalic anhydride for 6 hr and then distilling. Alternatively, if it is left in contact with freshly prepared anhydrous CuSO4 for several days about one half of the water is removed from 88% formic acid: distn. removes the remainder. Boric anhydride (prepared by melting boric acid in an oven at a high temperature, cooling in a desiccator, and powdering) is a suitable dehydrating agent for 98% formic acid; after prolonged stirring with the anhydride the formic acid is distd. under vac. Formic acid can be further purified by fractional crystn. using partial freezing.
75% Formic acid from Sodium Formate3
An aqueous solution of formic acid is obtained by distilling the sodium salt with dilute sulphuric acid:
2 HCO2Na + H2SO4 = 2 HCO2H + Na2SO4
Anhydrous formic acid is obtained from the aqueous solution (70-77%) by the addition of butyl formate followed by distillation. The first fraction is an azeotrope of ester and water, and then the excess of ester is removed from the formic acid by fractionation.
Do not mix concentrated sulfuric acid with sodium formate or anhydrous formic acid.
The formic acid will be oxidized to the very toxic gas carbon monoxide by sulfuric acid!
Formic acid from Lead Formate1
Anhydrous Formic acid can be obtained from Lead formate by reaction with hydrogen sulphide [this is given for information purposes only]. Note that several clandestine chemists have died as a result of inhaling hydrogen sulphide fumes. H2S is nasty.
To obtain lead formate, add about 100 mL of water to the distillate [from the prep. of formic acid] and stir powdered lead carbonate into the gently heated solution until no further effervescence occurs. The boil the mixture vigorously and filter at the pump. Evaporate the clear filtrate by direct boiling until crystals appear on the surface, and then allow to cool, finally chilling in ice-water. The lead formate separates as colourless crystals; filter off, wash with a small quantity of cold water, and dry. Yield, about 6 gms.
Lead formate is only slightly soluble in cold water, and insoluble in hot absolute alcohol: it can therefore be readily distinguished from lead acetate or "sugar of lead" because, quite apart from chemical tests, the acetate is readily soluble in cold water and moderately soluble in alcohol.
Lead formate separates from aqueous solution without water of crystallisation. It is therefore frequently used for the preparation of anhydrous formic acid. For this purpose, the powdered lead formate is placed in the inner tube of an ordinary jacketed condenser, and there held loosely in position by plugs of glass-wool. The condenser is then clamped in an oblique position and the lower end fitted into a receiver closed with a calcium chloride tube. A current of dry hydrogen sulphide is passed down the inner tube of the condenser, whilst steam is passed through the jacket. (ie. the reaction takes place at 100°C) The formic acid which is liberated [Pb(HCOO)2 + H2S → 2 HCOOH + PbS] collects in the receiver, and is then purified from dissolved hydrogen sulphide by redistillation over a further quantity of lead formate.
Many aliphatic compounds are oxidised by concentrated nitric acid, the carbon atoms being split off in pairs, with the formation of oxalic acid. This disruptive oxidation is shown by many carbohydrates, e.g., cane sugar, where the chains of secondary alcohol groups, present in the molecule break down particularly readily to give oxalic acid.
Required: Cane sugar 30g; nitric acid, 150ml.
Owing to the copious evolution of nitrous fumes, this preparation must be carried out in a fume-cupboard having an efficient draught. Place 30g of coarsely powdered cane sugar (sucrose) in a 750 ml flat-bottomed flask, add 150ml of concentrated nitric acid and heat the flask on a boiling water-bath. As the mixture becomes warm, the greater part of the sugar dissolves and a vigorous but harmless reaction, accompanied by a tremendous evolution of nitrous fumes, takes place. Immediately the evolution of gas starts, remove the flask from the water-bath and place it on a wooden block or some similar non-conducting surface. When the reaction subsides (after about 15 minutes) pour the hot solution into an evaporating-basin, wash out the flask with about 20 e.e. of concentrated nitric acid, and then evaporate the acid solution on the water-bath until it has a volume of about 25ml. Some oxidation continues in the solution during the evaporation, which is comparatively rapid. Now add about 50 ml of water to the solution, and again evaporate to about 25ml. Cool the solution thoroughly in ice-water; oxalic acid rapidly crystallizes. When crystallization is complete, filter at the pump, and then recrystallize from a small quantity of hot water. Dry by pressing between pads of drying-paper, or in an atmospheric desiccator, but not in an oven where partial loss of water of crystallization may occur. Yield, 10-11g. The hydrated acid has m.p. 101°C; the anhydrous acid decomposes on heating.