Author Topic: Complete preparation of formic acid  (Read 3234 times)

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claude

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Complete preparation of formic acid
« on: August 25, 2004, 03:45:00 AM »
Considering the posts about the synthesis of formic acid, the one on Rhodium's site, and Mann and Saunders "Practical Organic Chemistry", is that a good resume ?

The glycerol is first heated to 175-180°C for five minutes, with stirring, to evaporate the water. Then, while the glycerol is still warm, 63g (50 mL) are added to a 250 mL distilling flask containing 40 of crystalline oxalic acid (the dihydrate forms (COOH)2,2H2O). A thermometer is addapted (and well immersed in the mixture), and a water-condenser is fitted to the flask. The mixture is heated to 110-120°C, and the temp must remain within these limits. There is a release of CO2, and formic acid begins to distill. When effervescence tends to subside, let the temp to fall to 70-80°C, add 40g of powdered oxalic acid, and continue the heating as above. 25-30mL of distillate are obtained in 1 hour of heating.

The reaction (according to the authors) is :

CH2(OH)-CH(OH)-CH2(OH) + (COOH)2 --> H2O + CH2(OH)-CH(OH)-CH2OCOCOOH (named A)

then when temp reaches 100°C, A is decomposed :

CH2(OH)-CH(OH)-CH2OCOCOOH --> CO2 + CH2(OH)-CH(OH)-CH2OCOH (named B)

And, on further heating, B is hydrolysed to glycerol and the formic acid :

CH2(OH)-CH(OH)-CH2OCOH + H2O --> CH2(OH)-CH(OH)-CH2(OH) + HCOOH

The first reaction is an esterification, which is an equilibrum, but the second one is entirely moved on the right side, due to the release of CO2. Consequently, the first reaction is also entirely moved on the right side.

Formic acid and water form an azeotrope which boils at 107.6°C. It's composed of 77.6% of formic acid and 22.4% of water.
So, the distillate from the reaction above (which is a dilute aqueous solution, concentration < 77.6%) is boiled in a flask fitted with a Vigreux column and a thermometer. Temp will reach until 107.6°C, wich is the azeotrope. Then stop the boiling. The flask contains 77.6% formic acid, which can be used in a performic oxydation.