In a
stainless-steel beaker of approximately 3-l. volume (180 mm. by 150 mm.) (Note
1), equipped with an
efficient mechanical stainless-steel stirrer and heated by an electric hot plate, are placed
84 g. (2 moles) of 97% sodium hydroxide pellets,
332 g. (5 moles) of 85% potassium hydroxide pellets (Note
2), and 50 ml. of water. The mixture is stirred and heated. When the temperature of the fluid mixture reaches 160°,
152 g. (1 mole) of vanillin is added in portions over a period of 2.5–3 minutes at a rate sufficient to maintain the reaction (Note
3). The temperature at this point is 190–195°. Stirring is continued, and heat is applied until the temperature reaches 240–245° (Note
4). The temperature is maintained at 240–245° for 5 minutes. The hot plate is removed, and the mixture is allowed to cool with stirring. When the mixture has cooled to about 150–160°, 1 l. of water is added, and the mixture is stirred until all the fusion mixture is dissolved. The solution is transferred to a
4-l. beaker, another 500 ml. of water is added, and
sulfur dioxide gas is introduced for 2 minutes (Note
5); the mixture is then completely acidified with
1.5 l. of 6 N hydrochloric acid. The acidified mixture is cooled in an
ice bath (5°) for 2 hours, and the crystalline precipitate is filtered, washed on the filter with two 100-ml. portions of ice water, and air dried. The tan crystals of
protocatechuic acid melting at
196–198° weigh
90–100 g. Extraction of the filtrate and washings with two
1-l. portions of ether yields an additional
45–55 g. of
protocatechuic acid melting at
190°. The total yield of crude
protocatechuic acid amounts to
135–153 g. (
89–99%) (Note
6).