||In a 200 ml three-neck flask, 34.5 g of dimethyl 5-(3-hydroxy-3-methylbutyl-1-yl)isophthalate (the compound represented by the formula (2)), 60 g of water, and 2.2 g of ascorbic acid (a reducing agent) were added in a nitrogen stream, and stirred. Added thereto was 60 g of a 25% aqueous solution of sodium hydroxide, and thereafter, the mixture was heated and the temperature thereof was elevated to 80 C. The mixture was reacted at 80 C. for 4 hours, and thereafter, cooled down to 50 C. Added thereto was activated carbon, and stirred for 30 minutes. Subsequently, the activated carbon was removed by suction filtration using celite, and water used for washing a filter element was also added thereto. 50.7 g of 35.5% concentrated hydrochloric acid was added to the aforementioned aqueous solution, and the inner temperature was once elevated to 80 C., and thereafter, gradually cooled down to room temperature over 4 hours. A precipitated product was taken out by suction filtration, and washed with 50 g of running distilled water and dried by air at 50 C. for 2 days, whereby 22.6 g of white powder of 5-ethynylisophthalic acid was obtained (yield percent: 95.2%).The white powder thus obtained was analyzed based on HPLC. The area ratio of light absorbance at 254 nm was 97.8%, and 1,4-bis(3,5-dicarboxylphenyl)-1,3-butadiyne that is an oxidation by-product was almost not detected.
||5-ethylnylisophthalic acid was synthesized without using ascorbic acid in Example 2, and was taken out by distillation under reduced pressure (yield: 22.2 g, percent yield: 93.5%).This product was analyzed based on HPLC, and the area ratio of the light absorbance at 254 nm was 81.1%, and 1,4-bis(3,5-dicarboxylphenyl)-1,3-butadiyne that is an oxidation by-product was detected at the area ratio of 16. 9%.