Method for producing ditrimethylolpropane

A method for producing ditrimethylolpropane represented by the following formula 1:

• which has a melting point of 109° C. , a phthalic acid resin color of 1.0, and a purity of 96 to 100%,
• the method consisting essentially of:
• a step of reacting n-butyl aldehyde and formaldehyde in the presence of a basic catalyst to obtain a reaction product solution;
• a step of extracting the reaction product solution with an organic extractant to obtain an extract;
• a step of distilling the extract to separate trimethylolpropane as a distillate and obtain a distillation still residue;
• and
• a step of adding an organic solvent to the distillation still residue in an amount of 0.6 to 10 times a weight of the distillation still residue, the organic solvent being at least one compound selected from the group consisting of acetone and methyl ethyl ketone, and treating the distillation still residue, in a single treatment operation at a crystallization temperature Y ° C. satisfying the following formula 3:
• (30×ln(X)−92)≦Y≦55 (3)
• wherein X is a percent by weight concentration of ditrimethylolpropane in the distillation still residue before adding the organic solvent, for 1 to 4.5 h, thereby crystallizing said ditrimethylolpropane having said melting point, said phthalic acid resin color and said purity.