Interaction of lactic acid and ethyllactate with aldehydes

Abstract

Lactic acid is important representative of alpha-hydroxycarboxylic acids. It combines properties both acids and alcohols. Interaction of alcohols with aldehydes that lead to formation of acetals is well known whereas only a few communication about interaction carbonic acids with aldehydes may be found. It is difficult to predict which hydroxyl group will react with alcohol. However, in case of esters of lactic acid alpha-hydroxyl is only available. We have investigated the interaction of lactic acid and ethyl lactate with aldehydes – ethanal and butanal over ion exchange resin as a strong acidic catalyst at a temperature range of 20–100 ⁰C and 0.1 MPa in a fixed bed flow reactor. Gas chromatography and ¹³C NMR have been used for reaction products identification. The reaction enters both an alcohol hydroxyl in the alpha position of lactic acid and hydroxyl of the carboxyl group to form hemiacetals. Subsequent reaction is elimination of water or alcohol and formation of oxolane cycle. The final reaction products are mainly complete internal cyclic acetals – oxolanones. In the reaction with the aldehyde, both lactic acid itself and its ethyl or methyl ester produce the same products. 5-methyl-2-propyl-1,3-dioxolan-4-one was formed in reaction of lactic acid or ethyl lactate or methyl lactate with butanal and 2,5-dimethyl-1, 3-dioxolan-4-one – in the reaction with ethanol. High selectivity toward 5-methyl-2-propyl-1,3-dioxolan-4-one (up to 80 %) is observed at low temperature (below 40 ⁰C) and lactate conversion is 38 %. At higher temperature selectivity decreases because of byproducts formation such as aldol condensation products as well as acetal of aldehyde and alcohol to be eliminated. In addition, formation of butylydenlactate was observed in minor quantities.