HOOCCH2COOH

HOOCCH2COOH refers to malonic acid, also known as propanedioic acid. It is a small dicarboxylic acid consisting of a central methylene group flanked by two carboxyl groups, giving the formula C3H4O4. In its neutral form, it is a white crystalline solid.

Malonic acid is soluble in water and melts at around 135–137 °C, before it decomposes. It is a diprotic acid with two dissociable protons. Reported pKa values are approximately 2.83 for the first deprotonation and 5.69 for the second. The molecule can lose its two protons stepwise, forming malonate ions under basic conditions.

In chemistry, malonic acid is notable for its role in malonic ester synthesis. Deprotonation of the central methylene once, then alkylation at this activated methylene allows introduction of various substituents, followed by hydrolysis and decarboxylation to yield substituted acetic acids. Malonic acid esters also participate in Knoevenagel condensations and related carbon–carbon bond-forming reactions. A classic reaction is the decarboxylation of malonic acid on heating, which yields acetic acid with evolution of carbon dioxide (HOOC-CH2-COOH → CH3COOH + CO2).

Preparation typically involves hydrolysis or saponification of malonate esters, such as diethyl malonate, followed by neutralization to give the free acid. It can also be prepared from malonyl chloride or via oxidation routes of malonate derivatives, depending on the synthetic context.

Applications of malonic acid span organic synthesis, where it serves as a versatile building block for constructing substituted acetic acids and complex molecules. Safety considerations include handling as a hazardous chemical; appropriate precautions are recommended in laboratory use.