Malonic Acid

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Malonic Acid IUPAC Name

Malonic acid is a dicarboxylic acid with structural formula CH2(COOH)2 and chemical formula C3H4O4. The name malonic acid originated from the word ‘Malon’ which is Greek for ‘apple’. The IUPAC name of malonic acid is Propanedioic acid. Methane Dicarboxylic acid is another name for malonic acid. The ester and salts of malonic acid are called malonates. The dicarboxylic acid has organic reactions similar to the monocarboxylic acid where amide, ester, anhydride, and chloride derivatives are formed. Lastly, the malonic ester malonate as a coenzyme A derivative malonyl CoA that is as important a precursor as Acetyl CoA in the biosynthesis of fatty acids. 

Synthesis of Malonic Acid 

  • The synthesis of malonic acid usually begins with chloroacetic acid. It is also synthesized by cyanoacetic acid or by acid saponification reaction of malonates. From monochloroacetic acid, it is produced by sodium or potassium cyanide. 

  • The sodium carbonate primarily breaks down to give sodium salt which reacts with sodium cyanide to give sodium salt of cyanoacetic acid by the process of nucleophilic substitution.

  • Further, via hydrolyzation, the nitrile group binds with sodium malonate, whose acidification results in the production of malonic acid. 

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Malonic Acid Structural Formula

The structural formula of malonic acid can be given as

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The malonic acid Lewis structure has been found by the X-ray crystallography method. The malonic acid structure CH2(COOH)2 has two carboxylic acids. The salts and esters of malonic acid (malonates) have structures similar to malonic acid.    

Properties of Malonic Acid 

  • Malonic acid molecular weight: 104.061 g.mol-1

  • The density of malonic acid is 1.619 g/cm3.

  • It appears as a crystalline powder that is white or colourless.

  • At the boiling point above 140oC the compound decomposes. The melting point is 135-137o C.

  • If heated to decomposition under fire it emits carbon oxide fumes and acrid irritating smoke.

  • Acidity pKa = 2.85 at 25oC. pKa1 = 2.83, pKa2 = 5.69 

  • The molar heat of combustion is 864 kJ/mol. The heat of vaporization is 92 kJ/mol.

  • It is soluble in water. Solubility 763 g/L.

  • It has a white crystal or crystalline powder structure. The compound is naturally occurring and can be found in many vegetables, fruits. The dicarboxylic acid compound was first prepared by Victor Dessaignes by the oxidation reaction of malic acid. 

Malonic Acid Uses

  • Malonic acid acts as a precursor for conversion to 1,3-propanediol, which is a compound used in polyesters and polymers with the huge market size.

  • Malonic acid is used for the preparation of cinnamic acid, a compound used for the formation of cin metacin which is an anti-inflammatory. The malonates are used in syntheses of B1 and B6, barbiturates, and several other valuable compounds. 

  • It is used in cosmetics as a buffering and as a flavouring agent in food.

  • Malonic acid is used as a component of alkyd resins, used in coating applications to protect from UV rays, oxidation, and corrosion.

  • Malonic acid is a building block to many valuable compounds in food and drug applications, pharmaceutical, electronics industry, fragrances, specialty polymer, specialty solvents, and many more.   

Health Hazards of Malonic Acid

  • Upon exposure, Malonic acid can cause a range of symptoms in the eyes, skin, and respiratory tract. It can irritate eyes, skin. The compound if inhaled can irritate the respiratory tract. It may also cause serious eye damage or eye irritation. Hence, if exposed eyes should be washed with normal water or with a saline solution. If redness or irritation is observed in the affected skin, the area should be immediately washed with water. The contaminated clothes should be removed.  

  • If ingested it may cause gastrointestinal irritation such as nausea, vomiting, etc. The mouth should be immediately rinsed with water several times and further medical help should be taken.

  • It is a strong irritant that can affect both skin and mucous membranes. 

  • If heated the compound releases toxic fumes of carbon dioxide and monoxide and gases that may irritate the respiratory tract if inhaled.  

FAQ (Frequently Asked Questions)

Q1. What is Knoevenagel Condensation of Malonic Acid?

This condensation reaction is named after Emil Knoevenagel. It is a nucleophilic addition reaction. In this reaction malonic acid or its diesters react with an aldehyde or ketone’s carbonyl group. This reaction is then followed by a dehydration reaction.

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Malonic acid when subjected to this reaction gives the desired product through a second step where removal or loss of carbon dioxide occurs. This modification step is called Doebner modification. An example of this so-called Doebner reaction would be the reaction between malonic acid and acrolein. The reaction product of this reaction is trans-2,4-Pentadienoic acid. Instead of two, this product has only one carboxylic acid group, due to the loss of carbon dioxide in the second step.

Q2. Write about the Role of Malonic Acid as an Inhibitor.

Malonic acid has a key role in biochemistry as a competitive inhibitor. It is the competitive inhibitor of the enzyme succinate dehydrogenase. In the respiratory electron transport chain, the compound malonic acid binds with the active site of the enzyme succinate dehydrogenase without any reaction. The competitive inhibitor resembles the actual substrate and binds at the active site of the enzyme.

The malonate and succinate exhibit very similar structure that causes this competitive inhibition. This inhibits the usual reaction that takes place upon binding with the substrate succinate. Malonic acid lacks the group -CH2CH2- thus inhibiting the dehydrogenation reaction. This inhibition reaction of the enzyme succinate dehydrogenase results in a decrease in cellular respiration.