Malic acid is a naturally occurring organic acid found in many fruits and vegetables. It has a tart taste and is used as a food additive to enhance the flavour of certain products. In addition to its culinary use, malic acid is also used in skin care products and dietary supplements due to its potential health benefits, such as improved energy metabolism and reduced muscle fatigue.

• IUPAC name for malic acid is 2-hydroxy butane dioic acid.
• Chemical formula of malic acid is C₄H₆O₅

Let’s learn about Malic Acid its structure, properties and others in detail.

Structure of Malic Acid

Malic acid is a four-carbon dicarboxylic acid, meaning it contains two carboxyls (-COOH) functional groups. The chemical formula for malic acid is C₄H₆O₅, which indicates that it consists of four carbon atoms, six hydrogen atoms, and five oxygen atoms.

The carbon atoms in malic acid are bonded in a chain, with the carboxyl groups located at opposite ends of the chain. The hydroxyl group is located on one of the middle carbon atoms, adding to the reactivity and versatility of the molecule.

The chemical formula of malic acid can be written in a few different ways to represent its molecular structure and properties. In addition to the empirical formula (C₄H₆O₅), malic acid can also be written using a Lewis structure or a line-angle formula. These formulas provide different levels of detail and insight into the molecule’s structure and behaviour.

The image given below shows the structure of the Malic Acid,

 

The condensed structural formula of malic acid can be represented as follows HOOC-CH₂-CH(OH)-COOH

In this condensed formula, the carbon atoms are represented by the letter “C”, the oxygen atoms are represented by the letter “O”, and the hydrogens are not explicitly shown. The carboxyl group (-COOH) is attached to the first carbon atom, and the hydroxyl group (-OH) is attached to the third carbon atom.

Physical Properties of Malic Acid

The physical properties of Malic Acid can be studied with the help of the table given below,

Chemical Properties Malic Acid

Various chemical properties of Malic Acid are,

• Malic acid is a weak organic acid with a pKa of 3.4
• Malic acid is a reactive molecule due to the presence of both a hydroxyl group and a carboxyl group, which can participate in various chemical reactions.
• Malic acid can undergo decarboxylation, a reaction that results in the loss of a carboxyl group and the formation of a new molecule
• Malic acid can be reduced to form the alcohol lactol or the aldehyde tartaric aldehyde
• Malic acid can undergo isomerization to form tartaric acid
• Malic acid can form complexes with metal ions such as calcium and magnesium
• Malic acid can undergo hydration, a reaction in which water is added to the molecule.

Various chemical reactions of Malic Acid are,

Esterification Reaction

Esterification is a reaction in which an acid reacts with an alcohol to form an ester and water. In the case of malic acid, esterification can occur as follows

C₄H₆O₅ + ROH -> C₄H₆O₄-R + H₂O

where “R” represents an alcohol molecule, and “C₄H₆O₄-R” represents the resulting ester.

In this reaction, the hydroxyl group (-OH) from the alcohol reacts with the carboxyl group (-COOH) from the malic acid to form an ester bond (-COOR) and water. The resulting ester is characterized by its distinctive flavour and fragrance and is commonly used in the food and fragrance industries.

Esterification reactions are typically catalyzed by acids or bases, and the conditions and reaction rate can be controlled by adjusting the concentration of reactants, the presence of catalysts, and the temperature.

Reaction With Base

Malic acid can react with a base to form its conjugate base, salt, and water. The reaction can be represented as follows

C₄H₆O₅ + MOH -> C₄H₆O₄-M + H₂O

Where “M” represents the base and “C₄H₆O₄-M” represents the salt formed by the reaction.

In this reaction, the hydroxyl group (-OH) from the base reacts with the carboxyl group (-COOH) from the malic acid to form the salt and water. The reaction is an example of proton transfer, as the hydrogen ion (proton) from the carboxyl group is transferred to the hydroxyl group of the base.

Methods of Preparations of Malic Acid

Various methods of preparation of Malic Acid are,

Reaction of Fumaric Acid with Sodium Hydroxide

The reaction of fumaric acid with sodium hydroxide refers to the chemical reaction where fumaric acid (C₄H₄O₄) and sodium hydroxide (NaOH) are combined to form malic acid (C₄H₆O₅) and sodium ions (Na⁺) and hydroxide ions (OH^–).

The overall equation for the reaction can be represented as

C₄H₄O₄ + 2 NaOH → C₄H₆O₅ + 2 Na⁺ + 2 OH^–

In this reaction, the fumaric acid molecule reacts with the sodium hydroxide to form a salt and a neutralized organic acid. The sodium ions and hydroxide ions form sodium hydroxide, which is a strong base commonly used in various industrial and laboratory processes.

Reaction of Sodium Bisulfite with Tartaric Acid

The reaction of sodium bisulfite with tartaric acid refers to the chemical reaction where sodium bisulfite (NaHSO₃) and tartaric acid (C₄H₆O₆) are combined to form malic acid (C₄H₆O₅) and sodium sulfate (Na₂SO₄).

The overall equation for the reaction can be represented as

C₄H₆O₆ + NaHSO₃ → C₄H₆O₅ + Na₂SO₄ + H₂O

In this reaction, the sodium bisulfite acts as a reducing agent and reacts with the tartaric acid to form malic acid and sodium sulfate. The reaction is an example of a reducing esterification reaction, where the tartaric acid molecule is reduced and the sodium bisulfite is oxidized. The water (H2O) is produced as a byproduct of the reaction.

Uses of Malic Acid

Various uses of Malic Acid are,

• Malic acid has several industrial and commercial uses, including
• Food and Beverage Industry Malic acid is used as a flavour enhancer and acidity regulator in foods and beverages, such as soft drinks, fruit juices, candies, and chewing gums.
• Pharmaceutical Industry Malic acid is used in the production of some pharmaceutical products, such as antacids and multivitamin supplements.
• Cosmetics Industry Malic acid is used in skin care products and personal care items as an exfoliant, to help remove dead skin cells and improve the appearance of skin.
• Agricultural Industry Malic acid is used as a plant growth regulator and to improve the yield and quality of crops.
• Photography Malic acid is used in the development of photographs in the silver halide process.
• Textile Industry Malic acid is used in the dyeing process of textiles to improve the colourfastness and brightness of the dyes.
• Chemical synthesis Malic acid can be used as a starting material in the synthesis of various chemical compounds, such as chiral compounds and polymers.

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FAQs on Malic Acid

Question 1: What is the chemical formula for malic acid?
Answer

 The chemical formula for malic acid is  C₄H₆O₅.

Question 2: What is the IUPAC name for malic acid?

Answer

 The IUPAC name for malic acid is 2-Hydroxy Butane Dioic Acid.

Question 3: What are some uses of malic acid?

Answer 

Malic acid is used in the food and beverage industry as a flavour enhancer and acidity regulator, in the pharmaceutical industry for antacids and multivitamin supplements, in cosmetics as an exfoliant, in agriculture as a plant growth regulator, in photography, in the textile industry, and as a starting material in chemical synthesis.

Question 4: What is the esterification reaction of malic acid?

Answer

The esterification reaction of malic acid occurs when an acid reacts with an alcohol to form an ester and water. The reaction can be represented as,

C₄H₆O₅ + ROH -> C₄H₆O₅-R + H₂O

where “R” represents an alcohol molecule and “C₄H₆O₅-R” represents the resulting ester.

Question 5: What happens when malic acid reacts with a base?

Answer

When malic acid reacts with a base, it forms its conjugate base, salt, and water. The reaction can be represented as 

C₄H₆O₅ + MOH -> C₄H₆O₅-M + H₂O

where “M” represents the base and “C₄H₆O₅-M” represents the salt formed by the reaction.