The second name for malic acid is hydroxyntaric. It is a member of the hydroxy dicarboxylic acid class. The compound was first obtained by Karl Scheele (a Swedish chemist) from unripe apples (which determined its name) in 1785. Also in nature, it is found in grapes, barberry, mountain ash, raspberry, etc. In the form of salts, called malates, the substance is contained in tobacco. The maximum content of hydroxycyclic acid together with citric acid in unripe green apples reaches 1.2%.
Malic Acid: Formula
This chemical compound has the following formula:
Under normal conditions, malic acid is a colorless crystalline powder that dissolves well in alcohol (in 100 ml — 35.9 g) and in water (in 100 ml — 144 g). The molecular weight of the compound is 134.1 g / mol.
The picture below shows the spatial structure of the malic acid molecule. The carbon atoms are indicated in black, oxygen in red, hydrogen in white.
Malic acid exists in the form of a racemate (an optically inactive compound) and two stereoisomers. The latter are such compounds in the molecules of which between atoms there is the same sequence of chemical bonds, but there are differences in their arrangement in space relative to each other. In detail this issue deals with stereochemistry. In malic acid, there are two stereoisomers; it was by their example that P. Walden was the first to show in 1896 that interconversions of enantiomers are possible. The study of this phenomenon served as the foundation for the subsequent creation of a theory of the reaction of the so-called nucleophilic substitution at the carbon atom (saturated).
Substance is obtained in two ways: natural and chemical. The first offers extraction from fruits and berries. Synthetic malic acid is obtained by several reactions:
1. Hydration of maleic or fumaric acid. A prerequisite is a temperature of 100-150 ° C. The reaction equation is as follows:
HOOCCH = CHCOOH + H2O → HOOC-CH2 -CH (OH) - COOH
2. Hydrolysis of bromo- or chloro-succinic acid. Malic acid is extracted using ether. It uses plant material.
Apple acid. Interaction Reactions
1. Oxidation with concentrated sulfuric acid (H2 SO4 ) to form coumalinic acid. The reaction takes place in two stages:
Noos-SN2 -CH (OH) - COOH + H2 SO4 → HOOC-CH2 -CHO + HCOOH
As a result, aldehyde malonic and formic acids are formed. The latter compound decomposes to form carbon monoxide and water:
HCOOH → CO + H2O
Aldehyde malonic acid is immediately transformed into cumalin acid.
Noos-SN2 -CH (OH) - COOH + HCl → HOOC-CH2-CHCl-COOH
The resulting substance is called 2-chloroanthin.
3. Malic acid is susceptible to oxidation (in particular, when using KMnO4):
Noos-SN2 -CH (OH) - COOH + KMnO4 → HOOC-CH2-CO-COOH
The resulting acid is called 2-oxo-succinic (oxyl-acetic).
4. Interaction with acetyl chloride to form 2-acetoxysuccinic acid:
Noos-SN2 -CH (OH) - COOH + CH3COCl → HOOC-CH2-CH (OCOCH3) -COOH
With gradual heating, malic acid decomposes to form a series of intermediate products. At a temperature of 100 ° C, the formation of anhydrides occurs (they are similar to lactides). With an increase to 140-150 ° C, they are converted to fumaric acid. With a rapid increase in temperature to 180 ° C, maleic anhydride is obtained.
Summarizing all the data, we can say that the chemical properties of malic acid are the same as those of other hydroxy acids.
Malic acid is involved in the Krebs cycle. It represents the main stage in the respiration of all cells that use oxygen and is an intermediate link between glycolysis and the electric transport chain. The main role of the tricarboxylic acid cycle (Krebs) is the synthesis of reduced coenzymes FAD * H2 and OVER * N. They are subsequently used to produce ATP, ADP and phosphate. Hydroxy-succinic acid is formed by hydration with fumaric acid. Its subsequent oxidation with NAD + completes the Krebs cycle. The catalyzing enzyme is malate dehydrogenase.
Spheres of use
Obtained commercially malic acid is quite widely used:
- In the food industry, it is known under the code E296. The substance is used as a preservative, flavor enhancer and acidity regulator. Main application: carbonated drinks, fruit juices, confectionery, wine, canned food. It is worth emphasizing that in small quantities, malic acid has a positive effect on the body.
- In cosmetology. Oxycytic acid has antioxidant, whitening, exfoliating and moisturizing properties, which allows it to be used as part of anti-cellulite and skin whitening agents, peelings. In addition, it is added to toothpastes and oral care products.
- In pharmacology, malic acid (the formula given above) is used in the composition of expectorant and laxatives.