Classification and major representatives
Heterofunctional compounds containing a carboxyl group are classified according to the nature of the second functional group. The first category includes keto acids, which contain a carbonyl (keto) group. The second category includes hydroxy acids, which contain a hydroxyl group. The presence of an acidic group can be identified using indicators or by measuring the pH of the medium.
Inside a living organism, these substances are rarely found in a free state. Since the intracellular environment is mildly alkaline, the acids dissociate and exist in the form of anions. The main natural representatives:
- Pyruvic acid (keto acid). Its salts are pyruvates.
- Lactic acid (hydroxy acid). Forms salts known as lactates.
- Oxaloacetic acid (dicarboxylic keto acid). Salts are oxaloacetates.
- Malic acid (dicarboxylic hydroxy acid). Salts are malates.
- Citric acid (tribasic hydroxy acid). Forms citrates. The presence of three –COOH groups at once makes it a prime example of tribasic acids.
The presence of additional functional groups pulls electron density away and enhances the acidic properties of the compound. It is precisely this effect that determines the sharp sour taste of lemons or unripe apples.
Spatial structure and enantiomerism
An important property of many heterofunctional acids is their ability to exist as enantiomers. The essence of this phenomenon lies in spatial asymmetry: the molecular forms relate to each other like right and left hands, being non-superimposable mirror images.
The condition for such asymmetry to arise is the presence of an asymmetric carbon atom. This atom (chiral center) must be covalently bonded to four different substituents. As an example, we can consider the structure of lactic acid. Its asymmetric atom is located at the center of an imaginary tetrahedron, toward the vertices of which bonds are directed with the groups (–COOH, –CH3, –OH, –H).
During spatial rotation, one such tetrahedron cannot be superimposed on another if the substituents are different. This proves the existence of two independent molecular forms — $D$-lactic and $L$-lactic acids. Malic acid possesses similar enantiomerism.
Similar asymmetry requirements are applicable to other classes of substances as well. For example, for optical isomerism to arise in saturated alcohols, the molecule must contain at least four carbon atoms. The simplest such alcohol is butan-2-ol, where the asymmetric carbon atom is bonded to a hydrogen atom, a hydroxyl group, a methyl radical, and an ethyl radical.
Chemical properties of hydroxy acids
The chemical behavior of hydroxy acids displays the properties of both functional groups. The most characteristic processes are esterification reactions.
1. Intermolecular esterification The carboxyl group of a hydroxy acid is capable of reacting with alcohols in the presence of an acid catalyst (e.g., sulfuric acid). The reaction of 2-hydroxypropanoic (lactic) acid with ethanol yields ethyl lactate (ethyl lactate) and water is released.
2. Intramolecular esterification If functional groups are in the same molecule at a suitable distance, they interact with each other. The hydroxyl group reacts with the carboxyl group, closing the ring and forming an internal ester — lactone. The lactone ring always includes the oxygen atom of the ester group. A characteristic example is the cyclization of 4-hydroxybutanoic acid with the elimination of water.
Effect of structure on acid strength and hydrolysis
The structure of the hydrocarbon radical directly affects the acid strength and the chemical equilibrium in salt solutions. If a halogen is present in the molecule, it exhibits a negative inductive effect (–I). By pulling electron density toward itself, the halogen increases the polarity of the O–H bond, which facilitates proton dissociation.
For this reason, monochloroacetic acid is much stronger than propionic acid. When propionic acid reacts with sodium monochloroacetate, the equilibrium shifts to the left (toward the starting materials), since the weaker propionic acid is unable to displace the strong monochloroacetic acid from its salt.
It is also important to distinguish acids from their functional isomers — esters. Molecules with the same empirical formula behave differently. For example, compounds with the formula $C_3H_6O_2$ can be esters (methyl ethanoate, ethyl methanoate) and readily undergo hydrolysis. Carboxylic acids do not enter into hydrolysis reactions.
The diversity of structures is clearly visible in the example of isomers with the formula $C_4H_8O_2$:
- Carboxylic acids: butanoic (butyric) and 2-methylpropanoic (isobutyric) acids.
- Esters: methyl propanoate, ethyl acetate, propyl formate, and isopropyl formate.
Briefly: answers to topical questions
Which keto acids are the most important representatives in the human body?
The most important representatives of keto acids in the human body are pyruvic, oxaloacetic, and α-ketoglutaric acids.
- Pyruvic acid is a keto acid that forms pyruvate anions.
- Oxaloacetic acid is a dicarboxylic keto acid that forms oxaloacetate anions.
- α-Ketoglutaric acid (α-ketoglutarate) is produced in the body in amounts of hundreds of grams per day and participates in transamination and oxidative decarboxylation reactions.
Which hydroxy acids are intermediate metabolites of the Krebs cycle?
Natural hydroxy acids indicated in the source as major metabolites include:
- Malic acid — a dicarboxylic hydroxy acid that forms malate salts.
- Citric acid — a tribasic hydroxy acid containing three carboxyl groups, forming citrate salts.
These acids participate in cellular glucose oxidation processes, including the Krebs cycle.
Class and exam questions
In what form do heterofunctional acids exist within body cells?
The internal environment of cells is mildly alkaline, which is why molecules ionize and exist in the form of salts (anions) — lactates, pyruvates, citrates.
What is required for enantiomers to arise?
The molecule must contain an asymmetric carbon atom covalently bonded to four different substituents.
How are lactones formed?
Lactones are formed as a result of intramolecular esterification: the hydroxyl and carboxyl groups of a single molecule react with each other, releasing water and forming a closed ring.
How does the equilibrium shift during the interaction of a weak acid with a salt of a strong acid?
The equilibrium shifts toward the formation of the weaker acid. For example, propionic acid will not be able to displace the stronger monochloroacetic acid from its salt.
What else to study on this topic
- The mechanism of the esterification reaction yielding ethyl lactate.
- Algorithm for calculating the molar mass of an acid from the volume of released carbon dioxide.
- The spatial arrangement of the asymmetric carbon atom tetrahedron.
- Evaluation of the negative inductive effect of substituents using examples.
- Conditions for the hydrolysis of carboxylic acid functional isomers.
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