Carboxylic acids: physical properties. S alts of carboxylic acids

2024 Author: Angel Austin | [email protected]. Last modified: 2023-12-17 05:14

Almost everyone has vinegar at home. And most people know that its base is acetic acid. But what is it from a chemical point of view? What other organic compounds of this series exist and what are their characteristics? Let's try to understand this issue and study the limiting monobasic carboxylic acids. Moreover, not only acetic acid is used in everyday life, but also some others, and the derivatives of these acids are generally frequent guests in every home.

Class of carboxylic acids: general characteristics

From the point of view of the science of chemistry, this class of compounds includes oxygen-containing molecules that have a special grouping of atoms - a carboxyl functional group. It looks like -COOH. Thus, the general formula that all saturated monobasic carboxylic acids have is: R-COOH, where R is a radical particle that can include any number of carbon atoms.

According to this, the definition of this class of compounds can be given as follows. Carboxylic acids are organic oxygen-containing molecules, which include one or more functional groups -COOH - carboxyl groups.

The fact that these substances belong specifically to acids is explained by the mobility of the hydrogen atom in the carboxyl. The electron density is distributed unevenly, since oxygen is the most electronegative in the group. From this, the O-H bond is strongly polarized, and the hydrogen atom becomes extremely vulnerable. It is easily split off, entering into chemical interactions. Therefore, the acids in the corresponding indicators give a similar reaction:

phenolphthalein - colorless;
litmus - red;
universal - red;
methylorange - red and others.

Due to the hydrogen atom, carboxylic acids exhibit oxidizing properties. However, the presence of other atoms allows them to recover, to participate in many other interactions.

Classification

There are several main features by which carboxylic acids are divided into groups. The first of these is the nature of the radical. According to this factor, they distinguish:

Alicyclic acids. Example: cinchona.
Aromatic. Example: benzoic.
Aliphatic. Example: acetic, acrylic, oxalic and others.
Heterocyclic. Example: nicotine.

If we talk about bonds in a molecule, then we can also distinguish two groups of acids:

marginal - all connections onlysingle;
unlimited - available double, single or multiple.

Also, the number of functional groups can serve as a sign of classification. So, the following categories are distinguished.

Single-basic - only one -COOH-group. Example: formic, stearic, butane, valeric and others.
Dibasic - respectively, two groups -COOH. Example: oxalic, malonic and others.
Multibasic - lemon, milk and others.

Further in this article we will discuss only the limiting monobasic carboxylic acids of the aliphatic series.

Discovery history

Winemaking has flourished since antiquity. And, as you know, one of its products is acetic acid. Therefore, the history of the popularity of this class of compounds dates back to the time of Robert Boyle and Johann Glauber. However, for a long time it was not possible to elucidate the chemical nature of these molecules.

After all, the views of vitalists dominated for a long time, denying the possibility of the formation of organics without living beings. But already in 1670, D. Ray managed to get the very first representative - methane or formic acid. He did this by heating live ants in a flask.

Later, the work of scientists Berzelius and Kolbe showed the possibility of synthesizing these compounds from inorganic substances (by distillation of charcoal). The result was acetic acid. Thus, carboxylic acids (physical properties, structure) were studied and the beginning was laid for the discovery of allother representatives of a number of aliphatic compounds.

Physical properties

Today, all their representatives have been studied in detail. For each of them, you can find a characteristic in all respects, including application in industry and being in nature. We will look at what carboxylic acids are, their physical properties and other parameters.

So, there are several main characteristic parameters.

If the number of carbon atoms in the chain does not exceed five, then these are sharp-smelling, mobile and volatile liquids. Above five - heavy oily substances, even more - solid, paraffin-like.
The density of the first two representatives exceeds one. Everything else is lighter than water.
Boiling point: the larger the chain, the higher the value. The more branched the structure, the lower.
Melting point: depends on the evenness of the number of carbon atoms in the chain. Even ones have it higher, odd ones have it lower.
Dissolves very well in water.
Able to form strong hydrogen bonds.

Such features are explained by the symmetry of the structure, and hence the structure of the crystal lattice, its strength. The simpler and more structured molecules, the higher the performance that carboxylic acids give. The physical properties of these compounds make it possible to determine the areas and ways of using them in industry.

Chemical properties

As we have already indicated above, these acids can exhibit different properties. Reactions withtheir participation is important for the industrial synthesis of many compounds. Let's denote the most important chemical properties that a monobasic carboxylic acid can exhibit.

Dissociation: R-COOH=RCOO^- + H⁺.
Shows acidic properties, that is, it interacts with basic oxides, as well as their hydroxides. It interacts with simple metals according to the standard scheme (that is, only with those that stand before hydrogen in a series of voltages).
Behaves like a base with stronger acids (inorganic).
Able to be reduced to primary alcohol.
Special reaction - esterification. This is an interaction with alcohols to form a complex product - an ether.
The reaction of decarboxylation, that is, the removal of a carbon dioxide molecule from a compound.
Able to interact with halides of elements such as phosphorus and sulfur.

It's obvious how versatile carboxylic acids are. Physical properties, as well as chemical ones, are quite diverse. In addition, it should be said that, in general, in terms of strength as acids, all organic molecules are rather weak compared to their inorganic counterparts. Their dissociation constants do not exceed 4, 8.

Methods of obtaining

There are several main ways in which saturated carboxylic acids can be obtained.

1. In the laboratory, this is done by oxidation:

alcohols;
aldehydes;
alkynes;
alkylbenzenes;
destruction of alkenes.

2. Hydrolysis:

esters;
nitriles;
amides;
trihaloalkanes.

3. Decarboxylation - removal of a CO molecule ₂.

4. In industry, synthesis is carried out by the oxidation of hydrocarbons with a large number of carbon atoms in the chain. The process is carried out in several stages with the release of many by-products.

5. Some individual acids (formic, acetic, butyric, valeric, and others) are obtained in specific ways using natural ingredients.

Basic compounds of saturated carboxylic acids: s alts

S alts of carboxylic acids are important compounds used in industry. They are obtained as a result of the interaction of the latter with:

metals;
basic oxides;
amphoteric oxides;
alkali;
amphoteric hydroxides.

Of particular importance among them are those that are formed between the alkali metals sodium and potassium and the highest saturated acids - palmitic, stearic. After all, the products of such interaction are soaps, liquid and solid.

Soaps

So, if we are talking about a similar reaction: 2C₁₇H₃₅-COOH + 2Na=2C₁₇ H₃₅COONa + H₂, the resulting product - sodium stearate - is by its nature the usual laundry soap used for washing clothes.

If you replace the acid withpalmitic, and the metal to potassium, you get potassium palmitate - liquid soap for washing hands. Therefore, it can be stated with confidence that s alts of carboxylic acids are in fact important compounds of an organic nature. Their industrial production and use is simply colossal in its scale. If you imagine how much soap each person on Earth spends, then it is easy to imagine these scales.

Esters of carboxylic acids

A special group of compounds that has its place in the classification of organic substances. This is a class of esters. They are formed by the reaction of carboxylic acids with alcohols. The name of such interactions is esterification reactions. The general view can be represented by the equation:

R, -COOH + R"-OH=R, -COOR" + H_{2 O.}

The product with two radicals is an ester. Obviously, as a result of the reaction, the carboxylic acid, alcohol, ester and water have undergone significant changes. So, hydrogen leaves the acid molecule in the form of a cation and meets with a hydroxo group that has split off from alcohol. The result is a water molecule. The group left from the acid attaches the radical from the alcohol to itself, forming an ester molecule.

Why are these reactions so important and what is the industrial significance of their products? The thing is that esters are used like:

food additives;
aromatics;
component of perfume;
solvents;
components of varnishes, paints, plastics;
drugs and more.

It is clear that their areas of use are wide enough to justify the volume of production in the industry.

Ethanoic acid (acetic)

This is a limiting monobasic carboxylic acid of the aliphatic series, which is one of the most common in terms of production volumes throughout the world. Its formula is CH₃COOH. Such prevalence is due to its properties. After all, the areas of its use are extremely wide.

It is a dietary supplement under the code E-260.
Used in the food industry for preservation.
Used in medicine for the synthesis of drugs.
Ingredient when making fragrance compounds.
Solvent.
Participant in the process of printing, dyeing fabrics.
A necessary component in the reactions of chemical synthesis of many substances.

In everyday life, its 80% solution is commonly called vinegar essence, and if you dilute it to 15%, you get just vinegar. Pure 100% acid is called glacial acetic acid.