Carboxylic acids: physical properties. Salts of carboxylic acids

Virtually all houses have vinegar. And most people know that its basis is acetic acid. But what is it from the 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, but also some others are used in everyday life, and the derivatives of these acids are generally frequent guests in every home.

Class of carboxylic acids: general characteristic

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 has the form -COOH. Thus, the general formula that all limiting monobasic carboxylic acids have is as follows: R-COOH, where R is a radical particle that can include any number of carbon atoms.

According to this, a definition of this class of compounds can be given. Carboxylic acids are organic oxygen-containing molecules that contain 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 OH bond is highly polarized, and the hydrogen atom becomes extremely vulnerable. It easily splits off, entering into chemical interactions. Therefore, the acids in the respective indicators give a similar reaction:

• Phenolphthalein - colorless;
• Litmus - red;
• Universal - red;
• Methyl orange - red and others.

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

Classification

It is possible to distinguish several basic features by which carboxylic acids are divided into groups. The first of these is the nature of the radical. The following factors are singled out for this factor:

• Alicyclic acids. Example: cynical.
• Aromatic. Example: benzoic.
• Aliphatic. Example: acetic, acrylic, oxalic and others.
• Heterocyclic. Example: nicotinic.

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

• Limiting - all connections are only single;
• Unlimited - available double, one or more.

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

1. Monobasic - only one -COOH-group. Example: formic, stearic, butane, valerian and others.
2. Dibasic - respectively, two groups -COOH. Example: oxalic, malonova and others.
3. Polybasic - lemon, lactic and others.

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

History of the discovery

Wine-making flourished since antiquity. And, as you know, one of his products is acetic acid. Therefore, the history of the fame of this class of compounds has its roots ever since the times of Robert Boyle and Johann Glauber. However, the chemical nature of these molecules could not be elucidated for a long time.

For a long time, the views of the vitalists dominated, which denied the possibility of the formation of organic matter 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 in a flask of living ants.

Later works of scientists Berzelius and Kolbe showed the possibility of synthesis of these compounds from inorganic substances (distillation of charcoal). As a result, acetic was obtained. Thus, carboxylic acids (physical properties, structure) were studied and the beginning was initiated for the discovery of all the remaining 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 for all parameters, including application in industry and being in nature. We will consider what the carboxylic acids are, their physical properties and other parameters.

So, we can distinguish several basic characteristic parameters.

1. If the number of carbon atoms in the chain does not exceed five, then this is a smelling, moving and volatile liquid. Above five - heavy oily substances, even more - solid, paraffinic.
2. The density of the first two representatives exceeds one. All the rest are lighter than water.
3. Boiling point: the larger the chain, the higher the value. The more branched structure, the lower.
4. Melting point: depends on the parity of the number of carbon atoms in the chain. In the even it is higher, the odd ones are lower.
5. They dissolve very well in water.
6. They are capable of forming strong hydrogen bonds.

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

Chemical properties

As we already indicated above, these acids can exhibit different properties. Reactions with their participation are important for the industrial synthesis of many compounds. We denote the most important chemical properties that a monobasic carboxylic acid can exhibit.

1. Dissociation: R-COOH = RCOO ^- + H ⁺ .
   
2. It exhibits acidic properties, that is , it interacts with basic oxides, as well as their hydroxides. With simple metals interacts according to a standard scheme (that is, only with those that stand before hydrogen in a series of stresses).
3. With stronger acids (inorganic) behaves as a base.
4. It is able to recover to the primary alcohol.
5. A special reaction is esterification. This interaction with alcohols to form a complex product - ether.
6. The decarboxylation reaction, that is, the cleavage from the compound of the carbon dioxide molecule.
7. It is able to interact with halides of elements such as phosphorus and sulfur.

Obviously, how many-sided are carboxylic acids. Physical properties, like chemical, are quite diverse. In addition, it should be said that in general, in strength as acids, all organic molecules are weak enough compared to their inorganic counterparts. Their dissociation constants do not exceed 4.8.

Methods of obtaining

There are several basic ways in which it is possible to obtain limiting carboxylic acids.

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

• Alcohols;
• Aldehydes;
• Alkynes;
• Alkylbenzenes;
• Destruction of alkenes.

2. Hydrolysis:

• Esters;
• Nitriles;
• Amides;
• Trihaloalkanes.

3. Decarboxylation - elimination of the CO ₂ molecule.

4. In the industry, synthesis is carried out by oxidizing 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, oily, valerian and others) are produced by specific methods using natural ingredients.

Basic compounds of the limiting carboxylic acids: salts

Salts 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 ;
• Alkalis;
• Amphoteric hydroxides.

Especially important among them are those that are formed between alkali metals, sodium and potassium and higher limiting 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: 2C ₁₇ H ₃₅ -COOH + 2Na = 2C ₁₇ H ₃₅ COONa + H ₂ ,

Then the resulting product - sodium stearate - is by nature a usual laundry soap used for washing clothes.

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

Esters of carboxylic acids

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

R ^, -COOH + R "-OH = R ^, -COOR" + H ₂ O.

A product with two radicals is an ester. Obviously, as a result of the reaction, carboxylic acid, alcohol, ester and water have undergone significant changes. So, hydrogen from the molecule of acid goes in the form of a cation and meets with a hydroxo group, split off from alcohol. As a result, a water molecule is formed. The grouping remaining from the acid adds a radical to the alcohol, forming an ester molecule.

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

• nutritional supplements;
• Aromatic additives;
• A component of perfume;
• Solvents;
• Components of varnishes, paints, plastics;
• Medicines and so on.

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

Ethanoic acid (acetic acid)

This is the ultimate monobasic carboxylic acid of the aliphatic series, which is one of the most common in terms of production throughout the world. Its formula is CH ₃ COOH. This prevalence is due to its properties. After all, the areas of its use are extremely wide.

1. It is a food additive under the code E-260.
2. Used in the food industry for conservation.
3. It is used in medicine for the synthesis of medicines.
4. Component in the preparation of fragrant compounds.
5. Solvent.
6. Participant in the process of printing, dyeing fabrics.
7. A necessary component in the reactions of chemical syntheses of many substances.

In everyday life, its 80-percent solution is called acetic essence, and if you dilute it to 15%, you will get just vinegar. A pure 100% acid is called glacial acetic acid.

Formic acid

The very first and simplest representative of this class. The formula is UNSO. It is also a food additive under the code E-236. Its natural sources:

• Ants and bees;
• nettle;
• needles;
• fruit.

Main areas of use:

• For preservation and preparation of animal feed;
• Is used to control parasites;
• For dyeing fabrics, etching parts;
• As a solvent;
• bleach;
• In medicine - for disinfection of instruments and equipment;
• For obtaining carbon monoxide in the laboratory.

Also in surgery, solutions of this acid are used as antiseptic agents.