Carboxylic acids: physical properties. Carboxylic Acid Salts

Almost everyone in the house has vinegar. And most people know that acetic acid is its base . 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, but also some others are used in everyday life, and even derivatives of these acids are generally frequent guests in every house.

Carboxylic Acid Class: General

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 saturated monobasic carboxylic acids have is: R-COOH, where R is a radical particle that can include any number of carbon atoms.

Accordingly, 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 relate 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 strongly polarized, and the hydrogen atom becomes extremely vulnerable. It is easily split off, entering into chemical interactions. Therefore, acids in the corresponding indicators give a similar reaction:

• phenolphthalein is 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, to participate in many other interactions.

Classification

We can distinguish 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, there are:

• Alicyclic acids. Example: Hinna.
• Aromatic. Example: benzoic.
• Aliphatic. Example: vinegar, acrylic, oxal and others.
• Heterocyclic. Example: nicotine.

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

• limit - all bonds are only single;
• unsaturated - in the presence of double, one or more.
  

Also, the number of functional groups can serve as a sign of classification. So, 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, malonic and others.
3. Polybasic - lemon, dairy and others.

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

Discovery story

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

Indeed, for a long time, the views of the vitalists dominated, which denied 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 works of scientists Berzelius and Kolbe showed the possibility of synthesizing these compounds from inorganic substances (distillation of charcoal). As a result, acetic was obtained. Thus, carboxylic acids (physical properties, structure) were studied and the foundation was laid for the discovery of all other representatives of a number of aliphatic compounds.

Physical properties

Today, all of their representatives have been studied in detail. For each of them, one can find a characteristic in all respects, including industrial applications and nature. We will consider what carboxylic acids are, their physical properties and other parameters.

So, there are several basic characteristic parameters.

1. If the number of carbon atoms in the chain does not exceed five, then these are sharply smelling, mobile, and volatile liquids. Above five - heavy oily substances, even more - solid, paraffin-like.
2. The density of the first two representatives exceeds unity. Everyone else is lighter than water.
3. Boiling point: the larger the chain, the higher the rate. The more branched the structure, the lower.
4. Melting point: depends on the parity of the number of carbon atoms in the chain. In even ones it is higher, in odd ones lower.
5. They dissolve very well in water.
6. 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 the molecules, the higher the rates that carboxylic acids produce. The physical properties of these compounds make it possible to determine the areas and methods of their use in industry.

Chemical properties

As we have already indicated above, these acids may exhibit different properties. Reactions with their participation are important for the industrial synthesis of many compounds. Let us 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. It interacts with simple metals according to the standard scheme (that is, only with those that stand up to hydrogen in a series of voltages).
3. With stronger acids (inorganic) it behaves like a base.
4. It is able to recover to primary alcohol.
5. A particular reaction is esterification. This interaction with alcohols with the formation of a complex product - ether.
6. The decarboxylation reaction, that is, cleavage of a carbon dioxide molecule from a compound.
7. It is able to interact with halides of elements such as phosphorus and sulfur.

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

Production methods

There are several basic methods by which saturated carboxylic acids can be prepared.

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

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

2. Hydrolysis:

• esters;
• nitriles;
• amides;
• trihaloalkanes.

3. Decarboxylation — cleavage of a CO ₂ molecule.

4. In industry, the 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, valerianic and others) are obtained by specific methods using natural ingredients.

Basic compounds of saturated carboxylic acids: salts

Carboxylic acid salts 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.

Of particular importance among them are those that are formed between the alkali metals sodium and potassium and higher limiting acids - palmitic, stearic. After all, the products of this 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 its nature, a common laundry soap used to wash laundry.

If you replace the acid with palmitic and the metal with potassium, you get potassium palmitate - a liquid soap for washing hands. Therefore, it is safe to say that carboxylic acid salts are actually 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 spends on Earth, 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 during the reaction of carboxylic acids with alcohols. The name of these interactions is esterification reactions. A 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 underwent significant changes. So, hydrogen leaves the acid molecule in the form of a cation and is found with a hydroxo group that is cleaved from alcohol. As a result, a water molecule is formed. The group remaining from the acid joins the radical from the alcohol to form 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 as:

• nutritional supplements;
• aromatic additives;
• a component of perfume;
• solvents;
• components of varnishes, paints, plastics;
• medicines and stuff.

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

Ethanoic acid (acetic)

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

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

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

Formic acid

The very first and simplest representative of this class. Formula - UNS. It is also a food supplement 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;
• used to combat parasites;
• for dyeing fabrics, pickling parts;
• as a solvent;
• bleach;
• in medicine - for the disinfection of instruments and equipment;
• to produce carbon monoxide in the laboratory.

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