Ketones: chemical properties and definition

Organic chemistry is a very complicated science, but interesting. After all, the compounds of the same elements, in different numbers and sequences, contribute to the formation of substances of different properties. Let's consider the compounds of the carbonyl group called "ketones" (chemical properties, physical characteristics, methods of their synthesis). And also compare them with other substances of the same kind - aldehydes.

Ketones

This word is a common name for a whole class of substances of organic nature, in molecules of which the carbonyl group (C = O) is linked to two carbon radicals.

In their structure, ketones are close to aldehydes and carboxylic acids. However, they contain at once two atoms C (carbon or carbon) connected with C = O.

Formula

The general formula for the substances in this class is as follows: R ₁ -CO-R ₂ .

To make it more understandable, as a rule, it is written like this.

In it, C = O is a carbonyl group. A R _{1 and} R ₂ Are the carbon radicals. In their place, there may be different compounds, but they must contain carbon.

Aldehydes and ketones

The physical and chemical properties of these groups of substances are quite similar. For this reason, they are often treated together.

The fact is that aldehydes also contain a carbonyl group in their molecules. At them with ketones even the formulas are very similar. However, if C = O is attached to 2 radicals in the substances under consideration, then it has only one aldehyde, instead of the second one - a hydrogen atom: R-CO-H.

An example is the formula for a substance of this class, formaldehyde, better known to everyone as formalin.

Based on the formula CH ₂ O, it can be seen that its carbonyl group is associated not with one but with two atoms of H.

Physical properties

Before you deal with the chemical properties of aldehydes and ketones, it is worth considering their physical characteristics.

• Ketones are low-melting solids or liquids with volatility. The lowest representatives of this class are highly soluble in H ₂ O and interact well with solvents of organic origin.
  Individual representatives (for example, CH ₃ COCH ₃ ) are remarkably soluble in water, and in absolutely any proportions.
  Unlike alcohols and carboxylic acids, ketones have a higher volatility, with the same molecular weight. This is facilitated by the inability of these compounds to create bonds with H, as can H-CO-R.
• Different types of aldehydes can reside in different aggregate states. So higher R-CO-H are insoluble solids. The lower ones are liquids, some of which are perfectly mixed with H ₂ O, but some of them are only soluble in water, but not more.
  The simplest of the substances of this type - formic aldehyde - is a gas that has a pungent odor. This substance is highly soluble in H ₂ O.

The most famous ketones

There are many R ₁ -CO-R ₂ substances, but not many of them are known. First of all, it is dimethyl ketone, which we all know as acetone. Also his colleague-solvent - butanone or as it is correctly called - methyl ethyl ketone.

Among other ketones, whose chemical properties are actively used in industry - acetophenone (methyl phenyl ketone). Unlike acetone and butanone, its smell is rather pleasant, because of what it is used in perfumery.

For example, cyclohexanone refers to typical representatives of R ₁ -CO-R ₂ , and is most commonly used in the manufacture of solvents.

It is impossible not to mention the diketons. This name is R ₁ -CO-R ₂ , in which not one, but two carbonyl groups in the composition. Thus, their formula looks like: R ₁ -CO-CO-R ₂ . One of the simplest, but widely used in the food industry representatives of diketones is diacetyl (2,3-butanedione).

The listed substances are just a small list of ketones synthesized by scientists (chemical properties are discussed below). In fact, there are more of them, but not all have been used. Especially it is worth considering that many of them are toxic.

Chemical properties of ketones

Substances of this category are able to react with various substances. This is precisely their specific chemical properties.

• Ketones are capable of attaching H (hydrogenation reaction) to themselves. However, for the production of this reaction, the presence of catalysts in the form of metal atoms of nickel, cobalt, cuprum, platinum, palladium and others is necessary. As a result of the reaction, R ₁ -CO-R _{2 is} evolved to secondary alcohols.
  Also, when interacting with a hydrogen in the presence of alkali metals or amalgam Mg from ketones, glycols are obtained.
• Ketones that have at least one alpha-hydrogen atom, as a rule, are affected by the tautomerization of the keto-enol. It is catalysed not only with acids, but also with bases. Usually the keto form is a more stable phenomenon than the enol. This equilibrium makes it possible to synthesize ketones by hydration of alkynes. The relative stabilization of the enol keto form by conjugation results in a rather strong acidity of R ₁ -CO-R ₂ (when compared with alkanes).
• These substances can react with ammonia. However, they proceed very slowly.
• Ketones interact with hydrocyanic acid. As a result, α-hydroxynitriles occur, the saponification of which promotes the appearance of α-hydroxy acids.
• The reaction with alkyl magnesium halides leads to the formation of secondary alcohols.
• Accession to NaHSO ₃ Promotes the emergence of hydrosulphite (bisulfite) derivatives. It is worth remembering that only methyl ketones can react in a fatty series.
  In addition to ketones, aldehydes can similarly interact with sodium hydrosulfite.
  When heated with a solution of NaHCO ₃ (baking soda) or mineral acid, the derivatives of NaHSO ₃ can decompose, accompanied by the release of a free ketone.
• In the course of the reaction, R ₁ -CO-R ₂ With NH ₂ OH (hydroxylamine) ketoximes are formed and as a by-product - H ₂ O.
• In reactions involving hydrazine, hydrazones are formed (the ratio of the substances taken is 1: 1) or azine (1: 2).
  If the product (hydrazone) produced by the reaction reacts with potassium hydroxide under the effect of the reaction, N and the ultimate hydrocarbons will be released. This process is called the Kizhner reaction.
• As already mentioned above, aldehydes and ketones have chemical properties and the production process are similar. In this case, the acetals R ₁ -CO-R ₂ are more complex than R-CO-H acetals. They appear as a result of the action on ketones of ethers of orthoformic and orthosilicic acids.
• In conditions with a higher concentration of alkalis (for example, when heated with concentrated H₂SO₄), R ₁ -CO-R ₂ undergo intermolecular dehydration with the formation of unsaturated ketones.
• If alkalis are present in the reaction with R ₁ -CO-R ₂ , ketones are subjected to aldol condensation. As a result, β-ketoalcohols are formed that can easily lose the H ₂ O molecule.
• The chemical properties of ketones are quite indicative in the case of acetone, which reacted with mesityl oxide. In this case, a new substance called "forron" is formed.
• Also to the chemical properties of the organic matter under consideration is the Leukart-Wallach reaction, which promotes the reduction of ketones.

From which R1-CO-R2

Having become acquainted with the properties of the substances under consideration, it is worthwhile to find out the most common methods for their synthesis.

• One of the most known reactions for the preparation of ketones is the alkylation and acylation of aromatic compounds in the presence of acid catalysts (AlCl ₃ , FeCI ₃ , mineral acids, oxides, cation exchange resins, etc.). This method is known as the Friedel-Crafts reaction.
• Ketones are synthesized by the hydrolysis of ketimines and vic-diols. In the case of the latter, the presence of mineral acids as catalysts is necessary.
• Also for the preparation of ketones, hydration of homologues of acetylene or, as it is called, Kucherov's reaction, is used.
• Reactions Guben-Gesha.
• Cyclization of Ruzicka is suitable for the synthesis of cycloketones.
• These substances are also extracted from tertiary peroxy ester using the Krieg rearrangement.
• There are several ways to synthesize ketones during oxidation reactions of secondary alcohols. Depending on the active compound, four reactions are identified: Swern, Kornblum, Kori-Kim and Parik-Dering.

Scope of application

Having dealt with the chemical properties and the production of ketones, it is worth knowing where these substances are used.

As mentioned above, most of them are used in the chemical industry as solvents for varnishes and enamels, as well as for the production of polymers.

In addition, some R ₁ -CO-R ₂ Have not bad proved themselves as flavors. In this capacity, ketones (benzophenone, acetophenone and others) are used in perfumery and cooking.

Also, acetophenone is used as a component for making sleeping pills.

Benzophenone, due to its ability to absorb harmful radiation, is a frequent ingredient in anti-sun cosmetics and, at the same time, a preservative.

Effects of R1-CO-R2 on the body

Having learned that the compounds are called ketones (chemical properties, application, synthesis and other data about them), it is worth familiar with the biological characteristics of these substances. In other words, to find out how they act on living organisms.

Despite the rather frequent use of R ₁ -CO-R ₂ In industry, it is always worth remembering that such compounds are very toxic. Many of them have carcinogenic and mutagenic properties.

Special representatives are capable of causing irritation on mucous membranes, up to burns. Alicyclic R ₁ -CO-R ₂ Can affect the body as drugs.

However, not all substances of this kind are harmful. The fact is that some of them take an active part in the metabolism of biological organisms.

Also ketones are original markers of violations of carbon metabolism and insulin deficiency. In the analysis of urine and blood, the presence of R ₁ -CO-R ₂ indicates various metabolic disorders, including hyperglycemia and ketoacidosis.