Carbonyl group. Aldehydes and ketones

Aldehydes and ketones

The word aldehyde was coined as an abbreviation of the Latin alcohol dehydrogenatus — dehydrated alcohol, the most popular aldehyde is formaldehyde, resins are made from it, medicines are synthesized and as a preservative. The aldehyde formula - R-CHO, a compound in which a carbonyl group is connected to hydrogen and a radical.

The word ketone comes from the word acetone, a junior compound from the ketone family. Ketones are used as solvents, drugs and for the synthesis of polymers. The ketone formula is R-C(O)-R, a compound in which a carbonyl group is connected to two radicals.

Structure and properties of the carbonyl group

The carbonyl group is based on the bond of a carbon atom and an oxygen atom by means of α- and π-bonds. The resonant structure of the group determines the high polarity of the connection and the electron cloud is shifted to oxygen side: C^δ+=O^δ-. The introduction of electronegative elements in reduces the polarity of the bond, increasing the positive charge of the molecule. Nucleophilic substituents increase the negative charge of oxygen.

The carbon atom in the carbonyl group is a strong electrophile (attaches electrons), so most reactions of aldehydes and ketones are carried out by nucleophilic reagents (Lewis bases). Logically, an oxygen atom It is a strong nucleophile, and reactions with an oxygen atom are possible with the use of electrophiles (Lewis acids).

Reaction of a carbonyl group with a Lewis base
(R)(R)C^δ+=O^δ- + B: → (R)(R)C(B)-O
Reaction of the carbonyl group with Lewis acid
(R)(R)C^δ+=O^δ- + Y: → (R)(R)C-O-Y

In addition, the undivided electrons of oxygen endow it with weak base properties, so those aldehydes and cetones that do not dissolve in water are dissolved in concentrated sulfuric acid.

Physical properties of the carbonyl group

The high polarity of the C=O bond forms a high dipole moment, which is why the carriers of the carboxyl group they have a higher boiling point compared to hydrocarbons.

The undivided electrons in the oxygen atom form a hydrogen bond with water molecules, therefore, starting from five carbon atoms in radicals, aldehydes and ketones are poorly soluble in water or do not dissolve at all.

Aldehydes and ketones having up to 12 carbon atoms are liquids. Aliphatic compounds with a carbonyl group they have a density of about 0.8, so they float on the surface of water, cyclohexanone has a density of about one, aromatic aldehydes and ketones have a density slightly higher than that of water.

Reactions of aldehydes and ketones

Water connection

During the reaction of water with aldehydes and ketones, diols (glycols, diatomic alcohols) are formed. The reaction proceeds using a catalyst - acid or base and is two-sided:

RR-CO + H-OH ↔ R^R\C^/OH-OH

Attachment of nucleophilic carbons

Important nucleophilic compounds reacting with aldehydes and ketones are organometallic compounds (organic compounds in whose molecules there is a bond of a metal atom with a carbon atom/atoms). Some of the representatives of organometallic compounds are Grignard reagents (the general formula is R-Mg-X), in reactions with aldehydes and ketones form alcohols:

RH-C=O + R-C^-H₂-Mg⁺-Cl^-→ RH-C-(O-MgCl)(CH₂-R)
RH-C-(O-MgCl)(CH₂-R) + H-OH→ RH-C-CH₂R +OH-Mg-Cl

Oxidation of aldehydes and ketones

During oxidation, aldehydes are at an intermediate stage between alcohols and carboxylic acids:

In the presence of hydrogen and oxygen:
R-CH₂-OH ↔ R-C(=O)-H ↔ R-COOH

Aldehydes are easily oxidized, which allows the use of softer oxidizers than simple oxygen. Aromatic aldehydes undergo oxidation more easily than aliphatic ones. The problem of oxidation of aldehydes is in formation of by-products.

Ketones are oxidized with difficulty, for the oxidation of ketones it is necessary to use strong oxidizers and a large amount of heat. As a result of oxidation, the C-C bond breaks and an acid is formed (there is an exception):

In the presence of KMnO₄, H and a large amount of heat:
CH₃-C(=O)-CH₂CH₃ → CH₃-C(=O)-OH + CH₃CH₂-C(=O)-OH

The exception is the oxidation with selenium dioxide, SeO₂, the methyl group following the carbonyl group is oxidized, converting to another carbonyl group. For example, methylethyl ketone is oxidized to diacetyl:

Oxidation of methyl ethyl ketone to diacetyl:
CH₃CH₂-C(=O)-CH₃ + SeO₂ → CH₃-C(=O)-C(=O)-CH₃ + H₂O + Se

The ease with which aldehydes are oxidized makes it easy to distinguish them from ketones, soft oxidizers are used for this, such as: Tollens reagent (diamine silver hydroxide, Ag(NH₃)₂OH), Fehling reagent (alkaline solution Cu copper ions in KNaC Ferrotic Salt₄H₆O₆·4H₂O) and Benedict's solution (copper ions with citrate and sodium carbonate). Aromatic aldehydes react with Tollens reagent, but do not react with reagents Benedict and Fehling, which is used to determine the amount of aliphatic and aromatic aldehydes.

Polymerization of aldehydes

Paraldehyde

Acetaldehyde has a boiling point of 20°C, which makes it difficult to store and use. When processing acetaldehyde acid at low temperature, acetaldehyde combines into a cyclic triple molecule - paraldehyde, with a boiling point of 120 °C. Paraldehyde depolymerizes with a little heat, releasing three acetaldehyde molecules.

Formaldehyde

For convenience of transportation and storage, formaldehyde is sold not in the form of gas, but in the form of formalin - an aqueous solution containing 37-40% paraformaldehyde, OH(CH₂O)_nH, with an average value of n=30. Paraformaldehyde is a white amorphous substance, solid, obtained by slow evaporation of formalin at low pressure. Polymerization occurs due to the addition of each to a friend of formaldehyde molecules:

CH₂=O + H₂O ↔ [HOCH₂OH]
[HOCH₂OH] + n[HCHO] → HO-(CH₂O)_n+1-H

Polymer Derlin (polyoxymethylene) is a good linear plastic with a high molecular weight, derlin has excellent strength and elasticity characteristics.