Aldehydes and ketones incorporate a carbonyl functional group> C = O and belong to the class of carbonyl compounds. They are also called oxo compounds. Despite the fact that these substances belong to the same class, due to the structural features, they are nevertheless divided into two large groups.
In ketones, a carbon atom from the group> C = O is connected to two identical or different hydrocarbon radicals, usually they have the form: R-CO-R '. This form of the carbonyl group is also called a keto group or an oxo group. In aldehydes, carbonyl carbon is connected to only one hydrocarbon radical, and the remaining valency is occupied by a hydrogen atom: R-. This group is called aldehyde. Due to these structural differences, aldehydes and ketones behave slightly differently when interacting with the same substances.
Carbonyl group
The C and O atoms in this group are in the sp 2 hybridized state. Carbon due to sp 2 -hybrid orbitals has 3 σ-bonds located at an angle of about 120 degrees in one plane.
The oxygen atom has a much greater electronegativity than the carbon atom, and therefore pulls together the moving π-bond electrons in the group> C = O. Therefore, an excess electron density δ - appears on the O atom, and on the contrary, on the C atom, its decrease δ + . This explains the properties of aldehydes and ketones.
The double bond C = O is stronger than C = C, but at the same time more reactive, due to the large difference in the electronegativities of carbon and oxygen atoms.
Nomenclature
As for all other classes of organic compounds, there are various approaches to the name of aldehydes and ketones. In accordance with the IUPAC nomenclature, the presence of the aldehyde form of the carbonyl group is indicated by the suffix -al, and the ketone -one. If the carbonyl group is the highest, then it determines the numbering order of C atoms in the main chain. In the aldehyde carbonyl carbon atom is the first, and in ketones the C atoms are numbered from the edge of the chain to which the group> C = O is closer. A related need is to indicate the position of the carbonyl group in ketones. Do this by writing down the appropriate number after the -on suffix.
Homological series of aldehydes and ketones| N-sleep | methanal | CH 3 —CO — CΗ 3 | propanone |
| CH 3 - SON | ethanal | CH 3 —CO — CΗ 2 —CΗ 3 | butanone |
| CH 3 —CΗ 2 —COΗ | propanal | CH 3 —CO — CΗ 2 —CH 2 —CΗ 3 | pentanone-2 |
| Η 3 -Η 2 -Η 2 -Η | butanal | CH 3 —CΗ 2 —CO — CΗ 2 —CH 3 | pentanone 3 |
| Η 3 - (Η 2 ) 3 -Η | pentanal | CH 3 —CO — CΗ 2 —CH 2 —CΗ 2 —CH 3 | hexanone-2 |
| Η 3 - (Η 2 ) 4- | hexanal | CΗ 3 —CΗ 2 —CO — CH 2 —CΗ 2 —CH 3 | hexanone 3 |
If the carbonyl group is not the oldest, then according to IUPAC rules its presence is indicated by the prefix -oxo for aldehydes and -oxo (-keto) for ketones.
For aldehydes, trivial names are widely used, obtained from the name of the acids into which they are able to transform during oxidation with the replacement of the word "acid" with "aldehyde":
- Η 3- acetic aldehyde;
- CΗ 3 —CH 2 —CHOH propionic aldehyde;
- Η 3 -CH 2 -CH 2 -CHOH butyric aldehyde.
For ketones, radical functional names are common, which consist of the names of the left and right radicals connected to the carbonyl carbon atom, and the words "ketone":
- CΗ 3 —CO — CH 3 dimethyl ketone;
- CΗ 3 —CΗ 2 —CO — CH 2 —CH 2 —CH 3 ethyl propyl ketone;
- C 6 Η 5 -CO-CΗ 2 -CΗ 2 -CΗ 3 propyl phenyl ketone.
Classification
Depending on the nature of hydrocarbon radicals, the class of aldehydes and ketones is divided into:
- limit - C atoms are connected to each other only by single bonds (propanal, pentanone);
- unsaturated - between atoms C there are double and triple bonds (propenal, penten-1-one-3);
- aromatic - contain in their molecule a benzene ring (benzaldehyde, acetophenone).
By the number of carbonyl and the presence of other functional groups are distinguished:
- monocarbonyl compounds - contain only one carbonyl group (hexanal, propanone);
- dicarbonyl compounds - contain two carbonyl groups in the aldehyde and / or ketone form (glyoxal, diacetyl);
- carbonyl compounds also containing other functional groups, which, in turn, are divided into halocarbonyl, hydroxycarbonyl, aminocarbonyl, etc.
Isomerism
The most characteristic of aldehydes and ketones is structural isomerism. Spatial is possible when an asymmetric atom is present in the hydrocarbon radical, as well as a double bond with various substituents.
- Isomerism of the carbon skeleton. It is observed in both types of carbonyl compounds under consideration, but begins with butanal in aldehydes and with pentanone-2 in ketones. Thus, the butanal CH 3 —CΗ 2 —CΗ 2 —CHOH has one isomer, the 2-methylpropanal CΗ 3 —CΗ (CΗ 3 ) —CHOH. And pentanone-2 CΗ 3 -CO-CΗ 2 -CΗ 2 -CΗ 3 is isomeric to 3-methylbutanone-2 CΗ 3 -CO-CΗ (CΗ 3 ) -CΗ 3 .
- Interclass isomerism. Oxy compounds with the same composition are isomeric with each other. For example, the composition 3Η 6 corresponds to the propanal 3 -Η 2- and propanone Η 3 --Η 3 . And the molecular formula of aldehydes and ketones C 4 H 8 O is suitable for the butanal of CH 3 -CΗ 2 -CΗ 2 -CH and the butanone of CH 3 -CO-CΗ 2 -CΗ 3 .
Also interclass isomers for carboxylic compounds are cyclic oxides. For example, ethanal and ethylene oxide, propanone and propylene oxide. In addition, unsaturated alcohols and ethers can also have a common composition and oxo compounds. So, the molecular formula C 3 H 6 O have:
- Η 3 -Η 2- - propanal;
- Η 2 = Η-Η 2 - - allyl alcohol ;
- Η 2 = Η-- 3 - methyl vinyl ether.
Physical properties
Despite the fact that the molecules of carbonyl substances are polar, unlike alcohols, aldehydes and ketones do not have mobile hydrogen, which means that they do not form associates. Therefore, their melting and boiling points are slightly lower than those of the corresponding alcohols.
If we compare the aldehydes of the same composition of ketones, then the last t bales are slightly higher. With increasing molecular weight t PL and t bales of oxo compounds regularly increase.
Lower carbonyl compounds (acetone, formaldehyde, acetic aldehyde) are highly soluble in water, while higher aldehydes and ketones are dissolved in organic substances (alcohols, ethers, etc.).
The smell of oxo compounds is very different. Their lower representatives have pungent odors. Aldehydes containing from three to six C atoms smell very unpleasant, but their higher homologs are endowed with floral aromas and are even used in perfumes.
Addition reactions
The chemical properties of aldehydes and ketones are due to the structural features of the carbonyl group. Due to the fact that the C = O double bond is strongly polarized, it easily transforms into a simple single bond under the action of polar agents.
1. Interaction with hydrocyanic acid. The addition of HCN in the presence of traces of alkali occurs with the formation of cyanohydrins. Alkali is added to increase the concentration of CN - ions:
R-COH + NCN -> R-CH (OH) -CN
2. The addition of hydrogen. Carbonyl compounds can easily be reduced to alcohols by adding hydrogen via a double bond. In this case, primary alcohols are obtained from aldehydes, and secondary alcohols are obtained from ketones. Reactions are catalyzed by nickel:
3 - + 2 -> 3 -Η 2 -Η
Η 3 --Η 3 + Η 2 -> 3 -Η (Η) -Η 3
3. The addition of hydroxylamines. These reactions of aldehydes and ketones are catalyzed by acids:
3 - + NH 2 OH -> Η 3 -Η = N-OH + 2
4. Hydration. The addition of water molecules to oxo compounds leads to the formation of gem diols, i.e. such dihydric alcohols in which two hydroxyl groups are attached to one carbon atom. However, such reactions are reversible, the resulting substances immediately decompose with the formation of starting materials. In this case, electron-withdrawing groups shift the equilibrium of reactions towards products:
> C = O + Η 2 <―>> C (OΗ) 2
5. The addition of alcohols. Various products can be obtained during this reaction. If two alcohol molecules are attached to the aldehyde, then acetal is formed, and if only one, then hemiacetal. The condition for the reaction is heating the mixture with an acid or a water-removing agent.
R- + -R '-> R- () --R'
R-COH + 2NO-R '-> R-CH (O-R') 2
Aldehydes with a long hydrocarbon chain are prone to intramolecular condensation, resulting in the formation of cyclic acetals.
Qualitative reactions
It is clear that with a different carbonyl group in aldehydes and ketones, their chemistry is also different. Sometimes it is necessary to understand which of these two types the obtained oxo compound belongs to. Aldehydes are oxidized more easily than ketones; this happens even under the influence of silver oxide or copper (II) hydroxide. In this case, the carbonyl group changes to carboxylic and carboxylic acid is formed.
The reaction of a silver mirror is called the oxidation of aldehydes with a solution of silver oxide in the presence of ammonia. In fact, a complex compound is formed in the solution, which acts on the aldehyde group:
Ag 2 O + 4NH 3 + H 2 O -> 2 [Ag (NΗ 3 ) 2 ] Η
Η 3 -Η + 2 [Ag (NΗ 3 ) 2 ] Η -> 3 --NH 4 + 2Ag + 3NH 3 + 2
More often write the essence of the reaction in a simpler way:
Η 3 -Η + Ag 2 O -> Η 3 -Η + 2Ag
During the reaction, the oxidizing agent is reduced to metallic silver and precipitates. In this case, a thin silver coating similar to a mirror forms on the walls of the reaction vessel. It is for this reaction that got its name.
Another qualitative reaction, indicating a difference in the structure of aldehydes and ketones, is the action of fresh Cu (OΗ) 2 on the —COH group. It is prepared by adding alkalis to solutions of divalent copper salts. In this case, a blue suspension forms, which, when heated with aldehydes, changes color to red-brown due to the formation of copper (I) oxide:
R- + Cu (OΗ) 2 -> R-Η + Cu 2 O + Η 2
Oxidation reactions
The oxo compounds can be oxidized with a KMnO 4 solution when heated in an acidic environment. However, ketones in this case are destroyed with the formation of a mixture of products that do not have practical value.
A chemical reaction reflecting this property of aldehydes and ketones is accompanied by discoloration of the pinkish reaction mixture. At the same time, carboxylic acids are obtained from the vast majority of aldehydes:
CH 3 -CHA + KMnO 4 + H 2 SO 4 -> CH 3 -CHA + MnSO 4 + K 2 SO 4 + H 2 O
During this reaction, formaldehyde is oxidized to formic acid, which decomposes under the action of oxidizing agents to form carbon dioxide:
- + KMnO 4 + H 2 SO 4 -> 2 + MnSO 4 + K 2 SO 4 + 2
Aldehydes and ketones are characterized by complete oxidation during combustion reactions. In this case, CO 2 and water are formed. The combustion equation of formaldehyde is:
+ O 2 -> 2 + 2
Getting
Depending on the volume of products and the purpose of their use, methods for producing aldehydes and ketones are divided into industrial and laboratory. In the chemical industry, carbonyl compounds are produced by the oxidation of alkanes and alkenes (petroleum products), the dehydrogenation of primary alcohols, and the hydrolysis of dihaloalkanes.
1. Obtaining formaldehyde from methane (when heated to 500 ° C in the presence of a catalyst):
Η 4 + 2 -> + Η 2 .
2. Oxidation of alkenes (in the presence of a catalyst and high temperature):
2Η 2 = Η 2 + 2 -> 2 3 -
2R-Η = Η 2 + 2 -> 2R-Η 2 -Η
3. The removal of hydrogen from primary alcohols (catalyzed by copper, heating is necessary):
Η 3 -Η 2 - -> 3- + Η 2
R-CH 2 -OH -> R-COH + H 2
4. Hydrolysis of dihaloalkanes with alkalis. A prerequisite is the attachment of both halogen atoms to the same carbon atom:
Η 3 -C (Cl) 2 H + 2NaOH -> Η 3 -Η + 2NaCl + 2
In small quantities under laboratory conditions, carbonyl compounds are obtained by hydration of alkynes or by oxidation of primary alcohols.
5. The addition of water to acetylenes occurs in the presence of mercury sulfide in an acidic environment (Kucherov reaction):
Η≡Η + Η 2 -> 3 -Η
R-≡Η + Η 2 -> R-- 3
6. The oxidation of alcohols with a terminal hydroxyl group is carried out using metallic copper or silver, copper oxide (II), as well as potassium permanganate or dichromate in an acidic medium:
R-Η 2 -Η + 2 -> R- + 2
The use of aldehydes and ketones
Formic aldehyde is necessary to obtain phenol-formaldehyde resins obtained during the reaction of its condensation with phenol. In turn, the resulting polymers are necessary for the production of a variety of plastics, particle boards, glue, varnishes and much more. It is also used to obtain medicines (urotropin), disinfectants and is used to store biological preparations.
The main part of ethanal goes to the synthesis of acetic acid and other organic compounds. Some amounts of acetaldehyde are used in pharmaceutical manufacturing.
Acetone is widely used to dissolve many organic compounds, including varnishes and paints, some types of rubbers, plastics, natural resins and oils. For these purposes, it is used not only pure, but also in a mixture with other organic compounds in the composition of solvents of grades R-648, R-647, R-5, R-4, etc. It is also used for degreasing surfaces in the manufacture of various parts and mechanisms. Large quantities of acetone are required for pharmaceutical and organic synthesis.
Many aldehydes have pleasant aromas, which is why they are used in the perfume industry. So, citral has a lemon smell, benzaldehyde smells like bitter almonds, phenylacetic aldehyde brings hyacinth flavor to the composition.
Cyclohexanone is needed for the production of many synthetic fibers. Adipic acid is obtained from it, which in turn is used as a raw material for caprolactam, nylon and capron. It is also used as a solvent for fats, natural resins, waxes and PVC.