Aldehydes include organic compounds that are characterized by a double bond between carbon and oxygen atoms and two single bonds of the same carbon atom with a hydrocarbon radical denoted by the letter R and a hydrogen atom. The group of atoms> C = O is called the carbonyl group, it is characteristic of all aldehydes. Many aldehydes have a pleasant smell. They can be obtained from alcohols by dehydrogenation (removal of hydrogen), due to which they have received a common name - aldehydes. The properties of aldehydes are determined by the presence of a carbonyl group, its location in the molecule, as well as the length and spatial branching of the hydrocarbon radical. That is, knowing the name of a substance reflecting its structural formula, one can expect certain chemical as well as physical properties of aldehydes.
There are two main ways of naming aldehydes. The first method is based on the system used by the International Union (IUPAC), often referred to as systematic nomenclature. It is based on the fact that the longest chain, in which a carbonyl group is attached to the carbon atom, serves as the basis for the name of the aldehyde, that is, its name comes from the name of a related alkane by replacing the suffix βan with the suffix βal (methane β matanal, ethane β ethanal , propane - propanal, butane - butanal and so on). Another method for forming the name of aldehydes uses the name of the corresponding carboxylic acid into which it will turn as a result of oxidation (methanal - formic aldehyde, ethanal - acetic aldehyde, propanal - propionic aldehyde, butanal - butyraldehyde, and so on).
The main structural differences between alkanes and aldehydes, which determine the physical properties of aldehydes, are the presence of a carbonyl group in the latter (> C = O). It is this group that is responsible for differences in the physical and chemical properties of substances with the same number of carbon atoms or the same molecular weight. These differences are explained by the polarization of the carbonyl group> Ξ΄ + = Ξ΄-. That is, the electron density from the carbon atom is shifted towards the oxygen atom, which makes oxygen partially electronegative, and carbon partially electropositive. The Greek letter delta (Ξ΄) is often used to indicate the polarity of the carbonyl group, this indicates a partial charge. The negative end of one polar molecule is attracted to the positive end of another polar molecule. These can be molecules of the same or the other substance. Aldehydes differ from ketones in that in the latter the carbon of the carbonyl group is connected by single bonds with two hydrocarbon radicals. In this regard, it is also interesting to compare the physical properties of aldehydes and ketones.
It is the polarity of the group> C = O that affects the physical properties of aldehydes: melting point, boiling point, solubility, dipole moment. Hydrocarbon compounds consisting only of hydrogen and carbon atoms melt and boil at low temperatures. For substances with a carbonyl group, they are significantly higher. For example, butane (CH3CH2CH2CH3), propanal (CH3CH2CHO) and acetone (CH3COCH3) have the same molecular weight of 58, and the boiling point of butane is 0 Β° C, while for propanal it is 49 Β° C, and for acetone it is 56 Β° C. The reason for the big difference is that polar molecules are more able to attract each other than nonpolar molecules, so they need more energy to break, and therefore a higher temperature is required for these compounds to melt or boil.
With an increase in molecular weight, the physical properties of aldehydes change. Formaldehyde (HCHO) is a gaseous substance under normal conditions, acetaldehyde (CH3CHO) boils at room temperature. Other aldehydes (with the exception of those with high molecular weight) are liquids under normal conditions. Polar molecules do not mix easily with nonpolar ones, because polar molecules attract each other, and nonpolar molecules cannot squeeze between them. Therefore, hydrocarbons do not dissolve in water, since water molecules are polar. Aldehydes, in the molecules of which the number of carbon atoms is less than 5, dissolve in water, but if the number of carbon atoms is more than 5, dissolution does not occur. The good solubility of low molecular weight aldehydes is due to the formation of hydrogen bonds between the hydrogen atom of the water molecule and the oxygen atom of the carbonyl group.
The polarity of molecules formed by various atoms can be quantified by a number called the dipole moment. Molecules formed by identical atoms are not polar and do not have a dipole moment. The vector of the dipole moment is directed towards the element standing in the periodic table (for one period) to the right. If the molecule consists of atoms of one subgroup, then the electron density will shift towards the element with a lower serial number. Most hydrocarbons do not have a dipole moment or its value is extremely small, but for aldehydes it is much higher, which also explains the physical properties of aldehydes.