Physical properties of aldehydes

Aldehydes include organic compounds characterized by a double bond between the carbon and oxygen atoms and two single bonds of the same carbon atom with a hydrocarbon radical designated by the letter R and a hydrogen atom. A group of atoms> C = O is called a 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 the carbonyl group, its location in the molecule, and the length and spatial branching of the hydrocarbon radical. That is, knowing the name of the substance that reflects its structural formula, we 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), it is often called the 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 derives from the name of the related alkane due to the substitution of the suffix -an for the suffix -al (methane-mananal, ethane-ethanal , Propane - propanal, butane - butanal and so on). Another method of forming the name of aldehydes uses the name of the corresponding carboxylic acid to which it will turn as a result of oxidation (methanaldehyde formic, ethanal-aldehyde acetic, propanal-aldehyde propionic, butanal-aldehyde oil, etc.).

The main structural differences between alkanes and aldehydes, which determine the physical properties of aldehydes, is the presence in the last carbonyl group (> 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 mass. These differences are explained by the polarization of the carbonyl group> Cδ + = Oδ-. That is, the electron density from the carbon atom is shifted toward the oxygen atom, which makes the oxygen partially electronegative, and the carbon is partially electropositive. To denote the polarity of the carbonyl group, the Greek delta (δ) is often used, this indicates a partial charge. The negative end of one polar molecule is attracted to the positive end of another polar molecule. It can be molecules of the same or another substance. Aldehydes differ from ketones in that in the last carbon of the carbonyl group it is connected by single bonds to two hydrocarbon radicals. In this connection 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. In substances with a carbonyl group, they are much higher. For example, butane (CH3CH2CH2CH3), propanal (CH3CH2CHO) and acetone (CH3COCH3) have the same molecular weight equal to 58, and the boiling point of butane is 0 ° C, while for propane it is 49 ° C and acetone is 56 ° C. The reason for the big difference is that polar molecules are more likely to attract each other than nonpolar molecules, so more energy is required to break them, and therefore a higher temperature is required to make these compounds melt or boil.

As the molecular weight increases, the physical properties of the aldehydes change. Formaldehyde (HCHO) is a gaseous substance under normal conditions, acetaldehyde (CH3CHO) boils at room temperature. Other aldehydes (with the exception of high molecular weight representatives) are liquids under normal conditions. Polar molecules do not mix easily with non-polar molecules, because polar molecules are attracted to each other, and non-polar molecules are not able to squeeze between them. Therefore, hydrocarbons do not dissolve in water, since the water molecules are polar. Aldehydes, in which the number of carbon atoms is less than 5, dissolve in water, but if the number of carbon atoms is greater than 5, dissolution does not occur. The good solubility of aldehydes with a low molecular weight 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 different atoms can be quantitatively expressed 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 toward the element in the Mendeleev's periodic table (for one period) to the right. If a molecule consists of atoms of one subgroup, then the electron density will shift towards the element with a smaller ordinal number. Most hydrocarbons do not have a dipole moment or its magnitude is extremely small, but for aldehydes it is much higher, which also explains the physical properties of aldehydes.