Definition and examples of polar bond in chemistry

There are three basic types of chemical bonds that hold atoms together, which are the ionic bond , the covalent bond , and the metallic bond . Additionally, covalent bonds can be divided into several classes depending on the number of electrons involved in the bond, the origin of the electrons (whether they come from one or both atoms), and the uniformity of the electron density distribution around them. both cores. Polar bonding is defined as a type of covalent bond in which the atoms do not share electrons equally, because they have different electronegativities .

It must be remembered that a covalent bond is one in which one or more pairs of valence electrons are shared between two atoms, which holds them together.

The reason why they are called polar bonds is that, in this type of bond, the electron density is slightly shifted towards the more electronegative element, so it acquires a partially negative charge (represented by the symbol δ-) while that the other atom acquires a partially positive charge (represented by the symbol δ+). Viewed this way, the link is an electric dipole, since it has a positive and a negative pole.

The polar bond and the electronegativity difference

The electronegativity of an atom is a number that represents its ability to attract electrons when chemically bonded to another atom. This property is measured on a scale that goes from 0.65 for francium to 4.0 for fluorine, which are the least and most electronegative elements, respectively.

Electronegativity is closely linked to chemical bonding and, in fact, determines in many cases the type of bond that will be formed between two atoms of different elements. If the difference is large, the bond will be ionic, and if it is very small or there is no difference, then the bond will be covalent. But if the difference is intermediate, then we will be in the presence of a polar bond.

But this brings up a very important question: How do you know when the difference is large enough to define an ionic bond, or small enough to define a pure covalent one?

In view of the fact that the ionic and covalent character does not change abruptly but rather gradually, the limits between one and the other type of bond are somewhat blurred. However, chemists established the following convention that allows a more clear definition of what a polar covalent bond is:

Polar bonds and dipole moment

It has already been clarified that polar bonds are electric dipoles. Electric dipoles are characterized by something called the dipole moment, which is a vector represented by the Greek letter μ (mu), pointing from the less electronegative to the more electronegative atom.

The magnitude of the dipole moment is given by the product of the charge on the poles and the length of the dipole (in this case, the length of the bond). In the case of polar bonds, the dipole moment is proportional to the difference in electronegativities between the two bonded atoms.

The polar bond and polarity

When a molecule has only one polar bond, then the molecule as a whole has a dipole moment, and the molecule is said to be polar . Polarity is a very important property in molecular compounds since it determines properties such as solubility in different solvents, melting and boiling points, among other properties.

It should be noted, however, that the fact of having polar bonds does not ensure that a molecule is polar. When a molecule has more than one polar bond, the total polarity of the molecule will be given by the sum of the dipole moments of all its polar bonds . These dipole moments add as vectors. For this reason, it may be the case that the dipole moments of the different polar bonds cancel each other, and the molecule as such will be nonpolar, despite having polar bonds. If they do not cancel, then the molecule will be polar.

Examples of polar bonds

Polar bonds occur, in most cases, between non-metallic elements. As a general rule, the further apart they are on the periodic table, the greater the difference in electronegativities between the two atoms and, therefore, the greater the dipole moment of the bond, ie the bond will be more polar.

Here are some examples of representative polar bonds that arise very frequently in organic chemistry:

the OH bond

There are many molecular compounds that have OH bonds. The most notorious is, of course, water, whose molecular formula is H ₂ O, and which has two OH bonds. However, there are countless other compounds with this type of bond including alcohols, phenols, carboxylic acids, and many more.

The electronegativity difference between oxygen and hydrogen is 1.24 which makes it

CO link

The CO bond is another very common example in many organic compounds including alcohols, ethers, acids, and many more. The electronegativity difference between carbon and oxygen is 0.89. This bond is responsible for the polarity of ethers, and is partially responsible for the polarity of many other compounds.

CN link

Amines, amides, and countless other compounds, including DNA and all proteins contain multiple CN bonds. With an electronegativity difference of 0.49, this bond is close to the borderline between polar bonding and nonpolar covalent bonding.

NH link

The electronegativity difference between nitrogen and hydrogen is 0.84, making this a fairly polar bond. In fact, this polarization of the bond means that the hydrogen attached to the nitrogen can form part of a special type of covalent bond between three nuclei called a hydrogen bond, which is responsible for many properties of the compounds that can form them.

C=O bond

This is an important example as it highlights the fact that covalent bond polarity is a concept independent of bond order. A bond can be polar or nonpolar regardless of whether it is a single, double, or triple bond.

In view of this, the C=O bond is still polar, regardless of the fact that it is a double bond. However, there is a difference in polarity, since the electronegativities of the elements depend on hybridization. In this case, both carbon and oxygen are sp ² hybridized , making them both more electronegative, but there is still a difference in electronegativities between the two.

The HF Link – An exception to the rule

As mentioned above, the boundaries between covalent and ionic character are blurred, and the definition of polar bond in terms of electronegativity difference may present exceptions. A very common one is hydrogen fluoride or HF.

For this compound, the difference in electronegativities is 1.78. This, according to the previous definition, would place HF within ionic compounds. However, what makes a compound ionic or covalent is not only its difference in electronegativity, but also (and, in fact, mainly) its physical and chemical properties.

The ionic bond is characterized by being very strong and by generating crystalline solids with very high melting and boiling points. However, HF is a gas at room temperature, since its boiling point is only 19.5 ºC. Compare with the boiling point of sodium chloride which is 1,465 ºC.

Also, HF is made up of two nonmetals instead of a nonmetal and a metal, as is the case with ionic compounds. For these two reasons, HF is considered a polar covalent compound , despite the high difference in electronegativities between hydrogen and fluorine.

SH link – Other exception

The SH bond is an example of a covalent bond that is considered polar, despite not meeting the electronegativity difference condition. In this case, the difference is 0.38, which would put it in the group of nonpolar covalent bonds, however, chemists agree that the bond is, in fact, polar.