## Polarity in molecules

Why is water mixed homogeneously with alcohol and not able to mix with oil? Solubility is a physical property that is directly related to the polarity of molecules. In this part, we will learn to establish if a molecule is polar or non-polar (apolar). Polarity is a property of molecules that represents the separation of electrical charges within the molecule, depending on the number and type of links it has.

The covalent bond between two atoms can be polar or apolar. This depends on the type of atoms that make it up: if the atoms are equal, the link will be apolar (since no atom attracts more electrons). But, if the atoms are different, the bond will be polarized towards the most electronegative atom, since it will be the one that attracts the pair of electrons with more force. Consider the H-H and H-F link, we see that in the H-H bond no atom is more electronegative than the other. Therefore, the pair of electrons does not polarize and we can say that the dipole moment (μ) is zero. In the case of the H-F bond, fluorine is more electronegative than hydrogen. Therefore, the pair of electrons is attracted to fluorine. We can represent this polarization of the link by means of an arrow, which ALWAYS points to the most electronegative atom. In the case of H-F, the dipole moment (μ) is different from zero.

## Dipole moment

The dipole moment is a quantitative measure of the polarity of a molecule. In the presence of an electric field, those polar molecules (that is, those with a dipole moment other than zero) are aligned in the direction of the field, while the apolar molecules are not affected.

In the case of molecules with more than two atoms, the dipole moment will depend on the polarity of all its bonds and on the molecular geometry. The presence of polar links does NOT necessarily imply that the molecule is polar.

##### JEE advanced important topics : Inorganic Chemistry

Example 1. Let’s analyze the CO2 molecule.

It is a linear molecule, as we have previously determined, where oxygen is more electronegative than carbon. Therefore, there will be a dipole vector oriented towards each of the oxygens.

However, both dipoles have the same magnitude but opposite sense. If we add such dipole vectors, they will be eliminated, giving a total dipole moment of zero. Therefore, the CO2 molecule is apolar.

Example 2. Let’s see what happens now in the water molecule (H2O).

It is a similar case to example 1, only that, in this case, the vectors point to the central atom, since it is the most electronegative.

However, in this case we must remember that the water molecule is not linear, but angular. Therefore, such vectors, when added together, will not be eliminated, but will give a resulting vector.

Therefore, the water molecule is polar: it has a resulting dipole moment.

#### Remember

Each bond made up of different atoms generates a dipole vector, which points to the most electronegative atom.

A molecule will have as many vectors as there are covalent bonds.

If the vectors do not vanish, then the molecule will be polar.

Molecules with tetrahedral geometry can also be apolar, although it is more difficult to represent such vectors in a three-dimensional molecule.

Example 3. Let’s look at the methane molecule (CH4).

Carbon is slightly more electronegative than hydrogen.

At the time of adding the vectors, these will be canceled, due to the symmetry of the molecule. Therefore, the methane molecule is apolar. Something different happens in the molecule of chloromethane (CH3Cl), where a hydrogen atom has been replaced by chlorine: the symmetry is lost, and therefore, the vectors do not cancel out. CH3Cl is polar.

#### Remember

Molecules with polar bonds in which the distribution of atoms is not symmetric, are POLAR.

On the other hand, those molecules that are linked to identical atoms, distributed symmetrically, are APOLARES.

And why is polarity important? Because in chemistry it is said that “like is mixed with like”. Those polar substances tend to mix with each other, while the apolar prefer to mix with apolar substances. Oil is an apolar substance, and we can verify this by mixing oil with water (polar substance): two phases are generated. The same polarity does not exist and, therefore, they do not mix.

Exercise 4

Analyze the molecule of sulfur dioxide (SO2). For the molecule in question, it determines and / or establishes the following:

Structure of Lewis

Formal charges

Hybridization of the central atom

Geometry of the molecule

If it is polar or apolar

## Hydrogen bond

Hydrogen bonds are formed when a “donor” atom donates the hydrogen atom covalently bound to it to an “acceptor” electronegative atom. Typical donors are oxygen in -OH groups (eg Ser, Thr, Tyr) or in water and nitrogen in -NH3 + (as in Lys, Arg) or in -NH- (as in the peptide bond, in Trp, His, Arg, in nitrogenous bases). The electronic pairs isolated from these same donors can serve as hydrogen bond acceptors. Likewise, the pairs of carbonyl oxygens = O (such as that of the peptide bond) and those of the nitrogens with three covalent bonds = N- (as in His, Trp or nitrogenous bases) can be acceptable. The latter can not be donors, because they lack hydrogens.

##### JEE advanced important topics : Photo Electric-effect

Jeffrey classifies the H bonds with donor-acceptor distances from 2.2 to 2.5 Å as “strong, predominantly covalent”, those from 2.5 to 3.2 Å as “moderate, mainly electrostatic” and the from 3.2 to 4.0 Å as “weak, electrostatic”. They correspond energies of 40 to 14, 15 to 4 and <4 kcal / mol respectively. The majority of H bonds in proteins fall into the category of moderates, since strong H bonds require chemical groups or conditions that are infrequent in proteins. Hydrogen atoms in moderate H bonds are often not in the straight line connecting the donor and the acceptor, so the donor-acceptor distance slightly underestimates the length of the H bond. The average distance between donor and acceptor in the secondary structure elements of the proteins is close to 3.0 Å, as is that between bases in the pairing of Watson and Crick. Since many pdb files lack hydrogen atoms, the presence of an energetically significant H bond can be deduced when a potential donor and acceptor are located less than 3.5 Å apart from each other.