Dipole-Dipole Force: Basics With Examples

Usually, non-metals share electrons between themselves to form a covalent bond. The resulting molecule is dipolar only when the electrons are shared unequally between the atoms. Dipole-dipole forces are the attractive electrostatic forces formed between the positive end of one polar molecule and the negative end of the other polar molecule. Dipole-dipole forces play a vital role in the stability of biomolecules, and we can also use them to study the structure of various molecules.

How do dipoles form?

Dipolar molecules have two centers or “poles” of charge: a partial positive and a partial negative charge on the atoms present. But why does this happen?

Let us learn through example. We will consider HCl as our model molecule, which is made of hydrogen(H) and chlorine(Cl). Imagine H and Cl in a tug-of-war, both trying to pull the shared electron pair towards itself. To predict the winner, we need to know who’s stronger. Thankfully, a scientist named Linus Pauling gave us the answer to it: electronegativity. Electronegativity is the tendency of an atom to pull the shared electron pair toward itself. Chlorine, with a value of 3.16, outdoes hydrogen’s(2.20) electronegativity and pulls the shared pair of electrons toward itself, gaining a partial negative charge. In contrast, hydrogen, having lost some of its hold on the shared pair, has a partial positive charge. Molecules with partial positive and negative charges are known as polar compounds.

Hcl
HCl is a polar compound with partial positive and negative charge

Other examples of dipolar molecules are water (H2O) and hydrogen fluoride(HF). Since we now know what dipoles are and how they form, we can move to understanding how two dipoles interact with one another.

How does dipole-dipole force occur?

Once again, we will use HCl as an example. We already know that HCl is a polar compound with a positive and negative end. When two HCl molecules are close enough, the partially positive hydrogen of one of the molecules attracts the partially negative Cl atom of the other molecule. This interaction between the two molecules is an example of dipole-dipole force.

dipole

Examples of dipole-dipole force

1. In H2O: Water is a polar molecule as well. Here, oxygen has a slightly negative charge, while the two hydrogen atoms have a slightly positive charge in a water molecule.

dipole

When two water molecules are close enough, the partially positive H-atom of one water molecule bonds with the partially negative O-atom of another.

dipole
Dipole-dipole interaction in water. As you’ll see later, this type of bond has a special name

2. In HF: Hydrogen fluoride (HF) is another compound where dipole-dipole forces are seen. The hydrogen carries a partial positive charge, and fluorine has a partial negative charge. Thus, the H-atom from one molecule of HF interacts with the F-atom of another.

3. Formation of acid: Till now, we have seen dipole-dipole forces act on identical molecules. The question is: Can two different polar molecules have a dipole-dipole interaction? Let’s put H2O and HCl to the test:

HCl

Because of the dipole-dipole interactions, HCl is broken down into H+ and Cl- in an aqueous solution. On one hand, slightly negative O-atom of water pulls the slightly positive H-atom of HCl. On the other hand, the Cl ion is pulled by the slightly positive H-atoms of water.

This leads to the formation of H3O+ ions. Thus, the compound HCl donates a H+ to water. This is one of the fundamental properties of acid. Hence, dipole-dipole forces play a major role in making acids.

Properties of dipole-dipole forces

1. Dipole-dipole forces occur only between two polar molecules. Sometimes, a dipole can induce a neutral atom and polarize it before interacting.

2. It is weaker than ionic (bonding between two ions) and covalent (bonds formed by sharing electrons) bonds.

Other types of dipole-based interactions

1. Dipole-induced dipole interaction: Have you ever tried to use a comb to pick up tiny bits of paper? Dipole-induced dipole interactions are almost the same stuff. A polar molecule with a dipole (the comb) induces a polarity in the non-polar, neutral molecule (the paper) and forms a dipole-induced dipole interaction. Let’s see an example involving water, a polar molecule, and xenon, a non-polar, neutral molecule.

HCl
HCl

Note how the noble gas xenon has polarized. The slight negative charge of oxygen induced a positive charge at the near end. This happens because the negative charge on oxygen repels the electrons of xenon towards the opposite end, resulting in a transient polarity.

2. Ion-dipole interaction: The chemistry of dissolving salt in water lies in ion-dipole interactions. Ion-dipole interactions occur between a charged ion and a neutral or weakly polar molecule. When we dissolve sodium chloride (NaCl) or salt in water, it dissociates into Na+ and Cl Since water is polar, the negative oxygen pulls the sodium ions toward itself, and the negative chloride ions are pulled toward the positive end.

3. Hydrogen bonding: We have already encountered an example of hydrogen bonding. Can you guess which one? If you guessed water, congrats! Hydrogen bonds are formed when the hydrogen attached to a highly electronegative atom (like N, O, F) interacts with another highly electronegative atom like F or Cl. In the case of water, H is bonded to highly electronegative O-atom, making it slightly positively charged and ready to interact with other electronegative atoms. Hydrogen bonds stabilize the structure of DNA and proteins.

DNA
DNA has hydrogen bonds that give stability to the DNA molecule, Credit; Wikimedia/Zephyris

4. London dispersion force: The weakest force of the bunch, London dispersion forces occur when a non-polar molecule becomes temporarily polar and either interacts with another temporarily polar molecule or with an induced dipole.

Application of dipole-dipole forces

1. Some adhesives help in binding two substances using dipole-dipole interactions.

2. Dipole-dipole interactions help in the formation of acids.

3. Dipole-induced dipole force helps in forming clathrates. Clathrates contain many molecules of one kind, which can trap a second kind of molecule inside. For example, if water is frozen under high pressure, it forms a cage-like structure that traps xenon inside.

Dipole-dipole interaction is omnipresent and has been a part of life as we see it. With the knowledge of dipole-dipole forces, one can uncover a lot about the structure of a compound.

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