Dipole Dipole vs Dispersion | Dipole Dipole Interactions vs Dispersion Forces
Dipole dipole interactions and dispersion forces are intermolecular attractions between molecules. Some intermolecular forces are strong while some are weak. However, all of these intermolecular interactions are weaker than the intramolecular forces like covalent or ionic bonds. These bonds determine the behavior of molecules.
What is Dipole Dipole Interactions?
Polarity arises due to the differences in electronegativity. Electronegativity gives a measurement of an atom to attract electrons in a bond. Usually Pauling scale is used to indicate the electronegativity values. In the periodic table, there is a pattern as to how the electronegativity values are changing. Fluorine has the highest electronegativity value, which is 4 according to the Pauling scale. From left to right through a period, the electronegativity value increases. Therefore, halogens have larger electronegativity values in a period, and group 1 elements have comparatively low electronegativity values. Down the group, the electronegativity values decrease. When the two atoms that form a bond are different, their electronegativities are often different. Therefore, the bond electron pair is pulled more by one atom compared to the other atom, which is participating in making the bond. This will result in an unequal distribution of electrons between the two atoms. Because of the uneven sharing of electrons, one atom will have a slightly negative charge whereas the other atom will have a slightly positive charge. At this instance, we say that the atoms have obtained a partial negative or positive charge (dipole). The atom with a higher electronegativity gets the slight negative charge, and the atom with a lower electronegativity will get the slight positive charge. When the positive end of one molecule and the negative end of another molecule are close by, an electrostatic interaction will form between the two molecules. This is known as dipole dipole interaction.
What is Dispersion Forces?
This is also known as London dispersion forces. For an intermolecular attraction, there should be a charge separation. There are some symmetrical molecules like H2, Cl2 where there are no charge separations. However, electrons are constantly moving in these molecules. So there can be instant charge separation within the molecule if the electron moves toward one end of the molecule. The end with the electron will have a temporarily negative charge, whereas the other end will have a positive charge. These temporary dipoles can induce a dipole in the neighboring molecule and thereafter, an interaction between opposing poles can occur. This kind of interaction is known as an instantaneous dipole- induced dipole interaction. And this is a type of Van der Waals forces, which is separately known as London dispersion forces.
What is the difference between Dipole Dipole Interaction and Dispersion Forces?
• Dipole dipole interactions occur between two permanent dipoles. In contrast, dispersion forces occur in molecules where there are no permanent dipoles.
• Two non polar molecules can have dispersion forces and two polar molecules will have dipole dipole interactions.
• Dispersion forces are weaker than dipole dipole interactions.
• The polarity differences in the bond and electronegativity differences affect the strength of dipole dipole interactions. The molecular structure, size and number of interactions affect the strength of dispersion forces.