Question

How do dipole dipole forces arise?

Answer 1

Dipole-dipole forces are intermolecular forces resulting from the attraction of the positive and negative ends of the dipole moments in polar molecules.

Explanation:

Dipole-dipole forces arise from the unequal distribution of electrons between atoms in a compound.

For example, consider `CH_3Cl`. Chlorine is very electronegative, and so it pulls electrons away from the adjacent carbon atom toward itself, giving chlorine a partial negative charge and carbon a partial positive charge. We would refer to this molecule as polar because electrical charge is not symmetrically distributed.

A dipole moment ​is a measurement of the separation of the ends of a dipole, which are oppositely charged. Each molecular dipole moment has a positive and negative end.

Now, there is certainly more than one molecule of a compound in whatever substance you might be dealing with, which can be easy to forget. In a polar compound, the most stable arrangement has the positive end of one dipole close to the negative end of another. So in a solution of chloromethane, we would expect the molecules to orient in such a way that the partially negative-charged Cl of one molecule is aligned with the partially positive-charged C of another, and so on.

In summary, dipole-dipole forces are generally attractive intermolecular forces resulting from the attraction of the positive and negative ends of the dipole moments of polar molecules (1).

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1. Wade, L. G.; Simek, J. W. In Organic Chemistry; Pearson: Glenview, IL, 2013; pp 60.