Introduction to Dimethyl Sulfoxide (DMSO)

Dimethyl Sulfoxide, (CH₃)₂SO is an organosulfur compound. It is a colorless liquid that is a polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water. An aprotic solvent is a solvent that has ion dissolving power but lacks an acidic hydrogen, which is a hydrogen that can be donated to another molecule. Dimethyl Sulfoxide has an intermediate polarity, it does not display hydrogen bonding. DMSO has a unique property of being able to penetrate living tissues without causing significant damage, because of this property it is marketed with other skin treatments to provide better penetration of the product. 

Oxygen=Red, Sulfur= Yellow, Carbon= Gray, Hydrogen=White

The above geometric structure is of the type AX₃E. It is a polar covalent molecule with London Forces and Dipole-Dipole intermolecular forces. It is a triangular pyramid, with 109.5 degree angles between the central atom and terminal atoms. The molecules molar mass is 78 g/mole. 

Intermolecular Forces

(The above picture also shows the polarity of the bonds in the individual molecule as well.) 

If two Dimethyl Sulfoxide molecules were attracted to each other, Dispersion forces and Dipole-Dipole forces would occur. 

London Dispersion forces act on all molecules, it does not matter if it is a nonpolar or polar  molecule. Dispersion forces occur when there is an attraction between temporary or induced dipoles in adjacent molecules. It is a temporary dipole. London dispersion is caused by the movement of electrons around the two atoms, one end of a polar molecule can be temporarily positive and another end may be temporarily negative. In DMSO it occurs between the atoms. 

Dipole Dipole occurs in all polar molecules. Adjacent molecules line up so the negative pole of one molecule is as close as possible to the positive pole of its neighbor. It is a electrostatic attraction. Oxygen is more electronegative than Sulfur; therefore a dipole can be created between a Sulfur of one molecule and Oxygen of an adjacent molecule. (This is indicated by the dotted line) 

There is no Hydrogen bonding because Hydrogen bonding is a specific type of dipole-dipole where a bond is formed between a Hydrogen atom and a Nitrogen, Oxygen, or Fluorine atom. Hydrogen bonding is a special type of the dipole-dipole attraction.  This does not occur in DMSO. As mentioned in the introduction, DMSO is an aprotic solvent. An aprotic solvent is a solvent that has ion dissolving power but lacks an acidic hydrogen, which is a hydrogen that can be donated to another molecule. Thus, it cannot have hydrogen bonding.

Polarity

Polar molecules contain positive and negative poles.  This is a result of an unsymmetrical distribution of electrons. In DMSO, Oxygen is more electro-negative than sulfur. As a result of this, the electrons tend to move closer to the nucleus of the oxygen atom. This creates a dipole with the cloud of electrons concentrated more around the oxygen. Sulfur is more electro-negative than carbon. Therefore, there would also be a polarity between sulfur and carbon with the electron cloud being concentrated towards sulfur. Due to the multiple polar bonds in the molecule itself the overall molecule is considered polar. 

The differences between the electronegativity of atoms are:

Between sulfur and oxygen: .9 (moderately covalent)

Between sulfur and carbon: .1 (very covalent)

Between carbon and hydrogen: .3 (very covalent) 

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