Carbon disulfide (\(\text{CS}_2\)) is a compound frequently used in industrial processes as a solvent. Understanding whether a compound is polar or nonpolar is fundamental to understanding its behavior and how it interacts with other substances. Molecular polarity determines many of its physical properties, such as its solubility and boiling point. To determine \(\text{CS}_2\)‘s polarity, one must examine the distribution of electric charge, which depends on the specific atoms involved and the molecule’s three-dimensional shape.
Defining Polarity in Chemistry
The concept of polarity begins with the unequal sharing of electrons between atoms, a property known as electronegativity. When two atoms with a noticeable difference in electronegativity bond, this unequal sharing creates a partial negative charge (\(\delta^-\)) and a partial positive charge (\(\delta^+\)). This results in a polar covalent bond, which is called a bond dipole. Molecular polarity depends on the combined effect of all bond dipoles and the molecule’s geometry. If the bond dipoles are arranged symmetrically, they cancel each other out, resulting in a net zero dipole moment and a nonpolar molecule.
How Carbon Disulfide is Structured
Carbon disulfide is formed from one carbon atom and two sulfur atoms, with the carbon atom positioned centrally between the two sulfur atoms. The Lewis structure shows that the central carbon atom forms a double bond with each of the two sulfur atoms, resulting in the structure \(\text{S}=\text{C}=\text{S}\). In \(\text{CS}_2\), the carbon atom has two double bonds and no lone pairs of electrons. These two double bonds constitute two regions of electron density, which attempt to position themselves as far apart as possible to minimize electron repulsion. This specific arrangement forces the three atoms into a straight line, establishing a perfectly linear molecular geometry.
Determining the Final Polarity of \(\text{CS}_2\)
The first step in determining the overall polarity of carbon disulfide is to assess the polarity of its individual \(\text{C}-\text{S}\) bonds. Carbon and sulfur have Pauling electronegativity values that are very close, but a slight difference does exist. Sulfur is marginally more electronegative than carbon, meaning that the shared electrons in the \(\text{C}=\text{S}\) double bonds are pulled slightly toward each sulfur atom. This small difference in electron-attracting power establishes two separate bond dipoles, making each \(\text{C}=\text{S}\) bond slightly polar. The vector of each dipole points from the less electronegative carbon atom toward the more electronegative sulfur atom.
Since the molecule has polar bonds, the final determination rests entirely on its geometry. Because \(\text{CS}_2\) possesses a perfectly linear structure, the two \(\text{C}=\text{S}\) bond dipoles are oriented 180 degrees apart. The dipole moment created by the left \(\text{C}=\text{S}\) bond is exactly equal in magnitude and opposite in direction to the dipole moment created by the right \(\text{C}=\text{S}\) bond. Consequently, these opposing dipoles cancel each other out completely, resulting in a net molecular dipole moment of zero.
The cancellation of the bond dipoles confirms that carbon disulfide is a nonpolar molecule. This nonpolar nature directly explains its physical behavior; for instance, \(\text{CS}_2\) is readily soluble in other nonpolar substances like oils, benzene, and many organic solvents. In contrast, it exhibits very low solubility in highly polar solvents such as water, following the general chemical rule that “like dissolves like.”