The Structure of Propane Affects Its Properties

Propane is a gaseous hydrocarbon byproduct of the petroleum industry. Its chemical formula is C₃H₈. Propane's bulk properties depend in part upon its intermolecular forces-forces between any propane molecule and its neighboring propane molecules. These depend on a number of factors, including molecule shape and electrical charge distribution, which in turn is dependent upon elemental composition.

Elements and Structure

Propane's structural formula is H₃C-CH₂-CH₃. The two -CH₃ groups (one drawn as H₃C) are methyl groups. The central -CH₂- group is a methylene group. The hydrogen (H) atoms of each methyl group are spread in three dimensions like tripod legs. For the methyl on the left, they spread to the left-for those on the right, to the right. The central or number two carbon atom is tetrahedral. In the model associated with this article, the methyl groups attached to the methylene group point downward to the left and downward to the right, whereas the two individual hydrogen atoms are seen pointing upward, to the front and to the rear.

Elemental Composition and Charge Distribution

Propane is comprised of only the two elements, carbon and hydrogen. The difference in electronegativities between these two elements is minimal. There is only a slight negative-to-positive carbon-to-hydrogen charge along its chemical bond. What charge another molecule of methane sees is largely canceled out. To illustrate this, consider an even simpler molecule, methane (CH₄).

Methane's carbon is central and its hydrogen atoms are uniformly distributed about that carbon. The partial charge distribution-a dipole charge-between one carbon and hydrogen is effectively canceled out by symmetry-that is to say there is no dipole moment. The same thing is true for all of a methane molecule's carbon-to-hydrogen bonds. The net effect is almost zero.¹

The Molecular Shape of Propane

Although the methyl groups are nearly equivalent to a methane molecule, they are not completely so. Only three carbon-to-hydrogen bonds exist. The fourth bond is a carbon-to-carbon bond, whether it is carbon-1 to carbon-2 or carbon-2 to carbon-3. Thus there is a real but very small dipole moment for propane, extending downward from the central carbon, opposite the attached hydrogen atoms (note second image). Dipole charges increase molecular cohesive forces and the tendency to liquefy. Thus the boiling point of methane is minus 161 degrees Celsius, whereas that of propane is minus 42 degrees.

¹ A very slight positive charge is felt when another molecule of methane temporarily approaches one of the hydrogen atoms of the methane molecule.

References and Resources:

University of Southern Maine: Polarity