Video Transcript
The table shows the physical properties of three aromatic compounds: benzene, phenol, and toluene. Which column corresponds to phenol?
In the provided data table, each column includes physical property data for one of the three aromatic compounds. A compound is classified as aromatic if it contains a benzene-like ring structure. There are exceptions as some aromatic compounds contain rings with atoms other than carbon, contain rings with a different number of atoms, such as a five-membered ring, or contain multiple ring structures. The molecules must contain an alternating single carbon-to-carbon bond and double carbon-to-carbon bond pattern. The molecules are flat and have delocalized electron density spread out over the ring structure. In benzene, this is caused by the interactions between the unhybridized p orbitals of the carbon atoms in the ring.
There are a wide variety of aromatic compounds, but in this video, we will focus only on benzene and monosubstituted arenes. It would be helpful to sketch structural formulas for the three aromatic compounds in this problem to identify their differences. Benzene consists of a six-membered hydrocarbon ring with the alternating carbon-to-carbon single bond and carbon-to-carbon double bond pattern. Toluene is a monosubstituted arene, which means there is one substituent directly bonded to the aromatic ring structure. In toluene, the substituent is a methyl group. Phenol is also a monosubstituted arene, but the substituent is a hydroxy group.
Benzene, toluene, and phenol are all similar in size and molecular structure, but the physical properties of phenol are significantly different. Let’s find out why. We know that the intermolecular forces between molecules of a compound have an impact on certain physical properties. Benzene and toluene are composed entirely of nonpolar carbon-to-carbon and carbon-to-hydrogen bonds. Therefore, these molecules are nonpolar and weak London dispersion forces will be present between the molecules of each compound.
In contrast, the phenol molecule is polar. The oxygen-to-hydrogen single bond of the hydroxy group is highly polar because of the large difference in electronegativity between the oxygen and hydrogen atoms. Hydrogen bonding will occur between the hydrogen atom that is covalently bonded to the oxygen atom and a lone pair of electrons on the oxygen atom of a nearby phenol molecule. The hydrogen bonding between phenol molecules is much stronger than the dispersion forces between the molecules of benzene and toluene. Therefore, we’d expect that the melting point and boiling point of phenol will be greater than that of benzene and toluene. In fact, the melting point of phenol is high enough for it to be a solid at room temperature.
Now, the benzene ring is not very soluble in water due to its nonpolar character. So benzene and toluene are essentially insoluble in water. However, the very polar hydroxy group in the phenol molecule can hydrogen bond with water molecules, which significantly increases phenol solubility in water. In summary, due to the presence of hydrogen bonding, phenol will have a greater melting point, a greater boiling point, and greater solubility in water than both benzene and toluene. The data provided in column A has the greatest values for melting point, boiling point, and solubility. Therefore, column A corresponds to phenol.
