The dipole moment of phenol is smaller than that of methanol

Analysis of the causes of the smaller dipole moment of phenol than methanol

in chemistry, the dipole moment of a molecule is often used to describe the distribution of charges within a molecule. When discussing the polarity of molecules, the dipole moment is an important parameter, which can usually reflect the asymmetry of charges in molecules. Many people may ask why phenol has a smaller dipole moment than methanol. Today, we will analyze this question and understand the reasons behind it.

1. Phenol and methanol molecular structure comparison

We need to compare the molecular structure of phenol and methanol. Phenol (C6H5OH) consists of a benzene ring and a hydroxyl group (OH), which is attached to the benzene ring. Methanol (CH3OH) is a simple alcohol molecule consisting of a methyl group (CH3) and a hydroxyl group (OH).

From the structural point of view, the hydroxyl group in methanol is directly connected with a methyl group, forming a relatively simple and symmetrical molecular structure. Relatively speaking, the structure of phenol is more complex, and the existence of benzene ring makes the molecular structure become more asymmetric. Therefore, although they all have hydroxyl groups, the electron cloud of the benzene ring affects the charge distribution of the hydroxyl group, thereby changing the magnitude of the dipole moment.

2. Electronic effects

We need to discuss the influence of the electronic effect on the dipole moment of the molecule. The hydroxyl group in methanol forms a strong hydrogen bond with the hydrogen atom through its oxygen atom, and due to the high electronegativity of the oxygen atom, the hydroxyl group will pull the electron to the direction of the oxygen atom, thus forming a relatively strong dipole.

In the phenol molecule, the electron cloud of the benzene ring affects the electron distribution of the hydroxyl group. The π-electron system on the benzene ring is highly delocalized, which makes the electron distribution of the benzene ring more uniform and reduces the charge asymmetry around the hydroxyl group. Therefore, although there is also a strong dipole in the phenol molecule, the dipole moment of phenol is smaller than that of methanol due to the electronic effect of the benzene ring.

3. Intermolecular Forces Comparison

In addition to the electronic effect, the interaction force between molecules is also an important factor affecting the dipole moment. The interaction between methanol molecules is mainly through hydrogen bonding, which makes the dipole moment between methanol molecules more concentrated and can effectively enhance the dipole effect.

Relatively speaking, the intermolecular force between phenol molecules is more complex, in addition to hydrogen bonds, there are van der Waals force and π-π interaction. The presence of the benzene ring makes the interaction between molecules less direct than the hydrogen bond in methanol, so the dipole moment of phenol is smaller than that of methanol.

4. Numerical differences in dipole moments.

Specifically, the dipole moment of methanol is approximately 1.69 Debye, while the dipole moment of phenol is 1.28 Debye. This numerical difference again proves that the dipole moment of phenol is indeed smaller than that of methanol. In methanol, the molecular dipole moment is relatively large due to the symmetrical structure of hydroxyl group and methyl group. The effect of the benzene ring in phenol on the dipole moment makes the dipole moment value less than that of methanol.

5. Conclusion

The reason why the dipole moment of phenol is smaller than that of methanol is mainly related to its molecular structure, electronic effect and intermolecular force. The presence of the benzene ring makes the charge distribution of the phenol molecule more uniform, thereby reducing the magnitude of the dipole moment, while the methanol has a relatively large dipole moment due to its simpler molecular structure and strong hydroxyl polarity. This phenomenon is determined by both intramolecular and intermolecular interactions.