Ethylene glycol, HOCH2CH2OH, has zero dipole moment even though carbon-oxygen bonds are strongly polarized. Explain.
The correct answer and explanation is :
Correct Answer:
Ethylene glycol (HOCH₂CH₂OH) has a zero dipole moment because the individual bond dipoles within the molecule cancel each other out due to its molecular symmetry and specific three-dimensional arrangement.
Explanation (approx. 300 words):
Ethylene glycol, with the formula HOCH₂CH₂OH, is a diol containing two hydroxyl (–OH) groups attached to a two-carbon backbone. Each carbon is bonded to one hydroxyl group and two hydrogen atoms. The carbon-oxygen bonds in the hydroxyl groups are indeed polar, with oxygen being significantly more electronegative than carbon and hydrogen. This leads to a separation of charge, forming bond dipoles.
However, the net dipole moment of a molecule is determined not just by the presence of polar bonds but also by the geometry of the molecule — specifically, how these dipoles add up vectorially. In the case of ethylene glycol, the molecule can adopt a conformation in which the bond dipoles from each –OH group are oriented in opposite directions. This happens because the molecule is flexible due to single bonds (sigma bonds) that allow rotation.
In its lowest energy conformation, ethylene glycol adopts a gauche or skewed structure where the –OH groups are positioned such that their individual dipole moments oppose and effectively cancel each other. This results in no net molecular dipole moment — or a dipole moment so small it is effectively zero.
Furthermore, the molecule’s approximate symmetry in its spatial arrangement enhances this cancellation. The central C–C bond forms the axis of rotation, and the molecule can arrange itself such that the vector sum of all bond dipoles (including those from the C–O and O–H bonds) equals zero.
Therefore, despite the strong polarization of the C–O and O–H bonds, ethylene glycol’s overall geometry ensures that these dipole moments cancel out, resulting in a molecule with a net dipole moment of zero.