Question

Which one is most, and least polar among acetic acid, salicylic acid, and acetylsalicylic acid? I googled a lot, and the...


Which one is most, and least polar among acetic acid, salicylic acid, and acetylsalicylic acid? I googled a lot, and they said salicylic acid is more polar than acetylsalicylic acid because of the -OH group, and acetic acid is more polar than salicylic acid because salicylic acid has benzene ring. So it says acetic>salicylic>acetylsalicylic. But my another research says dipole moments of them are 1.74D, 2.65D, and 6.49D, and I know bigger dipole moments, more polar. So it says acetic<salicylic<acetylsalicylic. I'm confused by this. Which one is correct?

Answer

Polarity is a phenomenon in which partial charges are generated between an electropositive and electronegative atom. The polar bonds are due to the electronegativity difference between the bonded atoms. The dipole moment is the measure of charge separation by a specific distance. The dipole moment value is a net value that considers the direction of polarity in a molecule.

The acetic acid will be most polar as it has a polar hydroxy group attached to the aliphatic hydrocarbon chain. This will be followed by salicylic acid as it again has a polar -OH group attached to a less polar aryl benzene ring. The last in the trend will be acetylsalicylic acid, as it does not have a polar hydroxy group. The extent of polarity and dipole moment value are not related directly. 

This can be understood by the fact that carbon dioxide is a non-polar molecule but still, it has a dipole moment. Another example is carbon tetrachloride. The molecule highly has a highly polar bond between chlorine and carbon but overall the dipole moment is zero because the direction is opposite and gets canceled out. Therefore polarity of the molecule cannot be related to the dipole moment value, rather the structure and the resultant dipole are also considered. 

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