Question

Which one is most, and least polar among acetic acid, salicylic acid, and acetylsalicylic acid?...


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

The relation between polar or non-polar molecule and dipole moment is as follows:

The polar molecule has larger electronegativity difference between the atoms and has net dipole moment. The non-polar molecule has lesser electronegativity difference between the atoms and has zero dipole moment.

Or,

The larger the value of dipole moment, more polar will be the molecule and the lesser the value of dipole moment, less polar will be the molecule.

As the dipole moment of acetic acid, salicylic acid, and acetylsalicylic acid is 1.74 D,  2.65 D, and 1.58 D respectively. So, the order of polarity of molecule will be:

Salicylic acid > Acetic acid > Acetylsalicylic acid

Salicylic acid is a polar molecule because the carboxylic acid group and hydroxyl group are place asymmetrically in the molecule and it cause a net dipole moment that show it is polar.

The salicylic acid is more polar than acetylsalicylic acid because of the presence of -OH group and shows intramolecular and intermolecular hydrogen bonding whereas acetylsalicylic acid shows only intramolecular hydrogen bonding due to steric hindrance between -COOH and -OCOCH3 group.

Thus, the least polar and strong polar molecule is acetylsalicylic acid and salicylic acid respectively.

Answer: The least polar and strong polar molecule is acetylsalicylic acid and salicylic acid respectively.

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