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It is true that acetone is less polar than ethanol. I thought the dipole moment is proportional to polarity, and if so, the dipole moment of acetone should be lower than ethanol. But the dipole moment of acetone is higher than ethanol. Why is it higher?

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    $\begingroup$ Regarding your statement about differences in polarity, did you consider differences in hydrogen bonding capabilities? $\endgroup$
    – Karsten ♦
    Jul 27, 2019 at 14:10
  • $\begingroup$ I change comment because I see your point better. You are rather asking why ethanol is more polar than acetone instead of why acetone has higher dipole moment. . It is not the same question. But you got the answer. $\endgroup$
    – Alchimista
    Jul 28, 2019 at 9:57

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Dipole moment is just one of aspects of solvent polarity, as is well known in HPLC separation analytical technique where the following effects take part:

Dipole moment interaction, permanent and induced The above for the whole molecule and for particular bonds
Electron pair donor
Electron pair acceptor (including $\ce{H+}$ donation)

Depending on solute molecules specifics, the solvent polarity order is variable, as various molecules have different responses to above interactions.

See also Using solvent triangle in HPLC

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The bond dipole μ is given by:

$ \mu =\delta \,d$.

The bond dipole is modeled as +δ — δ- with a distance d between the partial charges +δ and δ-. It is a vector, parallel to the bond axis, pointing from minus to plus, as is conventional for electric dipole moment vectors.

In your case, acetone has a higher dipole moment than ethanol because it has a carbonyl group more towards the center and thus acetone has lesser boiling point than ethanol.

The definition is from Wikipedia, : https://en.m.wikipedia.org/wiki/Bond_dipole_moment

EDIT : Alcohol hydrogen bonding is the major effect here, as in the case of water. (by @Poutnik)

Thanks for the edit, @MathewMahindaratne

Think for yourself

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    $\begingroup$ Acetone has 3 carbons, ethanol 2 carbons. Major effect is alcohol hydrogen bonding, similar to water. $\endgroup$
    – Poutnik
    Jul 27, 2019 at 14:09
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    $\begingroup$ Who said acetone has higher boiling point than ethanol? $\endgroup$
    – Mathew Mahindaratne
    Jul 28, 2019 at 3:17
  • $\begingroup$ Read carefully. $\endgroup$
    – Shishir Maharana
    Jul 28, 2019 at 4:00
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    $\begingroup$ @ShishirMaharana Note that every question and answer has dedicated edit history visible not just to you. Correcting the mistake which another user helped you to discover and then suggest to "read carefully" isn't polite and constructive at all. If there were a flaw in my answer, I would rather be grateful and seek other means to improve my answer instead. $\endgroup$
    – andselisk ♦
    Jul 28, 2019 at 4:08
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    $\begingroup$ Thanks for paying attention though @andselisk. I am very grateful to your noble deed. I respect you to the highest degree sir. Thanks to all the advisors who helped me edit this answer! Thanks to Chemistry Stack Exchange! All hail Chemistry Stack Exchange! rofl!!! :) $\endgroup$
    – Shishir Maharana
    Jul 28, 2019 at 4:22

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