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Why is the dipole moment of water (1.85D) larger than ethanol (1.66D)?

I thought the inductive effect from ethyl would develop more delta - charge on oxygen.. hence increasing the charge and the dipole moment

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It's probably due to the slight difference in electronegativity between H (2.2) and C (2.55)

Here's some data on gas-phase dipole moments from the CRC Handbook:

  • Water - 1.85 D
  • Methanol - 1.7 D
  • Ethanol - 1.66 D
  • n-Propanol - 1.55 D
  • n-Butanol - 1.66 D

Using some incorrect data, I pointed to a possible odd-even effect. Using the correct gas-phase dipole moments, we see that these are all pretty similar (i.e., +/- ~0.2D).

Certainly water has the highest dipole moment, but H is slightly more electropositive than C, suggesting that as a likely cause.

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  • $\begingroup$ Side note - my initial thought was to the odd-even effect, so when I quickly found online data suggesting a large dipole moment for methanol and n-propanol, it seemed reasonable. Always better to go to quality sources. $\endgroup$ – Geoff Hutchison Feb 18 '16 at 19:54

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