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Below are the lewis structures for dimethyl ether and acetone. My chemistry textbook indicates that acetone has a larger dipole moment than dimethyl ether. I am familiar with dipole moments and the fact they are created by an unequal sharing of electron density. However I do not understand why acetone is more polar than dimethyl ether. My best guess is that having an extra carbon provides more electrons which allows the molecule to establish a larger difference in electron density between the oxygen and the rest of the molecule. I am unsure if this is correct and would greatly appreciate a more in depth explanation.

Dimethyl ether

Dimethyl ether

Acetone

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    $\begingroup$ Well the electron density is farther away from the core in acetone? $\endgroup$ – Kenny Lau Sep 16 '17 at 16:56
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    $\begingroup$ While it's nice of you to add pictures, I doubt anyone not knowing these structures already, would help you. $\endgroup$ – Mithoron Sep 16 '17 at 18:08
  • $\begingroup$ For acetone, hyperconjugation also comes into effect and the methyl groups donate electron density towards the carbonyl group as well, resulting in an even stronger dipole moment $\endgroup$ – Tan Yong Boon Sep 17 '17 at 4:29
  • $\begingroup$ However, the electron-donating effect of the methyl groups may actually reduce the strength of the dipole moment of the carbonyl group as it neutralises the partial positive charge on the central carbon. So I don't know which perspective is correct... $\endgroup$ – Tan Yong Boon Sep 17 '17 at 4:35
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The ether is fairly symmetrical though there is a bond angle of ~111°, "exposing" the oxygen on the side (and its electron cloud). Acetone, however, has an oxygen off to the side, with its electron cloud offset ~120 picometers (pm), making it more polar.

Remember that oxygen is a strong electron acceptor, so that there will be a "lump" of electron probability surrounding it.

bond lengths and angles of acetone & dimethyl ether

[Sorry about the hodgepodge illustration!]

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In a very simplistic view, the dipole moment can be considered the vector addition of all polarised bonds. In an initial step, we can even completely ignore the $\ce{C-H}$ bonds since they are almost unpolar.

In dimethyl ether, you have two polarised bonds that point more or less towards each other at an angle of about $111^\circ$. This is not quite opposite but a large part of the dipole moment will cancel itself out, leaving only a rather small component along the principal $C_2$ axis.

In acetone, the main component of the dipole moment is a bond on this principal $C_2$ axis, which here coincides with the $\ce{C=O}$ bond. No cancelling out can happen.

Additionally, there is only a single carbon atom bound to that oxygen of acetone so this carbon has to deal with the entirety of oxygen’s electronegativity. In dimethyl ether, two carbon atoms can share their electron density with oxygen making each individual dipole less strong.

Therefore, acetone has a higher dipole moment.

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  • $\begingroup$ Can we completely ignore C-H bonds? Wouldn't there be some hyperconjugation going on? $\endgroup$ – Tan Yong Boon Sep 17 '17 at 4:37
  • $\begingroup$ @TanYongBoon Hyperconjugation would put more electron density on or close to oxygen in this case so yes. $\endgroup$ – Jan Sep 17 '17 at 11:26

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