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In trans compound the dipole moment always cancels each other! Then how can it's dipole moment be greater than cis compound's dipole moment?

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    $\begingroup$ What cis and trans compounds are ou talking about? $\endgroup$ Oct 27, 2017 at 8:43
  • $\begingroup$ I took the case of 2,3-dichloro 2-fluoro Ethene ---in this compound trans compound's dipole moment is higher than cis compound's dipole moment. But why? $\endgroup$
    – Sarah jane
    Oct 27, 2017 at 8:59
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    $\begingroup$ In this compound there are no cis or trans. There are Z and E instead. $\endgroup$ Oct 27, 2017 at 9:17
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    $\begingroup$ How is there 3 is an ethene? $\endgroup$ Oct 27, 2017 at 9:50

1 Answer 1

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No.

The dipole moment of trans-cyclooctene ($\pu{0.82 D}$) is greater than cis-cyclooctene ($\pu{0.43 D}$). See On the Molecular Geometry of trans-Cycloöctene, J. Am. Chem. Soc. 1958, 80 (8), 1953–1955.

Blomquist, Burge and Sucsy2 have called attention to the fact that while with the cyclodecenes, the cis isomer has the higher density and index of refraction as is typical in acyclic systems, with the cyclooctenes, as first noted by Ziegler and Wilms,3 the reverse is true. From these refractive indices, by application of van Arkel’s dipole rule,4 the prediction could be made that the trans-cyclooctene would have a higher dipole moment than the cis.

[...]

The dipole moments of the cis and trans isomers of cyclodecene were previously determined,4 and were found to be normal. The conclusion, drawn from the dipole rule, that the trans-cyclooctene would have a higher dipole moment than the cis represents a situation without analogy.

[...]

The cis isomer was found to have $\mu_{cis} = \pu{0.43 \pm 0.06 D}$ which may be compared with the value found for cis-cyclodecene of $\pu{0.44 D}$.4 The trans isomer was found to have a dipole moment of $\pu{0.82 \pm 0.03 D}$, compared with the value zero found for trans-cyclodecene.4

(Van Arkel's rule is: The isomer with the higher dipole moment possesses the higher boiling point, refractive index, and density.)

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    $\begingroup$ This is a very good example that challenges assumptions nicely. $\endgroup$
    – Zhe
    Oct 27, 2017 at 12:05
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    $\begingroup$ "On the Molecular Geometry of trans-Cycloöctene" Nice o-umlaut you have there ;-) Also, have an upvote! $\endgroup$ Oct 27, 2017 at 16:01
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    $\begingroup$ @paracetamol I was surprised to see the umlaut. They use it throughout the article. $\endgroup$
    – DavePhD
    Oct 27, 2017 at 16:28
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    $\begingroup$ For general interest, the umlaut here essentially serves as a pronunciation guide---it's a somewhat antiquated way of distinguishing the diphthong 'oo' from two (separate) vowels next to each other 'o' + 'o', and you also see it in words like preëxistence, etc. $\endgroup$ Oct 27, 2017 at 17:10
  • $\begingroup$ And in this context it shouldn’t be called umlaut but trema $\endgroup$
    – Jan
    Nov 1, 2017 at 14:28

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