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Why is the C-S-C bond angle in thiirane (48.5 degrees) so much smaller than C-O-C bond angle in oxirane/epoxide (60 degrees).

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Source: https://coek.info/pdf-three-membered-ring-heterocycles-.html

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    $\begingroup$ Because the sulfur atom is big, maybe? $\endgroup$ Commented Oct 17, 2021 at 10:02
  • $\begingroup$ How does that cause the bond angle to be smaller? $\endgroup$
    – Anonymous
    Commented Oct 17, 2021 at 11:36
  • $\begingroup$ Because the non-bonding doublets are more cumbersome than the bonding doublets. Sulfur is a large atom. It has two bonding electrons that are stretched between carbon and sulfur, and so are not really cumbersome. But it also has two non-bonding doublets that repell one another and are attracted by the sulfur nucleus. They also repell the stretched bonding electrons. As a consequence the angle between them is smaller than it is in a ordinary triangle $\endgroup$
    – Maurice
    Commented Oct 17, 2021 at 11:46
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    $\begingroup$ Similar question, possibly a dupe: Why does bond angle decrease in the order H2O, H2S, H2Se? $\endgroup$
    – andselisk
    Commented Oct 17, 2021 at 12:57
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    $\begingroup$ This is pure planar geometry, no chemistry required. Try drawing a triangle with one side much smaller than the other two - what happens to the angle between the large sides? I guess the chemistry part comes in knowing that a C-C bond is much shorter than a C-S bond, as your table shows. $\endgroup$ Commented Oct 18, 2021 at 0:10

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