June 04, 2020, 01:27:20 AM
Forum Rules: Read This Before Posting


Topic: VSEPR and double bonds and lone pairs  (Read 2293 times)

0 Members and 1 Guest are viewing this topic.

Offline earthnation112

  • Regular Member
  • ***
  • Posts: 94
  • Mole Snacks: +2/-0
VSEPR and double bonds and lone pairs
« on: July 05, 2016, 09:32:27 AM »
Questions were asked on another forum with no reply, so I thought il try getting input here. My questions are:

According to VSEPR theory would I be right to assume that the first image (attached below) is more accurate and is the one that should be used rather than the second one, as the first image includes the double bonds present which can’t be shown using wedges and dashes?  Thanks

Also when drawing a molecule using VSEPR do you show lone pairs that are not on the central atom but on another atom? Or do you only show lone pairs on the central atom and ignore the rest but include them only when drawing a lewis dot structure? Thanks

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7321
  • Mole Snacks: +514/-86
  • Gender: Male
Re: VSEPR and double bonds and lone pairs
« Reply #1 on: July 05, 2016, 12:33:18 PM »
When we draw VSEPR structure we are interested about direction of bonds and electron pairs only.
Your left structure informs us about bonding and shows no information about directions of bond.
You should take into account all convention included in chemical nomenclature, eventually discuss with IUPAC experts about changing convention. Moreover, you stereochemical convention is out of date.
AWK

Offline earthnation112

  • Regular Member
  • ***
  • Posts: 94
  • Mole Snacks: +2/-0
Re: VSEPR and double bonds and lone pairs
« Reply #2 on: July 05, 2016, 01:25:14 PM »
When you mention direction of bond are you referring to the wedges and dashes which show whether something is in front or behind the planer?

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7321
  • Mole Snacks: +514/-86
  • Gender: Male
Re: VSEPR and double bonds and lone pairs
« Reply #3 on: July 05, 2016, 02:53:33 PM »
For out of date i meant a wedge of parralel lines (below plane); you used a bold wedge line (above plane) correctly).

IUPAC 1996 recomendations:
"Graphic Representation of Three-Dimensional Structures*
Structural diagrams which depict stereochemistry must be prepared with extra care to ensure there is no ambiguity. In general plain lines depict bonds approximately in the plane of the drawing; bonds to atoms above the plane are shown with a bold wedge - (starting from an atom in the plane of the drawing at the narrow end of the wedge); and bonds to atoms below the plane are shown with short parallel lines I I I I I I I I I . As an alternative a bold bond may be used instead of a bold wedge. A broken line- - - - -has been used instead of parallel lines but this is better reserved for a partial bond, delocalisation, or a hydrogen bond. The use of a wedge of parallel lines is not recommended as it is ambiguous".
AWK

Offline earthnation112

  • Regular Member
  • ***
  • Posts: 94
  • Mole Snacks: +2/-0
Re: VSEPR and double bonds and lone pairs
« Reply #4 on: July 05, 2016, 03:02:20 PM »
Thanks for the input, but ff you use wedges and dashes to show out of plane and in, how would one then show double bonds? As you show double bonds when using VSEPR right?

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7321
  • Mole Snacks: +514/-86
  • Gender: Male
Re: VSEPR and double bonds and lone pairs
« Reply #5 on: July 05, 2016, 03:09:23 PM »
Just draw one structure with bonds and the second with stereochemistry like you did in the first post.
AWK

Sponsored Links