"H - C = C - H" doesn't make sense chemically why?

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"H - C = C - H" doesn't make sense chemically why?

Post by MajesticQ » Tue Aug 12, 2008 4:14 pm

Help me understand why does the following structure fail to make sense chemically?

H - C = C - H

In my mind it would look something like this...


(I'm self-studying from Campbell's textbook, this is one of the questions at the end of the chapter. I read the chapter, can't make sense of it all though)

PS the right end of the image got cropped out here from some reason, but you can figure it out

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Post by canalon » Tue Aug 12, 2008 7:55 pm

There is adiiference between H-C=C-H (impossible) and H-C=C-H (triple liaison in the last one, double in the first one). In fact look what you have drawn ;-)

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Post by mith » Wed Aug 13, 2008 8:10 am

carbon has 4 valence
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Re: "H - C = C - H" doesn't make sense chemically why?

Post by blcr11 » Mon Aug 18, 2008 6:35 pm

As Mith points out, the full valence of carbon is 4. It will prefer to share its four electrons with its bonding partners. The structure as drawn has the carbons sharing only three electrons (one for each bond) which means that each carbon has a non-bonded electron. That makes it an acetylene diradical. Such a molecule may exist along some photochemical reaction path, and you can, I’m sure, calculate its energy, which is undoubedly high relative to acetylene, but it is not a stable electonic structure for H2C2. The stable structure for H2C2 is acetylene or ethyne with a triple bond between the two carbons, as Canalon said.

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