Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: antimatter101 on October 03, 2012, 12:39:13 AM
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The chemistry books say that ethane is the simplest polymer, but I can draw even simpler ones in my chemistry book, such as:
H H H H H H
/ / / / / /
H-N-N-N-N-N-N-H
and
H-O-O-O-O-O-O-H
Please explain.
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That is because N-N and O-O bonds are too unstable to form a long polymer chain.
E.g. hydrazine (H2N-NH2) and hydrogen peroxide (HO-OH) are used in rocket fuels.
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It is not about what you can draw, but about what can exist.
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What about this polymer? It has double bonds for every atom and a lesser weight than ethane.
O=C=C=C=C=C=C=C=C=O
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What about this polymer? It has double bonds for every atom and a lesser weight than ethane.
O=C=C=C=C=C=C=C=C=O
Is that a Carbon nanofiber? :)
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I don't think so. I searched google images and found images of hexagonal groups sharing atoms, pivoted in a cylindral structure. That is NOT a carbon nanofibre.
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I don't think so. I searched google images and found images of hexagonal groups sharing atoms, pivoted in a cylindral structure. That is NOT a carbon nanofibre.
That is carbon nanotube. Whereas molecules with C=C=C=C structure are called cumulenes. The longest cumulene so far synthesized has 7 carbon atoms in its cumulene chain (Bildstein et.al. Journal of Organometallic Chemistry 622 (2001) 135–142) and that is quite a bit shorter than anything that could be called a polymer.
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Thanks. I gave you an extra mole snack.
I still have a nick, though. The replies I received from my initial topic "why is ethane the simplest fibre" has been met with "these such fibres are too unstable to exist".
But why are they unstable? I haven't asked someone in a long time.
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The simple answer is that any other bond you can form to the molecules in the backbone of your fiber would be stronger than the bonds those molecules have to each other. For example, in your cumulene chain, the carbon-carbon double bond has about 600 kJ/mol of energy, while in the alkane chain, a carbon-carbon single bond has about 350 kJ/mol and each C-H bond has about 400 kJ/mol. For every double bond you replace with a single bond and two hydrogens, you are picking up nearly 1000 kJ/mol. That is a lot of chemical potential. Allene bonds are even more stressed than isolated single bonds, so even that 1000 kJ/mol is a low estimate.
As a general rule, carbon is the only element that can make networks and fibers of covalent bonds to itself that are stable enough to make fibers. If you will look at some of the bond energies on this table http://www.wiredchemist.com/chemistry/data/bond-energies-lengths (http://www.wiredchemist.com/chemistry/data/bond-energies-lengths), you will see that single bonds between identical elements tend to be rather weak, with the exception of carbon.
Actually, you can pick up enough stability from conjugation to make an alternating single-double bond carbon fiber, but I'm not sure that would be considered simpler - it looks like this:
-CH=CH-CH=CH-CH=CH-CH=CH-
And although it can be made and studied, it isn't particularly stable in terms of use.
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Thanks! Your bond data sheet gave me everything I need! I gave you an extra mole snack. Please reply more for more mole snacks!