Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: MopUSAF on November 12, 2006, 02:01:18 PM
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Hello. I was just wondering if, when told to show a specific example of carbocationn formation, it would be correct to show a rxn involving a strong acid and a ethene molecule. The pi-bonding electrons in the double bond move to the H+ of the dissociated acid, which results in 2 resonance contributor structures, with the positive charge on either of the two carbon atoms of the ethene. I realize that this probably isn't the BEST example of how a carbocation forms due to the fact that the primary carbocation is very unstable and doesnt like to form, but is this method incorrect? Thank you for any help you can give me.
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There are not two resonance structures for the your carbocation. The carbocation has two hydrogens while the other carbon has three hydrogens. You cannot move the positive charge to the other carbon without moving the hydrogens, so there is no resonance (although structures that differ only by the position of one hydrogen are known as tautomers, this is a different issue). Also, I would say this is a bad example of a carbocation because you won't really see it form under most conditions. Generally, if you are asked to give an example, you should give an example which is representative of what one would find most often (i.e. the formation of a stable carbocation such as a tertiary carbocation).
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I don't know if this is a good example, but the protonation of an alcane with a superacid seems to be a pretty simple and easily understandable example for the formation of a carbocation.
CH4 + H+?H3C+ + H2
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Do you mean H3CCH4+? That's actually a carbonium ion, which is different.
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Hmm... OK when describing one of the ways that a carbocation can form I said through an alkene and a strong acid, and then drew (forgive me if there's a better way to draw things on here, but hopefully you can get the point):
H(2)C==CH(2) + H(+) + Cl(-) -----> [ H(2)C(+)--CH(3) <-> H(3)C--C(+)H(2) ]
I drew the arrow pointing from the double bond of the ethene to the H+ proton, then the two resonance contributors with either of the carbons containing the positive charge (whichever carbon only had 2 hydrogens attached to it). Is this a totally incorrect mechanism for carbocation formation then?
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As has been said, those are not resonance structures because you are moving atoms (H) as well as electrons. They are tautomers.
It is the correct/accepted mechanism.
A vinyl allyl carbocation, eg H2C=CH-+CH2 is resonance stabilised.
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OK I see what you're saying now. Thanks for your help everyone!
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Isn't CH2=CH-CH2+ alyl carbocation? Two resonance structures possible then and the same if have alylic radical formed which is stable (i am not sure if the phrase resonat stabilized correct) and thats why you can't polymerize propilene by simple radical polymerization - this phenomena is known as alylic poisoning. Then Ziegler and later Natta found another solution ;)
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Isn't CH2=CH-CH2+ alyl carbocation?
Yes, my mistake, I got the terms mixed up.