ilwdx
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why the energy of sigma bond (C-C) follows this sequence?
CHCH higher than CH2=CH2 higher than CH3-CH3?
[Edited on 20-6-2004 by ilwdx]
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Chemtastic
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Are you asking why a triple bond possesses more energy than a double bond and a double bond more energy than a single bond?
If that's the case, think of it as the energy required to seperate electrons held in orbit (orbitals really) around both nuclei. Instead of a
non-zero probability of finding the electron orbiting one nucleus, there is a non-zero probability of finding it around either nucleus (it actually is
more likely found between, thus the electron density map overlaps).
A single bond (one sigma bond--s suborbitals), like that found in ethane, requires seperation of only 2 electrons. A double bond (one sigma bond and
one pi bond--p suborbitals) requires seperation of 4 electrons. A triple bond (one sigma bond and two pi bonds) requires seperation of 6 electrons.
While not entirely appropriate, think of it energetically as creating a 2+ ion vs. a 6+ ion.
That started to get dangerously close to a first-year quantum mechanics lecture
EDIT:
In case I wasn't entirely clear, the title of your post is somewhat misleading. Only the first bond (the single bond) is a sigma bond. The
second and third of the double and triple bonds are both pi bonds.
[Edited on 20-6-2004 by Chemtastic]
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ilwdx
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| Quote: | Originally posted by Chemtastic
Are you asking why a triple bond possesses more energy than a double bond and a double bond more energy than a single bond?
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O, I see, it is somewhat misleading, I don't mean that.
I mean the one sigma bond C-C inside the CH3CH3, CH2CH2, CHCH, not including the pi bond.
and the sequence is
CHCH higher than CH2CH2 than CH3CH3
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Marvin
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The question was clear, but unusually technical.
A guess here but for a triple bond the carbon atoms are much closer together than for a single bond.
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Chemtastic
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Yeah, Now That I See What You Mean
I would have to agree with Marvin. The triple bond being shorter than the double bond and the double bond shorter than the single would put the
electrons of the sigma bond closer to either nucleus. More attraction between them, so the bond energy is progressively greater for ethane >
ethene > ethyne
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ilwdx
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I agree, however I cannot think of a more convincing one
It is free-response question of AP test
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ilwdx
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Can't believe they still keep my account after 16 years. Basically, the answer is that in acetylene it's sp hybridization, sp2 in ethylene, and sp3
for ethane. The hybridized orbitals are increasingly larger due to the greater p character than s character of the orbitals. So, the sigma bonds
formed from the overlap of the sp-sp, sp2-sp2, and sp3-sp3 orbitals are increasingly longer.
All those comments above are correct in the sense that the electrons in these orbitals are closer to the two carbon nuclei in each bond - the
attraction is stronger when they are close. So, the stronger sigma bond in acetylene than ethylene than ethane.
[Edited on 3-7-2020 by ilwdx]
[Edited on 3-7-2020 by ilwdx]
[Edited on 3-7-2020 by ilwdx]
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