Energy gap between orbitals and stability

In summary, benzene has 6 pi electrons in each of its 6 pi orbitals while butadiene has 4 pi electrons in each of its 4 pi orbitals. This results in a decrease in the energy gap between the bonding and antibonding orbitals in conjugated double bonds.
  • #1
Tahmeed
81
4
We know that the lower the energy gap between bonding and antibonding orbital the easier it is to break the bond making it unstable. However, in a conjugated double bond, the overlapping of wavefunctions cause the energy gap between the bonding and antibonding orbital go down. Then why conjugated double bonded molecules are more stable??
I aint sure whether this should go to chemistry part of the forum or it is fine to post here. so sorry in advance if this is in wrong part.
 
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  • #2
Tahmeed said:
However, in a conjugated double bond, the overlapping of wavefunctions cause the energy gap between the bonding and antibonding orbital go down.
No, this is not true. With conjugation, the energy gap increases.
 
  • #3
DrDu said:
No, this is not true. With conjugation, the energy gap increases.
But i am pretty sure about it. This is why UV Visible spectometry works. You can check wikipedia.
 
  • #4
Tahmeed said:
We know that the lower the energy gap between bonding and antibonding orbital the easier it is to break the bond making it unstable. However, in a conjugated double bond, the overlapping of wavefunctions cause the energy gap between the bonding and antibonding orbital go down. Then why conjugated double bonded molecules are more stable??
I aint sure whether this should go to chemistry part of the forum or it is fine to post here. so sorry in advance if this is in wrong part.
I think this is the right section of the forum. It would still be fine if it was in general physics section in my opinion.

I don't think I understand you well, but what do you mean "more stable"? There are several ways you can consider a molecule stable such as entire energy of the system, inertness to photo-oxidization or reduction, decomposition point, etc.
DrDu said:
No, this is not true. With conjugation, the energy gap increases.
In some very minor cases (that I don't remember), you are right. But generally, with more conjugation, the energy gap decreases.
 
  • #5
It may be the case that the HOMO-LUMO gap decreases, but that is not the whole story. You have to think about the total pi electron energy. What MOs are produced by the overlap of the 6 pi orbitals in benzene, for example, or the four pi orbitals in butadiene? How are they filled? What is the pi electron energy compared to that of forming three (or two) separate pi bonds?
 

1. What is the energy gap between orbitals and how does it affect stability?

The energy gap between orbitals refers to the difference in energy levels between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This gap plays a crucial role in determining the stability of a molecule. A larger energy gap indicates a more stable molecule, as it requires more energy to promote an electron from the HOMO to the LUMO. This makes it more difficult for the molecule to undergo chemical reactions, making it more stable.

2. How does the energy gap between orbitals affect a molecule's reactivity?

The energy gap between orbitals directly impacts a molecule's reactivity. A smaller energy gap means that it takes less energy to promote an electron from the HOMO to the LUMO, making the molecule more reactive. This is because the electrons in the HOMO are more readily available to participate in chemical reactions. On the other hand, a larger energy gap makes it more difficult for electrons to participate in reactions, resulting in a less reactive molecule.

3. Can the energy gap between orbitals be manipulated?

Yes, the energy gap between orbitals can be manipulated through various methods such as changing the molecular structure or introducing substituents. These changes can alter the electron distribution within the molecule, affecting the energy levels of the HOMO and LUMO. This can ultimately impact the stability and reactivity of the molecule.

4. How does the energy gap between orbitals differ for different types of molecules?

The energy gap between orbitals can vary greatly between different types of molecules. For example, molecules with conjugated systems, such as benzene, tend to have smaller energy gaps between their orbitals compared to non-conjugated molecules. This is because conjugation allows for delocalization of electrons, making it easier to promote an electron from the HOMO to the LUMO.

5. How can the energy gap between orbitals be determined experimentally?

The energy gap between orbitals can be determined experimentally through various techniques, such as UV-visible spectroscopy and cyclic voltammetry. These methods involve measuring the energy required to promote an electron from the HOMO to the LUMO and can provide valuable information about a molecule's stability and reactivity.

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