Aromaticity and the Hückel 4n + 2 Rule - Chemistry LibreTexts
The molecule is cyclic (a ring of atoms); The molecule is planar (all You can see how this works with the molecular orbital diagram for the. There is also very little difference in electronic states between the D5h and D5d through the Fe atom there will be centro symmetric (g) and anti symmetric (u) combinations. •. The five The π-molecular orbitals of the cyclopentadienyl ring ( D. The cyclopentadienyl anion C5H5- is the deprotonated form of cyclopentadiene. 6 π electrons make it aromatic and it acts as a pentahapto ligand, bound by all 5 .
How do we know that it is fully conjugated? That is, how do we know that each atom in this molecule has 1 p orbital? Let's look at the following figure. Carbons are sp2 hybridized because they have 3 attached atoms and have no lone electron pairs. What about carbon 1? Another simple rule to determine if an atom is sp2 hybridized is if an atom has 1 or more lone pairs and is attached to an sp2 hybridized atom, then that atom is sp2 hybridized also.
This video explains the rule very clearly. Therefore, carbon 1 has a p orbital.
Molecular orbital theory: conjugation and aromaticity - Chemistry LibreTexts
Heterocyclic Aromatic Compounds So far, you have encountered many carbon homocyclic rings, but compounds with elements other than carbon in the ring can also be aromatic, as long as they fulfill the criteria for aromaticity.
These molecules are called heterocyclic compounds because they contain 1 or more different atoms other than carbon in the ring. A common example is furan, which contains an oxygen atom. We know that all carbons in furan are sp2 hybridized.
But is the oxygen atom sp2 hybridized? The oxygen has at least 1 lone electron pair and is attached to an sp2 hybridized atom, so it is sp2 hybridized as well. Notice how oxygen has 2 lone pairs of electrons.
2.2: Molecular orbital theory: conjugation and aromaticity
An sp2 hybridized atom only has 1 p orbital, which can only hold 2 electrons, so we know that 1 electron pair is in the p orbital, while the other pair is in an sp2 orbital. A Common Misconception A very common misconception is that hybridization can be used to predict the geometry, or that hybridization somehow involves an energy cost associated with 'promoting' electrons into the hybrid orbitals.
This is entirely wrong. Hybridization is always determined by geometry.
Interactions between Cyclopentadienyl Molecular Orbitals and Metal d Orbitals
You can only assign hybridization states to an atom if you already know its geometry, based on some experimental or theoretical evidence. Problems Using the criteria for aromaticity, determine if the following molecules are aromatic: Aromatic Character and Aromaticity. Cambridge University Press, Contributors Valence bond theory does a remarkably good job at explaining the bonding geometry of many of the functional groups in organic compounds.
There are some areas, however, where the valence bond theory falls short. It fails to adequately account, for example, for some interesting properties of compounds that contain alternating double and single bonds. In order to understand these properties, we need to think about chemical bonding in a new way, using the ideas of molecular orbital MO theory.
When we described the hydrogen molecule using valence bond theory, we said that the two 1s orbitals from each atom overlap, allowing the two electrons to be shared and thus forming a covalent bond. In molecular orbital theory, we make a further statement: Recall that an atomic orbital such as the 1s orbital of a hydrogen atom describes a region of space around a single atom inside which electrons are likely to be found. A molecular orbital describes a region of space around two or more atoms inside which electrons are likely to be found.
We saw this previously when we discussed hybrid orbitals: Molecular orbitals for H2 According to MO theory, one sigma orbital is lower in energy than either of the two isolated atomic 1s orbitals —this lower sigma orbital is referred to as a bonding molecular orbital.
The second, 'sigma star' orbital is higher in energy than the two atomic 1s orbitals, and is referred to as an antibonding molecular orbital. The bonding sigma orbital, which holds both electrons in the ground state of the molecule, is egg-shaped, encompassing the two nuclei, and with the highest likelihood of electrons being in the area between the two nuclei. Remember that we are thinking here about electron behavior as wave behavior.
When two separate waves combine, they can do so with constructive interference, where the two amplitudes build up and reinforce one another, or destructive interference, where the two amplitudes cancel one another out.
Bonding MOs are the consequence of constructive interference between two atomic orbitals, which results in an attractive interaction and an increase in electron density between the nuclei.
Following the same aufbau 'building up' principle you learned in General Chemistry for writing out electron configurations, we place the two electrons in the H2 molecule in the lowest energy molecular orbital, which is the bonding sigma orbital.
The bonding attracting MO is full, and the antibonding repulsing MO is empty. MO theory and conjugated pi bonds The advantage of using MO theory to understand bonding in organic molecules becomes more apparent when we think about pi bonds.Molecular Orbital Theory Heteronuclear Diatomic (Cyanide, CN-) Example
We start with two atomic orbitals: Each contains a single electron. There is increased electron density between the two carbon nuclei in the molecular orbital - it is a bonding interaction. Again using the 'building up' principle, we place the two electrons in the lower-energy, bonding pi molecular orbital. Next, we'll consider the 1,3-butadiene molecule.