Orbital theory - Molecular vs Hybrid orbitals

[christian0710]

Hi I have a question regarding Molecular orbital theory:
Is it correctly understood that if we combine 2 or more atoms the atomic orbitals can become 2 things: They can either become Hybrid orbitals which are the orbitals that form sigma bonds between atoms, or they can become Molecular orbitals which are orbitals allowing the electrons to float freely around in the molecule in conjugated molecules and they always form pi bonds?

An example would be 1,3-butadiene: it has 4 molecular p-orbitals and 2*3= 6 hybridized sp2 orbitals? So the p-orbitals maintain their shape (like the atomic p-orbitals) but now we call them molecular orbitals.

 

[DrClaude]

Hi I have a question regarding Molecular orbital theory:
Is it correctly understood that if we combine 2 or more atoms the atomic orbitals can become 2 things: They can either become Hybrid orbitals which are the orbitals that form sigma bonds between atoms, or they can become Molecular orbitals which are orbitals allowing the electrons to float freely around in the molecule in conjugated molecules and they always form pi bonds?

Not exactly. The molecular orbital theory you are talking about is better known as LCAO-MO: Linear Combination of Atomic Orbitals - Molecular Orbital. It is based on the idea that an MO can be approximated as the LC of AOs located on different atoms, but it says nothing about what those AOs should be. Most often, taking hydrogen-like AOs gives a good enough approximation to the MOs.

However, there are cases, and carbon is a good example, where these hydrogen-like AOs give predictions (such as bond angles) which make no sense. A good solution to this problem is hybridization: take the hydrogen-like orbitals, make linear combination of them into hybrid AOs, and use those in LCAO-MO theory.

An example would be 1,3-butadiene: it has 4 molecular p-orbitals and 2*3= 6 hybridized sp2 orbitals? So the p-orbitals maintain their shape (like the atomic p-orbitals) but now we call them molecular orbitals.

The sp2 orbitals are AOs, which combine into MOs to form the σ bonds. The un-hybridized (left over) p AOs combine into MOs to form π bonds.

 

[christian0710]

Hi and thank you for the reply:
So in the below example you mention that sp2 orbitals ARE atomic orbitals (how can sp,sp2,sp3 be atomic orbitals- I thought they were only hybrid orbitals?) which form to combine Molecular orbitals. I'm not sure I'm following what you wrote aboute the LCAO-MO theory. So is it a incorrectly understood that when atomic orbitals combine they form hybrid orbitals (which form sigma bonds), and the left over orbitals (which are not hybridized) Can or cannot overlap to form sigma bonds?

 

[christian0710]

Can you recommend a book i can read to understand it more clearly? I'm taking organic chemistry, and I wan't to understand how Molecular Orbital theory can descibe or account for all the chemical bonds and chemical interactions taking place in chemistry.

 

[DrClaude]

Hi and thank you for the reply:
So in the below example you mention that sp2 orbitals ARE atomic orbitals (how can sp,sp2,sp3 be atomic orbitals- I thought they were only hybrid orbitals?)

Atomic orbitals correspond to possible states for electrons in an atom. The hybirdized sp, sp² and sp³ orbitals are still orbitals calculated for one atom, and therefore are atomic orbitals.

I mentionned "hydrogen-like" orbitals because the s, p and d orbitals that you are familiar with are adapted from the solution you get for the hydrogen atom (only one electron). When considering atoms with more than one electron, exact solutions are not possible, and an approximate description for orbitals is used.

So is it a incorrectly understood that when atomic orbitals combine they form hybrid orbitals (which form sigma bonds), and the left over orbitals (which are not hybridized) Can or cannot overlap to form sigma bonds?

When atomic orbitals are combined on one atom, the result is called hybrid orbtials (which are still atomic orbitals). When atomic orbitals are combined on different atoms, the result is a molecular orbital. When molecular orbitals are occupied, this can lead to the formation of a bond (ut not necessarily, some molecular orbitals are anti-bonding).

In organic chemistry, you are mainly concerned with the hybridization of C (although N hybridization can also explain the bonding in some molecules). In most cases, the hybrid orbitals will form sigma bonds and the left-over p orbitals (for sp and sp²) will make pi bonds.

Can you recommend a book i can read to understand it more clearly? I'm taking organic chemistry, and I wan't to understand how Molecular Orbital theory can descibe or account for all the chemical bonds and chemical interactions taking place in chemistry.

You have to look at books on quantum chemistry. A good choice is the one by McQuarrie:http://books.google.se/books/about/Quantum_Chemistry.html?id=zzxLTIljQB4C&redir_esc=y

 

[DrDu]

One should also mention that hybrid orbitals are a concept from valence bond theory rather than molecular orbital theory.