Saturday, January 4, 2014

Burgi-Dunitz Angle

We all learn nucleophilic attack on aldehydes and ketones in organic chemistry courses. To be honest, I have never wondered the angle of the attack. I always thought that the nucleophile should attack perpendicular to the C=O bond axis. The reason I was thinking like this was that the R groups should cause steric hindrance and make the attack less likely at any other angle. I thought the electron rich bond and the lone pairs on oxygen couldn't have much effect on the attack. Looks like I was quite wrong. I should have considered hybridization and molecular orbitals too. To be clear, you can see the way I thought the attack happens below:

Last night,  I learned that nucleophiles attack the carbon at ~107 degrees to the C=O bond axis. So, my curiosity took me to the original paper by Burgi and Dunitz. 

I read the paper a few times and here is what I learned:

1. As the nucleophile attacks, the R groups bend and C-O bond becomes longer.
2. As the nucleophile approaches even more, RRCO becomes even more nonplanar. Looks like sp2 hybridized carbon becomes sp3 hybridized. 

One of the R's in aldehydes is hyrdogen. This also explains why aldehydes are more reactive than ketones. They have less steric hindrance in the system. (This is one of the reasons.)

I think this is the way it happens:

From molecular orbital theory approach; the electrons in the HOMO of the nucleophile interact with the antibonding orbital (LUMO) of the C=O bond. Both the bonding and antibonding orbitals are occupied now. This breaks the Pi bond and the electrons move to the most electronegative atom (oxygen) in the bond. 

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