Polarization of the C=O Bond

and its impact on reactivity

The C=O bond is highly polarized toward the oxygen.  This is primarily an effect of the oxygen atom's electronegativity; however, resonance effects accentuate this over what is seen in an ether:

res1.gif (5507 bytes) The resonance structures of C=O place a partial positive charge on carbon, which attracts the electron density of a nucleophile.

One way we can further understand the reactivity uses the results of molecular orbital calculations.  The highest occupied molecular orbital (HOMO) shows where electrophiles will attack; the lowest unoccupied MO will show where nucleophiles will attack.  Alternatively, the relative charge distribution diplayed in an electrostatic potential map can tell us similar things.


 A simplified diagram emphasizes the C=O bond:
The LUMO of the C=O bond is bigger on carbon, which also enhances reaction with a nucleophile.
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Load HOMO

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The HOMO and LUMO of the carbonyl group in cyclohexanone (click the link to visualize). Red and blue signify only different mathematical signs for the MO wavefunction. Both of these MOs mix in parts of the rest of the molecule (as all molecular orbitals do), but you can see three distinguishing features:
  • The composition of the pi bond, arising from interaction of p orbitals on the sp2-hybridized carbon and oxygen atoms.
  • In the HOMO, the density is greatest on oxygen, reflecting the polarization of the bond. Most of the electron density in the pi bond will reside on oxygen, rather than carbon. (Compare this to the equal distribution in a C=C pi bond.)
  • In the LUMO, the biggest part of the MO is on carbon. If we interact with a Lewis base, the carbon atom will be the primary point of interaction because of this, and the orientation of this orbital governs the trajectory of attack of the nucleophile.

Load ESP surface
Now load the electrostatic potential map for cyclohexanone (click link to load). 
Blue = positive potential;
Red = negative potential.		 The net effect of all of the MOs working together (though consistent with the HOMO & LUMO we illustrate) is to have electron density build up on oxygen, and decreased elsewhere in the molecule. Note especially the difference between axial and equatorial hydrogens on the alpha position--we'll see this is important in later chapters.

The most important reactivity feature of the carbonyl group is that (electon-rich) nucleophiles attack at the carbonyl carbon to form new bonds.

res3.gif (2212 bytes) Resonance in C=O
Now, look at what happens when an (electron-poor) electrophile reacts.  H+ bonds to the oxygen, accentuating the positive character of the carbonyl carbon, particularly when compared to the rest of the molecule. This accelerates attack of a nucleophile.
res2.gif (2309 bytes) Resonance in the protonated form is similar and enhances the positive charge on carbon.
Note when you load the ESP map, that the molecule as a whole is more blue (positive), centered on the added proton and the carbonyl carbon. Any Lewis acid has this effect of binding to oxygen, and activating the carbonyl carbon for nucleophilic attack.
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Aldehydes are generally more reactive than ketones to nucleophilic addition; a quick look at the ESP map confirms that the carbonyl is slightly more positive at carbon. 
(Why, do you think?)

Also, the steric congestion is far less at an aldehyde than a ketone.
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