Carbonyl groups polarity

Fig. 90. (A) Glycine-tyrosine bound to carboxypeptidase (443). Indirect attack of Glu-270 promotes the attack of a water molecule on the amido carbonyl group polarized by interaction with zinc. (B) Direct attack of Glu-270 on the amido carbonyl with formation of an anhydride.

In Chapter 10 we used NMR to convince you that a carbonyl group polarized a conjugated alkene we hope you find the NMR data even more convincing. Conjugate addition occurs to those very atoms whose electron deficiency we can measure by proton NMR. 

Three main concepts influence acidity when comparing methanol with formic acid (1) the carbonyl group polarizes the 0-H bond, (2) the OH of carboxylic acids hydrogen bonds to a greater extent, and (3) the conjugate base (formate, 9) is more stable than methoxide (2), driving the equilibrium to the right. These concepts are the same as those introduced in Chapter 2 to explain acid-base reactions and the importance of the acidity constant, K. ... 

The acidity of carboxylic acids is also partly the result of an inductive effect. The carbonyl group polarizes the H—O bond by attracting electrons through the O-bonding network. Withdrawing electron density from the O—H bond weakens it, increasing the acidity of the hydrogen atom. 

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