Ketones atomic charges

Dinur U and A T Hagler 1995. Geometry-Dependent Atomic Charges Methodology and Application to Alkcmes, Aldehydes, Ketones and Amides. Journal of Computational Chemistry 16 154-170. 

In order to predict the structure of the product, you must identify the factors that will tend to favor selective ketal formation. Consider selective carbonyl protonation first. Obtain energies and atomic charges, and display electrostatic potential maps of the alternative protonated ketones (protonated ketone A, protonated ketone B). Identify the more stable isomer. Compare geometries and draw whatever Lewis structures are needed to account for your data. Why is one isomer more stable than the other Is the more stable isomer also that in which the positive charge is better delocalized Will the more stable isomer undergo nucleophilic attack more or less easily than the other Explain. 

Figure 6.5. Comparison of NMR shifts, ppm from TMS, of ketone carbonyl-carbon atoms with the corresponding atomic charges, reported in Table 6.4. The atom numbering is that indicated in this Table. (Reproduced with permission from Ref. 41.)...

Atomic Charges—Methodology and Application to Alkanes, Aldehydes, Ketones, and Amides. 

The far right structural representation shows the + and - charges separated by three atoms instead of the usual two atoms (e.g., for the C==0 in aldehydes and ketones). Larger charge separation results in increased polarity and increased intermolecular forces, even greater than those due to hydrogen bonding. 

Many biological processes involve an associa tion between two species in a step prior to some subsequent transformation This asso ciation can take many forms It can be a weak associ ation of the attractive van der Waals type or a stronger interaction such as a hydrogen bond It can be an electrostatic attraction between a positively charged atom of one molecule and a negatively charged atom of another Covalent bond formation between two species of complementary chemical re activity represents an extreme kind of association It often occurs in biological processes in which aide hydes or ketones react with amines via imine inter mediates... 

The acid-catalvzed hydration reaction begins with protonation of the carbonyl oxygen atom, which places a positive charge on oxygen and makes the carbonyl group more electrophilic. Subsequent nucleophilic addition of water to the protonated aldehyde ot ketone then yields a protonated gem diol, which loses H+ to give the neutral product (Figure 19.5). 

The hydration reaction just described is typical of what happens when an aldehyde ot ketone is treated with a nucleophile of the type H-Y, where the Y atom is electronegative and can stabilize a negative charge (oxygen, halogen, or sulfur, for instance). In such reactions, the nucleophilic addition is reversible, with the equilibrium generally favoring the carbonyl reactant rather than the tetrahedral addition product. In other words, treatment of an aldehyde or... 

Both H and R can attack a ketone or aldehyde to give an alcohol. The main difference is the effect on the carbon skeleton. With H, the carbon skeleton does not change at all. But with R, the carbon skeleton gets larger. We are forming a C—C bond. We will soon see that this is very important for synthesis problems. For now, let s focus on how we can make R in the first place. After all, a negative charge on a carbon atom is not very stable (and therefore not trivial to make). 

There are four bases in DNA guanine, thymine, cytosine and adenine. Each has a ketone C=0 group in which the oxygen is quite electronegative and bears an excess negative charge <5 , and an amine in which the electropositive hydrogen atoms bear an excess... 

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