Aldehydes 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. 

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

They estimated the ene reactivity of trihaloaldehydes on the basis of the atomic charge and LUMO energy level by running an MO calculation on the aldehyde-fT" complexes as a model of aldehyde/chiral Lewis acid complexes [15J, The results from semi-empirical (MNDO and PM3) and ab initio (6-3IG ) calculations are listed in Table 1-1. 

Although the models found using regression and anndes contain different descriptors, there are obvious similarities for each model. The most important features found in all the models used for predicting toxicity of chemicals of this class seem to be the aldehyde groups, molecular connectivity, and atomic charge. 

MBH adducts and their derivatives derived from methyl acrylate and aldehydes undergo stereoselective cycloadditions with diazomethane and benzonitrile oxide to give the corresponding cycloadducts in good yields (Scheme 3.214). The stereochemical outcome can be explained by the so-called inside alkoxy elfect theory.However, in the case of diazomethane cycloadditions, electrostatic factors play a reduced role compared to the corresponding nitrile oxide reactions, while steric elfects are of major importance in governing the stereoselectivity. This dilferent behavior of the two 1,3-dipoles has been rationalized by analysis of the atomic charges, as calculated at the RHF/3-21G level of theory, for the transition structure of these reactions. 

Many biological processes involve an "association" between two species in a step prior to some subsequent transformation. This association can take many forms. It can be a weak association 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 reactivity represents an extreme kind of association. It often occurs in biological processes in which aldehydes or ketones react with amines via imine intermediates. 

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). 

In a typical run, bis(l,2-diphenylphosphino)ethane (DPPE) (0.022 g, 0.05 mmol) and 1,3 diene (32.5 mmol) are added to a portion of the co-condensate, containing 5.2 mg of rhodium (0.05 mg. atom) in 10 ml of mesitylene. The solution is introduced by suction into an evacuated, 80 ml stainless steel autoclave. Carbon monoxide is introduced to the desired pressure and the autoclave is rocked and heated at 80 °C. Hydrogen is rapidly charged to give 1 1 gas composition. When the pressure reaches the theoretical value corresponding to the desired conversion, the autoclave is cooled, depressurised, and the reaction mixture analyzed by GLC. The crude product is distilled. The aldehydes are obtained as pure samples by preparative GLC and characterized by H NMR spectroscopy and GC-MS analysis. 

Some cation-radicals can appear as hydrogen acceptors. Thus, fullerene Cgg is oxidized to the cation-radical at a preparative scale by means of photoinduced electron transfer. As in the case of anion-radical, the fullerene Cgo cation-radical bears the highly delocalized positive charge and shows low electrophilicity. This cation-radical reacts with various donors of atomic hydrogen (alcohols, aldehydes, and ethers) yielding the fullerene 1,2-dihydroderivatives (Siedschlag et al. 2000). 

You see in Chapter 10 that aldehydes and ketones contain a carbonyl group attached to carbon or hydrogen atoms. In the case of carboxylic acids and their derivatives, a carbonyl group is attached to an electronegative element such as oxygen, chlorine, or nitrogen. The presence of these elements tends to increase the 5+ charge on the carbonyl carbon, which makes the carbon atom more susceptible to nucleophilic attack. 

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