Electronegative oxygen

C. Calculate the dipole moment of cyclopropenone using the OPTIONS input to change the 1.1 matrix element to 2.0 for electronegative oxygen. Use the results to infer the direction of the dipole moment toward or away from the oxygen atotn. 

NMR The electronegative oxygen of an alcohol decreases the shielding of the car bon to which it is attached The chemical shift for the carbon of the C—OH is 60-75 ppm for most alcohols Carbon of a C—S group is more shielded than carbon of C—O... 

In this case, the ti-MOs resemble those of butadiene. Relative to the butadiene orbitals, however, the acrolein orbitals lie somewhat lower in energy because of the effect of the more electronegative oxygen atom. This factor also increases the electron density at oxygen relative to carbon. 

The additional electronegative oxygen atom in the sulfoxonium salts stabilizes these ylides considerably, relative to the sulfonium ylides. ... 

Every alternating atom in the PDMS chain is the electronegative oxygen, which makes it a polar polymer. The authors observed that at low loadings of PDMS, PVC is destabilized. This is to be expected according to the alternative model for degradation and stabilization of PVC put forward by Naqvi. But for compositions with 50% or more PDMS, both polymers are stabilized. No possible explanation can be given for this reversal in thermal stability at this time. 

When two resonance forms are nonequivalent, the actual structure of the resonance hybrid is closer to the more stable form than to the less stable form. I bus, we might expect the true structure of the acetone anion to be closer to the resonance form that places the negative charge on an electronegative oxygen atom than to the form that places the charge on a carbon atom. 

Organic acids are characterized by the presence of a positively polarized hydrogen atom (blue in electrostatic potential maps) and are of two main kinds those acids such as methanol and acetic acid that contain a hydrogen atom bonded to an electronegative oxygen atom (O-H) and those such as acetone (Section 2.5) that contain a hydrogen atom bonded to a carbon atom next to a C=0 bond (Q=C-C-H). 

The acidity of acetone and other compounds with C=0 bonds is due to the fact that the conjugate base resulting from loss of H+ is stabilized by resonance. In addition, one of the resonance forms stabilizes the negative charge by placing it on an electronegative oxygen atom. 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Since the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carbonyl carbon rehybridizes from sp2 to sp3 during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

As we saw in A Preview of Carbonyl Compounds, the most general reaction of aldehydes and ketones is the nucleophilic addition reaction. A nucleophile, Nu-, approaches along the C=0 bond from an angle of about 75° to the plane of the carbonyl group and adds to the electrophilic C=0 carbon atom. At the same time, rehybridization of the carbonyl carbon from sp2 to sp3 occurs, an electron pair from the C=0 bond moves toward the electronegative oxygen atom, and a tetrahedral alkoxide ion intermediate is produced (Figure 19.1). 

The conjugate addition of a nucleophile to an a,fi-unsaturated aldehyde or ketone is caused by the same electronic factors that are responsible for direct addition. The electronegative oxygen atom of the a,/3-unsaturated carbonyl compound withdraws electrons from the /3 carbon, thereby making it electron-poor and more electrophilic than a typical alkene carbon. 

Additionalstrongly electronegative oxygen atoms are bonded to X These tend to draw electrons away from the oxygen atom bonded to H. Thus we would predict that the ease of dissodation of a proton, and hence Km should increase in the following order, from left to right ... 

PI ace the following elements in order of increasing electronegativity oxygen, carbon, nitrogen, fluorine, silicon, phosphorus, sulfur. 

Several structural features, including electron transfer between atoms of different electronegativity, oxygen deficiency, and unsynchronized resonance of valence bonds, as well as tight binding of atoms and the presence of both hypoelectronic and hyperelectronic elements, cooperate to confer metallic properties and high-temperature superconductivity on compounds such as (Sr.Ba.Y.LahCuO,-,. 

Another seven-membered ring that shows some aromatic character is tropone (44). This molecule would have an aromatic sextet if the two C=0 electrons stayed away from the ring and resided near the electronegative oxygen atom. In fact, tropones are stable compounds, and tropolones (45) are found in nature. However, analyses of dipole moments, NMR spectra, and X-ray diffraction measurements show that tropones and tropolones display appreciable bond alternations. ... 

These molecules must be regarded as essentially nonaromatic, although with some aromatic character. Tropolones readily undergo aromatic substitution, emphasizing that the old and the new definitions of aromaticity are not always parallel. In sharp contrast to 44, cyclopentadienone (46) has been isolated only in an argon matrix below 38 Above this temperature, it dimerizes. Many earlier attempts to prepare it were unsuccessful. As in 44, the electronegative oxygen atom draws electron to itself, but in this case it leaves only four electrons and the molecule is unstable. Some derivatives of 46 have been prepared. ... 

The more electronegative oxygen atoms are outer atoms ... 

For a solution of ammonia in acetone, we must examine both components. Acetone has an electronegative oxygen atom with nonbonding pains, whereas NH3 has a polar N—bond. Consequently, a mixture of these two compounds displays hydrogen bonding between the hydrogen atoms of ammonia and oxygen atoms of acetone ... 

Acids consistently display low solubility in DMSO. In our Pfizer studies we found acids to be 4 times more likely to have poor DMSO solubility of less than 5pgmL than any other functionality. The reason is that DMSO with its very electronegative oxygens is a poor solvator for any compound with partial negative or full anionic character. 

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