Substituent, electronegative

Table Bl.11.1. Effect of an electronegative substituent upon methyl shifts in X- C H. ...

Electronegative substituents m a molecule can affect acidity even when they are not directly bonded to the lomzable proton Compare ethanol (CH3CH2OH) with a related compound m which a CE3 group replaces the CH3 group... 

The decreased shielding caused by electronegative substituents is primarily an inductive effect and like other inductive effects falls off rapidly as the number of bonds between the substituent and the proton increases Compare the chemical shifts of the pro tons m propane and 1 mtropropane... 

The deshieldmg effects of electronegative substituents are cumulative as the chem ical shifts for various chlorinated derivatives of methane indicate... 

The effect of structure on acidity was introduced m Section 1 15 where we developed the generalization that electronegative substituents near an lomzable hydrogen increase Its acidity Substituent effects on the acidity of carboxylic acids have been extensively studied... 

An electronegative substituent particularly if it is attached to the a carbon increases the acidity of a carboxylic acid As the data m Table 19 2 show all the mono haloacetic acids are about 100 times more acidic than acetic acid Multiple halogen sub stitution increases the acidity even more trichloroacetic acid is 7000 times more acidic than acetic acid ... 

Sections Electronegative substituents especially those within a few bonds of the... 

Sulfates —0—SO2—0— Primary alkyl salts Secondary alkyl salts 1415-1380 (s) 1200-1185 (s) 1315-1220 (s) 1140-1075 (m) 1270-1210 (vs) 1075-1050 (s) Electronegative substituents increase frequencies. Strongly influenced by metal ion Doublet both bands strongly influenced by metal ion... 

Halogenation reactions usually involve pyrroles with electronegative substituents. Mixtures are usually obtained and polysubstitution products, ie, tetrahalopyrroles, predominate. The monohalopyrroles are difficult to prepare and are not very stable in air or light. 

An electronegative substituent adjacent to a ring oxygen atom also shows a preference for an axial orientation. This is known as the anomeric effect , and is particularly significant to the conformations of carbohydrates (B-71MI20100, B-83MI20100). 

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

Carbanion-stabilizing effects have been calculated at several levels of theory. Table 7.6 gives some gas-phase data. The AMI and PM3 semiempirical calculations have also been done in water. The order NO2 > CH=0 > CN > Ph > CH2=CH is in accord with the experimental trends and reflects charge delocalization. The electronegative substituents F, OH, and NH2 are stabilizing by virtue of polar effects. The small stabilization provided by CH3 is presumabfy a polarization effect. 

The exceptions are formaldehyde, which is nearly completely hydrated in aqueous solution, and aldehydes and ketones with highly electronegative substituents, such as trichloroacetaldehyde and hexafluoroacetone. The data given in Table 8.1 illustrate that the equilibrium constant for hydration decreases with increasing alkyl substitution. 

In contrast to hydrogen-type ethers, a-haloethers, both linear and cyclic, are relatively easily cleaved by anhydrous hydrogen fluoride. Bis( 1,1 -difluoroalkyl) ethers are converted to 1,1,1 -trifluoroalkanes and alkanoyl fluorides The cleavage temperature depends on the substituents present ethers having no electronegative substituents other than a-fluorines are readily cleaved below 20 °C, 3-halo-1,1-di fluoroethers require approximately 70 °C, but 2-halo-1,1-difluoroethers are prac tically resistant toward hydrogen fluoride [/I (equation 1)... 

Sections Electronegative substituents, especially those within a few bonds of the 19.6-19.7 carboxyl group, increase the acidity of carboxylic acids. 

Anomeric effect (Section 25.8) The preference for an electronegative substituent, especially a hydroxyl group, to occupy an axial orientation when bonded to the anomeric carbon in the pyranose form of a carbohydrate. 

Ethylene has no dipole moment, and propene has only a very small one. The other systems have nontrivial dipole moments. Thus, the more electronegative substituents produce nontrivial dipole moments, in contrast to a single hydrogen atom or a methyl group. ... 

Tile CH carbon shift ranges between S 62.4 and 95.1 ppm and is typical of carbons carrying two electronegative substituents. Tliis could be a hint for the observed overall ease of substitution reaction at this center. However, electronic and structural properties of these substituents cause no characteristic differences in the S values. 

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