Polarized C-X bond

The polarized C-X bonds in aliphatic organic halides are electrophilic and can be attacked readily by various nucleophilies. These reactions... 

In 1998, Yang and coworkers reported a series of (7 )-carvone derived ketones (63) containing a quaternary center at and various substituents at (Fig. 22) [119]. The ees of fran -stilbene oxide varied with different para and meta substituents when 63b was used as the catalyst. The major contribution for the observed ee difference is from the n-n electronic repulsion between the Cl atom of the catalyst and the phenyl group of the substrate. The substitution at also influences the epoxidation transition state via an electrostatic interaction between the polarized C -X bond and the phenyl ring on franx-stilbene (Table 6, entries 3-7, 10-14). In 2000, Solladie-Cavallo and coworkers reported a series of fluorinated carbocyclic ketones... 

Alkyl halides react mainly by heterolysis of the polar C—X bond. NUCLEOPHIUC DISPLACEMENT... 

The polar C-X bond present in alkyl halides has a substantial dipole moment. Alkyl halides are poorly soluble in water, but are soluble in organic solvents. They have boiling points that are similar to alkanes of comparable molecular weight. Due to polarity, the carbon is an electrophilic centre and the halogen is a nucleophilic centre. Halogens are extremely weak nucleophilic centres and therefore, alkyl halides are more likely to react as electrophiles at the carbon centre. 

The left column of Scheme 35 repeats the ionization-dissociation scheme discussed in Section II.G. If carbon is connected to an electronegative element, one speaks of a covalent compound with a polarized C—X bond. This treatment is justified as there is an approximately tetrahedral environment of the corresponding carbon center. Diphenylmethyl chloride, for example, is never termed a contact ion pair. A well-defined ionization step, which was discussed in Section II.G, generates a carbocation... 

Alkyl halides are weakly polar molecules. They exhibit dipole-dipole interactions because of their polar C-X bond, but because the rest of the molecule contains only C-C and C-H bonds they are incapable of intermolecular hydrogen bonding. How this affects their physical properties is summarized in Table 7.1. 

The properties of alkyl halides dictate their reactivity. The electrostatic potential maps of four simple alkyl halides in Figure 7.5 illustrate that the electronegative halogen X creates a polar C - X bond, making the carbon atom electron deficient. The chemistry of alkyl halides is determined by this polar C-X bond. 

The polar C-X bond makes the carbon atom electron deficient in each CH3X molecule. 

Alkyl halides have a polar C—X bond, so they exhibit dipole-dipole interactions but are incapable of intermolecular hydrogen bonding (7.3). 

The polar C—X bond containing an electrophilic carbon makes alkyl halides reactive towards nucleophiles and bases (7.5). 

The validity of the Lemieux-Chii interpretation was entirely supported, and the picturesque rabbit-ear idea shown untenable, by a theoretical study that used an ab initio (Hartree-Fock) calculation, with fluoromethanol as the model compound. The calculations showed that the stable conformation has the C-F bond trans to one electron pair and gauche to another, whereas the conformation in which the C-F bond bisects the electron pairs is the energy maximum. Similar interpretations were established for hydrazine, hydroxylamine, and hydrogen peroxide. The concept can be stated in general terms as a destabilization of a conformation that places a polar (C-X) bond eclipsed between two electron pairs. 

The polar C-X bond present in alkyl halides has a substantial dipole moment. Alkyl halides are poorly soluble in water, but are soluble in organic solvents. They have boiling points that... 

Another variant of this HCN addition to polar C=X bonds is the Strecker synthesis of aminonitriles from ketones or aldehydes, HCN, and ammonia [Eq. (4)] [8]. A number of patents to Distler and co-workers at BASF do teach the use of aqueous media to facilitate isolation of pure products [9], and researchers at Grace [10], Stauffer [11], Mitsui [12], and Hoechst [13] have reportedly used aqueous media to control the rates of formaldehyde aminohydrocyanation to isolate intermediate addition products in good yields. The use of aqueous media has also proven advantageous when amides, rather than nitriles, are the desired products. 

Some of the classic examples where the anomeric effect was first recognized are, in fact, quite complicated. The true importance of the anomeric effect is often masked in these cases because other steric or stereoelectronic interactions are present at the same time. Again, it is the full picture and the balance of multiple effects that matter in molecules with multiple polar C-X bonds (Figure 6.53b). 

Oxo and imido compounds also react as nucleophiles with compounds containing polar C=X bonds, such as aldehydes, ketones, imines, and heterocumulenes. These reactions generally occur by a formal [2+2] cycloaddition process to generate a metala-cycle that breaks down to the more thermodynamically stable combination of products containing metal-ligand and carbon-heteroatom multiple bonds. Three examples of such reactions are shown in Equations 13.90-13.92. In the first two cases, the four-membered metallacycle was isolated or characterized by low-temp6rature NMR spectroscopy. ... 

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