Carbon-halogen bond polarity

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

Relatively simple notions of attractive forces between opposite charges are suffi cient to account for many of the properties of chemical substances You will find it help ful to keep the polarity of carbon-oxygen and carbon-halogen bonds m mind as we develop the properties of alcohols and alkyl halides m later sections... 

The carbon-halogen bond in an alkyl halide is polar... 

Secondary alkyl halides react by a similar mechanism involving attack on benzene by a secondary carbocation Methyl and ethyl halides do not form carbocations when treated with aluminum chloride but do alkylate benzene under Friedel-Crafts conditions The aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon-halogen bonds and these complexes are the electrophilic species that react with benzene... 

Ketene Insertions. Ketenes insert into strongly polarized or polarizable single bonds, such as reactive carbon—halogen bonds, giving acid hahdes (7) and into active acid haUdes giving haUdes of p-ketoacids (8) (46). Phosgene [77-44-5] (47) and thiophosgene [463-71-8] (48) also react with ketenes. 

The carbon-halogen bond of alkyl halides is polarized. 

Because the carbon-halogen bond is polar the OH attacks the S+ carbon atom and so the reaction is classified as a nucleophilic substitution. Reactions in which an -OH group replaces a halogen atom are also called hydrolysis reactions. 

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds. Nucleophilic substitution reactions are categorised into and on the basis of their kinetic properties. Chirality has a profound role in understanding the reaction mechanisms of Sj l and Sj 2 reactions. Sj 2 reactions of chiral all l halides are characterised by the inversion of configuration while Sj l reactions are characterised by racemisation. 

Apart from the carbon-halogen bond, the carbon-oxygen one is rather active toward the reductive cleavage due to its polarity, so different types of compounds bearing a carbon-oxygen bond are able to undergo this reaction. 

Several research groups ha ve been involved in the study of ET reactions from an electrochemically generated aromatic radical anion to alkyl halides in order to describe the dichotomy between ET and polar substitution (SN2). The mechanism for indirect reduction of alkyl halides by aromatic mediators has been described in several papers. For all aliphatic alkyl halides and most benzylic halides the cleavage of the carbon-halogen bond takes place concertedly with the... 

Catalysis of Nucleophilic Substitution Reactions. It has been known for many years that metal ions with a strong affinity for halogens will accelerate the reactions of alkyl halides with nucleophiles (Equation 3). It is assumed that the polarization of the carbon-halogen bond, as a consequence of coordination,... 

Ionic or polar reactions of alkyl halides rarely are observed in the vapor phase because the energy required to dissociate a carbon-halogen bond heterolyti-cally is almost prohibitively high. For example, while the heat of dissociation of chloromethane to a methyl radical and a chlorine atom is 84 kcal mole-1 (Table 4-6), dissociation to a methyl cation and a chloride ion requires about 227 kcal mole-1 ... 

Acyl halides have a rather positive carbonyl carbon because of the polarization of the carbon-oxygen and carbon-halogen bonds. Addition of a nucleophilic group such as the oxygen of an alcohol occurs rather easily. 

Elimination reactions can also occur when a carbon halogen bond does not completely ionize, but merely becomes polarized. As with the El reactions, E2 mechanisms occur when the attacking group displays its basic characteristics rather than its nucleophilic property. The activated complex for this mechanism contains both the alkyl halide and the alkoxide ion. 

We propose a mechanism of carbon-halogen bond-forming ligand coupling of diaryl(halo)-A3-iodanes, which involves a highly polarized apical-equatorial transition state. Ligand coupling mechanism of p-chlorophenyl(p-methyl-phenyl)bromo-A3-iodane 77, which exists as an equilibrium mixture of 77a and... 

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