Alkyl halides polar carbon-halogen bonds

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... 

Alkyl halides contain a halogen bonded to a saturated, sp3-hybridized carbon atom. The C-X bond is polar, and alkyl halides can therefore behave as electrophiles. 

It is the polar carbon-halogen bond that causes alkyl halides to undergo substitution and elimination reactions. There are two important mechanisms for the substitution reaction ... 

Nomenclature Physical properties Interesting alkyl halides The polar carbon-halogen bond General features of nucleophilic substitution The leaving group The nucleophile Possible mechanisms for nucleophilic substitution Two mechanisms for nucleophilic substitution The S 2 mechanism Application Useful Snj2 reactions... 

Secondary alkyl halides react with benzene by forming a secondary carbocation. However, primary alkyl halides do not form carbocations under Friedel—Crafts conditions. Instead, the alkyl group transfers directly to the aromatic ring from the Lewis acid—Lewis base complex, which has a highly polarized carbon halogen bond. 

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... 

The carbon-halogen bond of alkyl halides is polarized. 

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. 

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 ... 

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. 

The carbon-halogen bond is slightly polar. Overall, however, an alkyl halide is not much more polar than an alkane, so the physical properties of an alkyl halide are not very different from those of an alkane of similar molecular weight. For example, the boiling point of 1-chlorobutane (MW = 92.5 g/mol) is 78°C, whereas that of hexane (MW = 86 g/mol) is 69°C. In general, alkyl halides are insoluble in water. Because of the presence of the more massive halogen atom, the alkyl halide may be more dense than water. For example, when dichloromethane, a common laboratory solvent, and water are mixed, two layers are formed, with dichloromethane as the lower layer. 

We sow irt the preceding chapter that the carbon-halogen bond in alkyl holtdes its polar and that the carbon atom electron-poor. Thus, alkyl halides are electrophileii, and much of their chemistry involves polar reactions with nucleophiles and bases. 

We saw in the preceding chapter that the carbon-halogen bond in an alkyl halide is polar and that the carbon atom is electron-poor. Thus, alkyl halides are electrophiles, and much of their chemistry involves polar reactions with nucleophiles and bases. Alkyl halides do one of two things when they react with a nucleophile/base, such as hydroxide ion either they undergo substitution of the X group by the nucleophile, or they undergo elimination of HX to yield an alkene. 

Several factors influence whether a reaction will occur by an Sn1 or Sn2 mechanism carbocation stability, steric effects, strength of nucleophile, and the solvent. Tertiary halides tend to react by the SN1 process because they can form the relatively stable tertiary carbocations and because the presence of three large alkyl groups sterically discourages attack by the nucleophile on the carbon-halogen bond. The Sn2 reaction is favored for primary halides because it does not involve a carbocation intermediate (primary carbocations are unstable) and because primary halides do not offer as much steric hindrance to attack by a nucleophile as do the more bulky tertiary halides. Strong nucleophiles favor the Sn2 mechanism and polar solvents promote SN1 reactions. 

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