The Polar Carbon-Halogen Bond

What kind of reactions do alkyl halides undergo The characteristic reactions of alkyl halides are substitution and elimination. Because alkyl halides contain an electrophilic carbon, they react with electron-rich reagents— Lewis bases (nucleophiles) and Brpnsted—Lowry bases. 

In a substitution reaction of RX, the halogen X is replaced by an electron-rich nucleophile Nu . The C-X o bond is broken and the C-Nu o bond is formed. 

In an elimination reaction of RX, the elements of HX are removed by a Brpnsted-Lowry base B. 

Electrostatic potential maps of four halomethanes (CHslQ 

In a substitution reaction of RX, the halogen X is replaced by an electron-rich nucleophile 

The remainder of Chapter 7 is devoted to a discussion of the substitution reactions of alkyl halides. Elimination reactions are discussed in Chapter 8. 

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

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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 ... 

FIGURE 6.3 A substantial dipole moment exists in the polar carbon—halogen bond, X = F, Cl, Br, or I. 

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 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... 

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 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. 

Polar bonds determine the behavior of many classes of molecules. Recall that polarity is due to a difference in the electronegativity of two atoms bound to each other (Section 1-3). We have already introduced the haloalkanes, which contain polar carbon-halogen bonds as their functional groups. In Chapters 6 and 7 we shall explore their chemistry in depth. Another functionality is the hydroxy group, -O-H, characteristic of alcohols. The characteristic functional unit of ethers is an oxygen bonded to two carbon atoms... 

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. 

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. 

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... 

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