Alkyl halides Halogenation

Alkane Halogen Alkyl halide Hydrogen halide... 

Q Show how enols, enolate ions, and enamines act as nucleophiles. Predict the products of their reactions with halogens, alkyl halides, and other electrophiles. Show how they are useful in synthesis. 

Vinylcopper reagents react with a wide variety of electrophilic reagents such as halogens, alkyl halides, allylic halides, acid chlorides, epoxides, a,(3-unsaturated ketones, and a,p-acetylenic esters with complete retention of the double bond stereochemistry. To enhance the reactivity of vinylcopper intermediates toward carbon electrophiles, the coupling is often carried out in the presence of activators such as HMPT, DMPU, and/or P(OEt)3 (triethylphosphite). Some representative examples of stereospecific... 

Therefore, 33 provided a trace amount of ethylene oxide adduct (35), whereas 34 afforded 36 in a satisfactory yield. However, 33 gave adducts with halogens, alkyl halides, aldehydes, and ketones in good to excellent yields. The lithium cation in 33 is unable to participate in Li-coordinated activation of oxirane oxygen for the oxirane ring opening (Scheme 3.9). 

In the absence of a beta halogen, alkyl halides or 1,1-dihaloalkanes undergo coupling reactions indicative of radical intermediates. The presence of a beta halogen increases the rate of elimination by stabilising the forming radicaF . ... 

The (tM—C bond breaking may take place as a result of reactions with compounds containing active hydrogen atoms, with halogens, alkyl halides, hydrogen, metal... 

The Alkyl Halides. Ethyl bromide and iodide (see below) are typical alkyl halides. Compounds of this class are of very great importance in synthetic work, owing to the reactivity of the halogen atom. This is illustrated by the following reactions ... 

Alkyl mercuric halides. Grignard reagents, prepared from alkyl halides, react with a mercuric halide that contains the same halogen as the reagent to form alkyl mercuric halides ... 

Strictly speaking the alkyl halides are esters of the halogen acids, but since they enter into many reactions (t.g., formation of Grignard reagents, reaction with potassium cyanide to yield nitriles, etc.) which cannot be brought about by the other eaters, the alkyl halides are usually distinguished from the esters of the other inorganic acids. The preparation of a number of these is described below. 

If only the monocarboxybc acid is required, the ester after hydrolysis with potash may be strongly acidified with sulphuric acid and the mixture heated under reflux the mineral acid promotes decarboxylation at a temperature just above 100°. The net result is the replacement of the halogen atom of the alkyl halide by —CH COOH thus in the above example ... 

It IS convenient m equations such as this to represent generic alcohols and alkyl halides as ROH and RX respectively where R stands for an alkyl group In addition to con venience this notation lets us focus more clearly on the functional group transformation that occurs the OH functional group of an alcohol is replaced as a substituent on car bon by a halogen usually chlorine (X = Cl) or bromine (X = Br)... 

The lUPAC rules permit alkyl halides to be named m two different ways called func twnal class nomenclature and substitutive nomenclature In functional class nomencla ture the alkyl group and the halide (fluoride chloride bromide or iodide) are desig nated as separate words The alkyl group is named on the basis of its longest continuous chain beginning at the carbon to which the halogen is attached... 

Substitutive nomenclature of alkyl halides treats the halogen as a halo—(fluoro chloro bromo or lodo ) substituent on an alkane chain The carbon chain is numbered m the direction that gives the substituted carbon the lower number... 

The carbon that bears the functional group is sp hybridized m alcohols and alkyl halides Figure 4 1 illustrates bonding m methanol The bond angles at carbon are approximately tetrahedral as is the C—O—H angle A similar orbital hybridization model applies to alkyl halides with the halogen connected to sp hybridized carbon by a ct bond Carbon-halogen bond distances m alkyl halides increase m the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—I (216 pm)... 

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 rest of this chapter describes a completely different method for preparing alkyl halides one that uses alkanes as reactants It involves substitution of a halogen atom for one of the alkane s hydrogens... 

Selectivity is not an issue m the conversion of alcohols to alkyl halides Except for certain limitations to be discussed m Section 8 15 the location of the halogen sub stituent m the product corresponds to that of the hydroxyl group m the starting alcohol... 

Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide It IS one of the most useful methods for preparing alkenes by p elimination... 

The rate of elimination depends on the halogen the reactivity of alkyl halides increasing with decreasing strength of the carbon-halogen bond... 

Step (1) Alkyl halide dissociates by heterolytic cleavage of carbon-halogen bond (Ionization step)... 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

A proton and a halogen add to the double bond of an alkene to yield an alkyl halide Addition proceeds in ac cordance with Markovnikov s rule hy drogen adds to the carbon that has the greater number of hydrogens halide to the carbon that has the fewer hydro gens... 

Nucleophilic substitution reactions of alkyl halides are related to elimination reactions m that the halogen acts as a leaving group on carbon and is lost as an anion The... 

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

Table 8 1 illustrates an application of each of these to a functional group transfer matron The anionic portion of the salt substitutes for the halogen of an alkyl halide The metal cation portion becomes a lithium sodium or potassium halide... 

Notice that all the examples m Table 8 1 involve alkyl halides, that is compounds m which the halogen is attached to an sp hybridized carbon Alkenyl halides and aryl halides, compounds m which the halogen is attached to sp hybridized carbons are essentially unreactive under these conditions and the principles to be developed m this chapter do not apply to them... 

Alkoxide ion (RO ) The oxygen atom of a metal alkoxide acts as a nucleophile to replace the halogen of an alkyl halide The product is an ether... 

Carboxylate ion (RC—O ) An ester IS formed when the negatively charged oxygen of a carboxylate re places the halogen of an alkyl halide... 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

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... 

---