Halides, alkyl, halogen replacement

The interaction of diaryl tellurides and the esters of halogenated fatty acids, e.g. methyl bromacetate, gives diarvl-telluretin halide alkyl esters, e.g. (C6H5)2Te(Br).CH2.COOCH3. The halogen present may be chlorine, bromine or iodine, moist silver oxide replacing the bromine by the hydroxyl group. 

Syntheses of alkyl phenyl ethers, C,H, OR, are carried out by refluxing aqueous or alcoholic solutions of alkali phenolates with alkyl halides the yields vary with the nature of the alkyl halides (40-80%). The reactive halogen in benzyl halides is easily replaced by an alkoxyl group (95%). ° The choice of a solvent is sometimes important. Thus, in the preparation of the alkyl ethers of o- and p-hydroxybiphenyl from a mixttire of the phenol, alkyl halide, and powdered potassium hydroxide, high yields are obtained using acetone as a solvent, whereas, with alcohol as solvent, only small yields are obtained. Triarylmethyl chlorides react with alcohols directly (97%). ... 

Consider the following organic reaction, in which one halogen replaces another in an alkyl halide ... 

Particularly good yields are obtained from n-alkyl halides, but only up to about 25% of the halide can be replaced from secondary carbon atoms, and tertiary halogen compounds give olefins. 

Because radical chlorination of an alkane can yield several different monosubstitution products as well as products that contain more than one chlorine atom, it is not the best method for synthesizing an alkyl halide. Addition of a hydrogen halide to an alkene (Section 4.1) or conversion of an alcohol to an alkyl halide (a reaction we will study in Chapter 12) is a much better way to make an alkyl halide. Radical halogenation of an alkane is nevertheless still a useful reaction because it is the only way to convert an inert alkane into a reactive compound. In Chapter 10, we will see that once the halogen is introduced into the alkane, it can be replaced by a variety of other substituents. 

The replacement of an a hydrogen of an alkyl halide by halogen decreases Sn2 reactivity. Chloroform, however, is about one thousandfold more reactive in basic hydrolysis than methylene chloride . Every bromine-containing halo-form studied (Table 7) is at least 600 times as reactive toward hydroxide ions in 66.7% aqueous dioxan as methylene bromide ". Toward weakly basic nucleophiles, such as thiophenoxide ion, the predicted reactivity order is obeyed haloforms have been found to be less reactive than the corresponding methylene halides . The reaction of haloforms with sodium thiophenoxide is strongly accelerated, however, by the presence of hydroxide ions - . These observations are quite unexplainable in terms of scheme (22). 

Alkyl halides are compounds in which a halogen atom (fluorine, chlorine, bromine, or iodine) replaces a hydrogen atom of an alkane. For example, CH3CI and CH3CH2Br are alkyl halides. Alkyl halides are also called haloalkanes. The generic formula for an alkyl halide is R—X where X = fluorine, chlorine, bromine, or iodine. 

The halogen atom of an alkyl halide can be replaced by the hydrogen atom bonded to tin in tributyltin hydride (BUsSnH). The process, called dehalogenation, is a radical reaction, and it can be initiated by AIBN (2,2 -azobisisobutyronitrile). AIBN decomposes to form nitrogen gas and two isobutyronitrile radicals, which initiate the reaction. Write a mechanism for the reaction. 

The carbon-halogen bond in alkyl halide is polar. When a stronger nucleophile approaches an alkyl halide, the halogen atom along with its bonding electron pair is replaced by the incoming nucleophile, which forms a new bond with the carbon atom of the alkyl group. 

In the reduction of alkyl, alkenyl, and aryl halides, the halogen (X = F, Cl, Br, or I) rarely leaves without one or more of the electrons in the bond accompanying it (i.e., it rarely leaves as X+). Thus, even though the halogen generally leaves as an anion or a radical, there are four major reductive pathways that will be considered here. The first three differ from the fourth since, in them, the halogen is directly replaced by hydrogen. The fourth method is more circuitous there is electron transfer from a metal to the alkyl, alkenyl, or aryl halide and then, in a true second step, hydrolysis of the metal-carbon bond. 

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)... 

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—6 ) An ester is formed when the negatively charged oxygen of a carboxylate replaces the halogen of an alkyl halide. 

The haloalkanes (also called alkyl halides) are alkanes in which at least one hydrogen atom has been replaced by a halogen atom. Although they have important uses, many haloalkanes are highly toxic and a threat to the environment. The haloalkane 1,2-dichlorofluoroethane, CHC1FCH2C1, is an example of a chlorofluorocarbon (CFC), one of the compounds held responsible for the depletion of the ozone layer (see Box 13.3). Many pesticides are aromatic compounds with several halogen atoms. 

Duong and Gaudemer studied the alkylation of (presumably) [Co -(DMG)2X], where X is pyridine, aniline, or water, by the cis and trans isomers of )S-bromostyrene (PhCH=CHBr) and the methyl ester of )3-chloroacrylic acid (CHCl=CHCOOMe) in 50% aqueous methanol, and found that the configuration of the double bond remained unchanged, i.e., the halogen had simply been replaced by cobalt. They suggested that the reaction involved the addition of cobalt, followed by the elimination of the halide ion (apparently without rotation around the C—C bond), i.e.. 

The replacement of halogen by alkyl or aryl groups is included in this section. For the conversion of RX RH (X = halogen) see Section 160 (Hydrides from Halides and Sulfonates). 

Sn2 and SN2 Reactions with Halides and Sulfonates. Corey and Posner discovered that lithium dimethylcuprate can replace iodine or bromine by methyl in a wide variety of compounds, including aryl, alkenyl, and alkyl derivatives. This halogen displacement reaction is more general and gives higher yields than displacements with... 

An alkyl halide (also known as a haloalkane) is an alkane in which one or more hydrogen atoms have been replaced with halogen atoms, such as F, Cl, Br, or I. The functional group of alkyl halides is R—X, where X represents a halogen atom. Alkyl halides are similar in structure, polarity, and reactivity to alcohols. To name an alkyl halide, first name the parent hydrocarbon. Then use the prefix fluoro-, chloro-, bromo-, or iodo-, with a position number, to indicate the presence of a fluorine atom, chlorine atom, bromine atom, or iodine atom. The following Sample Problem shows how to name an alkyl halide. 

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