SYNTHESIS OF ALKYL HALIDES

Reactions of alcohols with hydrogen halides (Section 4.7) Alcohols react with hydrogen halides to yield alkyl halides. The reaction is useful as a synthesis of alkyl halides. The reactivity of hydrogen halides decreases in the order HI > HBr > HCI > HF. Alcohol reactivity decreases in the order tertiary > secondary > primary > methyl. 

The synthesis of alkyl halides from alkyl halides is called the Finkelstein reaction ... 

Thomson . Click Organic Interactive to use a web-based palette to design a synthesis of alkyl halides, beginning with alcohols. 

The reaction of halogenotriphenylphosphonium halides (triphenyl-phosphine dihalides) with alcohols is a useful method for the synthesis of alkyl halides (see Section II,2b p. 239). It has been found88 that (alkoxymethylene)dimethyliminium halides are formed in the reactions of these reagents with alcohols in N,N-dimethylformamide a possible mechanism is shown. Hydrolysis of the (alkoxymethylene)-dimethyliminium halide intermediate affords a formic ester, whereas... 

Several useful reactions for the synthesis of alkyl halides that we already have encountered are summarized below with references to the sections that supply more detail ... 

Decarboxylation of the silver salts of carboxylic acids in the presence of bromine or chlorine, the Hunsdiecker reaction, often is useful for the synthesis of alkyl halides ... 

Alkyl halides are important starting materials in the synthesis and manufacture of a variety of organic compounds. Free-radical halogenation (Scheme 4.12) is an important methodology for the synthesis of alkyl halides from alkanes. Brominations (X = Br) tend to be very selective because of the... 

Fig. Mechanism for the POCl3 dehydration of an alcohol. Synthesis of Alkyl Halides...

The reaction between an alcohol and the halide of a non-metal (usually phosphorus) is well known in the synthesis of alkyl halides. Usually the reaction is conducted with an excess of alcohol and/or in basic media thus... 

Although we discussed its mechanism at length in Section 4-3, free-radical halogenation is rarely an effective method for the synthesis of alkyl halides. It usually produces mixtures of products because there are different kinds of hydrogen atoms that can be abstracted. Also, more than one halogen atom may react, giving multiple substitutions. For example, the chlorination of propane can give a messy mixture of products. 

Cristol, S J, Firth, W C, Convenient synthesis of alkyl halides from carboxylic acids, J. Org. Chem., 26, 280 1961. 

Electrophilic addition reactions of alkenes lead to the synthesis of alkyl halides, vicinal dihalides, halohydrins, alcohols, ethers, and alkanes. 

Preparation of Alkyl Halides from Olefins. There are two general methods for the synthesis of alkyl halides (1) by the interaction of an alcohol with a halogen hydride—a procedure that may reasonably be discussed under esterification or halogenation and, also, under the Friedel-Crafts synthesis when a metal halide is used to catalyze the reactions— and (2) by the addition of a halogen hydride to an unsaturated hydrocarbon. This reaction may be catalyzed by metal halides and by sulfuric acid. In the latter instance, the ethylsulfuric acid first formed is converted to the halide by gaseous chlorine or chlorine liberated in situ by action of sulfuric acid on a halide. 

In general, the ease of synthesis of alkyl halides from olefins is dependent on the molecular weights of the hydrocarbons and the particular hydrogen halide. Thus, as was explained earlier in this chapter, the reaction becomes more difficult in passing from iodides through bromides to chlorides. Likewise, olefins containing several carbon atoms react much more readily than those of a lower order. 

SYNTHESIS OF ALKYL HALIDES VIA FREE-RADICAL HALOGENATION... 

Leadbeater NE, Torenius HM, Tye H (2003) Ionic liquids as reagents and solvents in conjunction with microwave heating rapid synthesis of alkyl halides from alcohols and nitriles from aryl halides. Tetrahedron 59 2253-2258... 

In this chapter, we see the synthesis of alkyl halides and alcohols from alkenes through addition reactions. Hydroboration allows us to do addition reactions in the anti-Markovnikov sense. The 8 2 and 8 1 reactions from earlier chapters allow us to do further transformations of the alcohols and halides. The new synthetic methods are summarized below. 

Several papers have appeared describing the synthesis of alkyl halides from organoboranes. Alkyl chlorides are obtained by the reaction of trialkylboranes with dichloramine-T, and in somewhat lower yield with iV,N-dichlorourethane/ whereas alkyl bromides are produced in excellent yield when the boranes derived from terminal alkenes and dicyclohexylborane are treated with either Bt2 or BrCl. Alkyl iodides are obtained from the same boranes by the use of ICl-NaQAc " or iodide ion-chloramine-T/ and this latter method has been applied to the radioiodination of olefins (with Vinyl iodides result from... 

SYNTHESIS OF ALKYL HALIDES THROUGH THE FORMATION OF HALOGEN-CARBON(SP ) BONDS... 

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