Functional group transformations alkyl halides

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)... 

Thionyl chloride and phosphorus tribromide are specialized reagents used to bring about particular functional group transformations For this reason we won t present the mechanisms by which they convert alcohols to alkyl halides but instead will limit our selves to those mechanisms that have broad applicability and enhance our knowledge of fundamental principles In those instances you will find that a mechanistic understand mg IS of great help m organizing the reaction types of organic chemistry... 

Chemical reactivity and functional group transformations involving the preparation of alkyl halides from alcohols and from alkanes are the mam themes of this chapter Although the conversions of an alcohol or an alkane to an alkyl halide are both classi tied as substitutions they proceed by very different mechanisms... 

This concludes discussion of our second functional group transformation mvolv mg alcohols the first was the conversion of alcohols to alkyl halides (Chapter 4) and the second the conversion of alcohols to alkenes In the remaining sections of the chap ter the conversion of alkyl halides to alkenes by dehydrohalogenation is described... 

Representative Functional Group Transformations by Nucleophilic Substitution Reactions of Alkyl Halides... 

Table 8.1 illustrates an application of each of these to a functional group transformation. 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. 

This chapter has a mechanistic emphasis designed to achieve a practical result. By understanding the mechanisms by which alkyl halides undergo nucleophilic substitution, we can choose experimental conditions best suited to carrying out a particular functional group transformation. The difference between a successful reaction that leads cleanly to a desired product and one that fails is often a subtle one. Mechanistic analysis helps us to appreciate these subtleties and use them to our advantage. 

Sulfonate esters are subject to the same limitations as alkyl halides. Competition from elimination needs to be considered when planning a functional group transformation that requires an anionic nucleophile, because tosylates undergo elimination reactions, just as alkyl halides do. 

Functional Group Transformation Alcohols can be prepared by nucleophilic substitution of alkyl halides, hydrolysis of esters, reduction of carboxylic acids or esters, reduction of aldehydes or ketones, electrophilic addition of alkenes, hydroboration of alkenes, or substitution of ethers. 

Functional Group Transformations Functional group transformations help us in the conversion of a functional group to an aldehyde or a ketone without affecting the carbon skeleton of the molecule. Aldehydes can be synthesised by the oxidation of primary alcohols, or by the reduction of esters, acid chlorides, or nitriles. Since nitriles can be obtained from alkyl halides, this a way of adding an aldehyde unit (CHO) to an alkyl halide ... 

Two reactions that lead to alkyl halides will be described in this chapter. Both illustrate functional group transformations, fii the first, the hydroxyl group of an alcohol is replaced by halogen on treatment with a hydrogen halide. 

The first mechanism the students encounter (Chapter 4) describes the conversion of alcohols to alkyl halides. Not only is this a useful functional-group transformation, but its first step proceeds by the simplest mechanism of all— proton transfer. The overall mechanism provides for an early reinforcement of acid-base chemistry and an early introduction to carbocations and nucleophilic substitution. 

There are other functional group transformations that lead to alkyl halides. A classical transformation converts carboxylates to allyl bromides (C—CO2 C—Br). This transformation is called the Hunsdiecker reaction (jt jjag also been called the Borodin reaction) 3 and it is most useful for the preparation of econdary halides. The silver salt of a carboxylic acid is heated with bromine to give the bromide via decarboxylation. An example is conversion of the silver salt of cyclohexane carboxylic acid (163) to bromo-cyclohexane.1 4 Phosphorus and bromine also react with acids in the Hell-Volhard-Zelinsky reaction. 

The condensation of enolates with alkyl halides or other carbonyl derivatives allows a wide variety of synthetic and functional group transformations in the carbon-carbon bond-forming process. Enolates are, therefore, among the most powerful synthetic intermediates known. In addition to generating a new carbon-carbon bond, the reaction proceeds with high diastereoselectivity in most cases, making it even more useful. 

The reactions to be described in the remainder of this chapter use either an alkane or an alcohol as the starting material for preparing an alkyl halide. By knowing how to prepare alkyl halides, we can better appreciate the material in later chapters, where alkyl halides figure prominently in key functional group transformations. Just as important, the preparation of alkyl halides will serve as our focal point as we examine the principles of reaction mechanisms. WeTl begin with the preparation of alkyl halides from alcohols by reaction with hydrogen halides. 

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