NAMING OF ALKYL HALIDES

We can classify alkyl halides as primary (1 ), secondary (2 ), and tertiary (3 ) alkyl halides. The general formuleis for these are as follows  

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Official Names of Alkyl Halides.—The official names of the alkyl halides are derived in exactly the same way as in the case of the hydrocarbons. The number of the carbon to which the halogen is joined, together with the name of the halogen, is used as a prefix to the official name of the hydrocarbon in which the halogen is substituted. These names are shown both in the table and in the scheme just given. 

Contrast the names of alcohols with the corresponding names of alkyl halides. If the —OH group were replaced by a chlorine atom, the names of these compounds would be 1-chloropropane and 2-chloropropane. 

The common names of alkyl halides consist of the name of the alkyl group, followed by the name of the halogen—with the ine ending of the halogen name replaced by ide (i.e., fluoride, chloride, bromide, iodide). 

Nitriles contain the —C=N functional group We have already discussed the two mam procedures by which they are prepared namely the nucleophilic substitution of alkyl halides by cyanide and the conversion of aldehydes and ketones to cyanohydrins Table 20 6 reviews aspects of these reactions Neither of the reactions m Table 20 6 is suitable for aryl nitriles (ArC=N) these compounds are readily prepared by a reaction to be dis cussed m Chapter 22... 

Reaction of alkyl halides 1 with hexamethylenetetramine 2 (trivial name urotropine) followed by a hydrolysis step, leads to formation of primary amines 3 free of higher substituted amines. This method is called the Delepine reaction, a comparable method is the Gabriel synthesis. 

Alkyl halides are encountered less frequently than their oxygen-containing relatives alcohols and ethers, but some of the kinds of reactions they undergo—nucleophilic substitutions and eliminations—are encountered frequently. Thus, alkyl halide chemistry acts as a relatively simple model for many mechanistically similar but structurally more complex reactions found in biornolecules. We ll begin in this chapter with a look at how to name and prepare alkyl halides, and we ll see several of their reactions. Then in the following chapter, we ll make a detailed study of the substitution and elimination reactions of alkyl halides—two of the most important and well-studied reaction types in organic chemistry. 

The validity of equation (12) has been checked for several families of alkyl halides for which D and E /x- are known (Ref. 32, see particularly figure 6 therein). It was thus found that for v = 0.1 V s the constant is equal to 0.3 eV at 20°C (expressing D in eV and the potentials in V). Equation (12) was then applied to the approximate determination of unknown BDEs in several series of compounds undergoing dissociative electron transfer, namely, TV-halosultams,32 sulfonium cations,33 vicinal dihalides,34 1,3-dihaloadamantes, 1,4-dihalo-bicyclo[2.2.2]octanes, and l,3-dihalobicyclo[l.l.l]pentanes.35 In the latter case, the mutual influence of the two halogens could be rationalized thanks to the conversion of the peak potential data to bond dissociation energies. 

Alkyl Halides and Triflates to Alkanes. The normal requirements for conversion of alkyl halides (and triflates) to alkanes using organosilicon hydrides are essentially the same as those needed for the reduction of the corresponding alcohols, namely, the substrates must generally be able to serve as precursors to... 

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. 

The typical reactions of carbocation intermediates were discussed in Chapter 7. The solvolysis of alkyl halides is an example of the involvement of carbocations in the SnI mechanism, in other words, where the final outcome is a nucleophilic substitution. The first step is a heterolytic cleavage of the C—X bond. Properties of X which favor heterolytic cleavage, namely electronegativity difference with carbon (the larger, the better) and the degree of overlap of the X orbital with the spn orbital of carbon (the smaller the better), have already been elucidated (Chapter 4). The transition state has partial... 

The alkyldiazenido complexes [MoX(N2R)(dppe)2] are generally synthesized by reaction of alkyl halides with [Mo(N2)2(dppe)2]. The reaction mechanism has been elucidated in some detail,140 and is believed to involve a step in common with substitution reactions of the bis(dinitrogen) species, namely rate-controlling reversible thermal loss of N2 (equation 14). The full reaction sequence is set out in equation (15). 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

A salt-type nomenclature is frequently used with alkyl halides in which the alkyl group s name precedes the name of the halide. In addition, halogen derivatives of methane have familiar non-systematic names. 

Alkyl halides are even less reactive than acyl halides, as indicated by the compilation of reaction rates of thiolate anions with various types of alkyl halides (282). Nevertheless, potentially useful affinity labels have been synthesized with alkyl halide substituents and have been shown to specifically inactivate several enzymes, albeit slowly the low reactivity of the alkyl halides may minimize nonspecific reaction. Adenosine 5 -(2-bromoethyl)phosphate has been characterized and reported to inactivate NAD -dependent isocitrate dehydrogenase (283). The 2 - and 3 -(2-bromoethyl)-AMP labels have also been synthesized, and model reactions of the bromoethyl-AMPs with cysteine, lysine, histidine, and tyrosine have been studied (284). More recently, esters of adenosine 5 -monophosphate have been prepared with ethyl, propyl, or hexyl moieties and bromo or chloro substituents at the w position (285). Yeast alcohol dehydrogenase exhibited enhanced inactivation by the hexyl derivative, but inactivation rates of other dehydrogenases were unremarkable. Two iodopropyl derivatives of cAMP have been described, namely, 1, A -(3-iodopropyleno)adenosine 3, 5 -cyclic monophosphate and 3 -0-(2-iodo-3-hydroxypropyl)adenosine 3, 5 -cyclic monophosphate the latter inactivates cAMP phosphodiesterase from human platelets, with a pseudo-first-order rate constant of 0.147 hr" (286). 

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