Alkyl halides table

These are best regarded as SN1 reactions, in which the leaving group is the carboxylate anion. The point is brought out very well by Moffat and Hunt s comparison79 of the solvolyses of /-alkyl trifluoroacetates in 70% aqueous acetone with the reactions of the corresponding /-alkyl halides (Table 26). The activation parameters, and the fate of the carbonium ion, as measured by the percentage of olefin formed by the parallel El reaction, are closely similar for the two types of substrate. 

The carbon-halogen bond distances in acyl halides increase in the direction F < Cl < Br < I, and are similar, but slightly larger than, those of the alkyl halides (Table 7). Nuclear quadrupole resonance frequencies of halogen compounds suggest that the charge density on the chlorine atom of an acyl chloride is greater than that on an alkyl chloride (Table 8). 

Considering the racemization in the cross-coupling process of enantiomerically pure alkyl halides (Table 5.8, entry 18), Fiirstner et al. partially support a theory delineating a metal-bound radical. However, they also call attention to several compounds set up for analogous 5-exo-trig cyclizations but that do not cyclize under identical conditions of the cross-coupling reaction [Scheme 5.18, reaction (3)]. Consequently, care has to be taken in generalizing this mechanistic proposal. 

As shown in Scheme 4.1, during an Sn2 reaction at sp3-hybridized carbon the nucleophile approaches the electrophile from the side opposite to the leaving group. Hence, if the remaining three substituents at the electrophilic carbon are large, the nucleophile will have to overcome repulsive forces, especially so if the nucleophile also is large. This steric repulsion is believed to be the main reason for the strong dependence of bimolecular substitution rates on the structure of simple alkyl halides (Table 4.1). 

Conformational and chiroptical properties of alkyl halides TABLE 11. Barriers to rotation in methyl- and tert-butyl compounds0... 

Alkali tellurides 13 and 14, or ditellurides 15, are potent nucleophiles and react rapidly with alkyl halides. Table 1 presents some representative examples of such reactions leading to dialkyl tellurides 3 and dialkyl ditellurides 4. Alkali organotellurolates 2 are also potent nucleophiles and are used to prepare symmetrical and unsymmetrical tellurides (Table 2). 

Although there is a considerable organic chemistry of all of the Group IIIA metals, the organic chemistry of aluminum is by far the most extensive and important. Various organic derivatives are known, and some of them are used industrially on a large scale. Several trialkylaluminum compounds are important, as are some of the mixed alkyl halides. Table 9.3 shows physical data for some of the aluminum compounds. 

A noticeable feature of the majority of reactions of diorganocuprates with alkyl halides (Table VI) is that molar ratios of up to 5 1, respectively, are employed for most effective coupling. However, good yields of coupled products can be obtained when molar equivalents of the reactants are used, a useful procedure for the more exotic copper... 

Becau e of the great versatility of nuclcophilK autwtitution reaetiO U, many kinds of producta can be prepared from alkyl halide. Table 1 l. l li semte eomnton nucleophiles in the approximate order of their reactivity showa the products of their reactions with b omomet.hdne. 

In contrast to the IIT mechanism, the clement effect on the Arens path is expected to be A (I) > A (Br) > A (C1) as is also the case with S 2 reactions of alkyl halides (Table 25) . This reactivity order has been observed in the reactions of l-halo-2-(2-thienyDacetylenes with sulphides and thiolates in methanol-water in which the C—X bond is presumably broken in the rate-determining step by attack on halogen (see Section Il.C.l.d). In both 5 n2 and halogen abstraction reactions, the magnitude of the element effect is quite large (Table 25). 

Alkyl halides have the same order of reactivity in S l reactions as they do in El reactions because both reactions have the same rate-determining step—dissociation of the alkyl halide (Table 11.5). This means that all alkyl halides that react under Sn1/E1 conditions will give both substitution and elimination products. Remember that primary alkyl halides do not undergo SnI/EI reactions because primary carbocations are too unstable to be formed. 

Table 11.1 lists some of the reaction conditions which have given prepara-tively useful yields of 3-alkylation. Entries 1-3 are typical alkylations using a magnesium salt and an alkyl halide. Even 2,3-disubstituted indoles are alkylated at C3 under these conditions (Entry 7). Entry 5 represents a more recently developed method in which an allylic alcohol and indole react in the... 

Table 4 2 lists the boiling points of some representative alkyl halides and alcohols When comparing the boiling points of related compounds as a function of the alkyl group we find that the boiling point increases with the number of carbon atoms as it does with alkanes... 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

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... 

The 8n2 mechanism is believed to describe most substitutions m which simple pri mary and secondary alkyl halides react with anionic nucleophiles All the examples cited in Table 8 1 proceed by the 8 2 mechanism (or a mechanism very much like 8 2— remember mechanisms can never be established with certainty but represent only our best present explanations of experimental observations) We 11 examine the 8 2 mecha nism particularly the structure of the transition state in more detail in 8ection 8 5 after hrst looking at some stereochemical studies carried out by Hughes and Ingold... 

There are very large differences m the rates at which the various kinds of alkyl halides— methyl primary secondary or tertiary—undergo nucleophilic substitution As Table 8 2 shows for the reaction of a series of alkyl bromides... 

As we have seen the nucleophile attacks the substrate m the rate determining step of the Sn2 mechanism it therefore follows that the rate of substitution may vary from nucleophile to nucleophile Just as some alkyl halides are more reactive than others some nucleophiles are more reactive than others Nucleophilic strength or nucleophilicity, is a measure of how fast a Lewis base displaces a leaving group from a suitable substrate By measuring the rate at which various Lewis bases react with methyl iodide m methanol a list of then nucleophihcities relative to methanol as the standard nucleophile has been compiled It is presented m Table 8 4... 

The haloalkane dehydrogenase is believed to act by using one of its side chain carboxylates to dis place chloride by an Sn2 mechanism (Recall the reac tion of carboxylate ions with alkyl halides from Table 8 1 )... 

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... 

Section 20 18 Nitnles are prepared by nucleophilic substitution (8 2) of alkyl halides with cyanide ion by converting aldehydes or ketones to cyanohydrins (Table 20 6) or by dehydration of amides... 

Although this reaction is useful for preparing a ammo acids (Table 22 3 fifth entry) it IS not a general method for the synthesis of amines Its major limitation is that the expected primary amine product is itself a nucleophile and competes with ammonia for the alkyl halide... 

Alkyl azides prepared by nucleophilic substitution of alkyl halides by sodium azide as shown m the first entry of Table 22 3 are reduced to alkylammes by a variety of reagents including lithium aluminum hydride... 

The carbon-halogen bonds of aryl halides are both shorter and stronger than the carbon-halogen bonds of alkyl halides In this respect as well as m their chemical behavior they resemble vinyl halides more than alkyl halides A hybridization effect seems to be responsible because as the data m Table 23 1 indicate similar patterns are seen for both carbon-hydrogen bonds and carbon-halogen bonds An increase m s... 

Table 7. Reactions of Primary Alkyl Halides with Potassium Fluoride and Hexadecyltributylphosphom um Bromide [6S]...

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