Substitution and elimination reactions

The complex of an alkyne with Co2(CO)e can also be used to protect an alkyne in the presence of an alkene (e.g., in hydroborations). The Co2(CO)e can then be removed from the alkyne by oxidation with Fe(III) ion. 

Copper(I) salts such as CuCN and ROCu undergo aromatic substitution reactions very readily with ordinary aryl halides. The mechanism has not been established with certainty. One reasonable possibility is an SrnI mechanism. Another reasonable possibility involves oxidative addition of Ar—to N=C—Cu(I) to give a Cu(III) complex, followed by reductive elimination of Ar—CN to give CuX. 

The only problem with the oxidative addition-reductive elimination mechanism is that Cu(in) is a relatively high energy species. The mechanism would be much more reasonable if the Cu cycled between Cu(0) and Cu(II) instead of Cu(I) and Cu(III). Such a mechanism can be proposed if an initial electron transfer from CuCN to CuCN to give [N=C—Cu(0)] is supposed. Oxidative addi- 

Other copper nucleophiles such as RaCuLi and R2Cu(CN)Li2 also undergo substitution reactions with aryl and alkenyl halides. Retention of double-bond geometry about alkenyl halides is observed. The mechanisms of the reactions of these other Cu(I) nucleophiles are likely very similar to the one shown for the reaction of CuCN and ArBr. 

In the Ullmann reaction, Cu metal promotes the coupling of Ar—I to give Ar—Ar. The reaction mechanism almost certainly involves oxidative addition of Cu(0) to Ar—I to give Ar—Cu(II), then reduction by another equivalent of Cu(0) to give Ar—Cu(I). This species can then react with Arl by the same mechanism that has just been written for other Cu(I) salts. 

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TosOH 4-methylbenzenesulfonic acid = p toluenesiilfonic acid, tosic acid X, Y leaving groups. e.g., halogen, RSOj, in substitution and elimination reactions... 

Conversion to p-toluenesulfonate esters (Section 8.14) Alcohols react with p-toluenesulfonyl chloride to give p-toluenesulfonate esters. Sulfonate esters are reactive substrates for nucleophilic substitution and elimination reactions. The p-toluenesulfonate group is often abbreviated —OTs. 

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. 

Thomson IOV Click Organic Interactive to use a web-based palette to design syntheses using substitution and elimination reactions. 

The phase-transfer catalysis method has also been utilized effectively for addition of dichlorocarbene to olefins,4 as well as for substitution and elimination reactions, oxidations, and reductions.18 The preceding procedure in this volume is another example.13... 

The electrophile is the compound being attacked by the nucleophile. In substitution and elimination reactions (which we will see in the next chapter), we generally refer to the electrophile as the substrate. 

In the previous chapter, we saw that a substitution reaction can occur when a compound possesses a leaving group. In this chapter, we will explore another type of reaction, called elimination, which can also occur for compounds with leaving groups. In fact, substitution and elimination reactions frequently compete with each other, giving a mixture of products. At the end of this chapter, we will learn how to predict the products of these competing reactions. For now, let s consider the different outcomes for substitution and elimination reactions ... 

An El reaction is generally accompanied by a competing SnI reaction, and a mixture of products is generally obtained. At the end of this chapter, we will explore the main factors that affect the competition between substitution and elimination reactions. 

Substitution and elimination reactions are almost always in competition with each other. In order to predict the products of a reaction, you must determine which mechanism(s) win the competition. In some cases, there is one clear winner. For example, consider a case in which a tertiary alkyl halide is treated with a strong base, such as hydroxide ... 

We mentioned that there are three main steps for predicting the products of substitution and elimination reactions. In the previous section, we explored the first step (determining the function of the reagent). In this section, we now explore the second step of the process in which we analyze the substrate and identify which mechanism(s) operates. 

We mentioned that predicting the products of substitution and elimination reactions requires three discrete steps ... 

Ionic Reactions—Nucleophilic Substitution and Elimination Reactions of Alkyl Halides... 

Table 6E Effects of reaction variables on substitution and elimination reactions...

Scheme4.25 Substitution and elimination reactions of alkyl halides with base.

Stepwise substitution and elimination reactions 67 Insight from recent work 69... 

In this chapter we review published results of studies of the kinetics and products of stepwise nucleophilic substitution and elimination reactions of alkyl derivatives, and we present a small amount of unpublished data from our laboratory. Our review of the literature is selective rather than comprehensive, and focuses on work that provides interesting insight into the factors that control the rate constant ratio ks/kp for partitioning of carbocations, and that provides an understanding of how the absolute rate constants ks and kp that constitute this ratio change with changing carbocation structure. 

Simple tertiary carbocations represent a benchmark against which to compare the reactivity of other a-methyl carbocations. Therefore, it is necessary to deal with complex questions about the mechanism for substitution and elimination reactions at tertiary aliphatic carbon in order to evaluate the rate constant... 

It was expected that values of ks/kp for partitioning of [1+] could be obtained from the yields of the products of acid-catalyzed reactions of [l]-OH and [2]. However, significantly different relative yields of these products are obtained from the perchloric acid-catalyzed reactions of [l]-OH and [2] in several mixed alcohol/water solvents.21 This demonstrates that the nucleophilic substitution and elimination reactions of these two substrates do not proceed through identical tertiary carbocation intermediates (Scheme 4). The observed... 

Table 1 includes partitioning data only for carbocations that are sufficiently stable to form in the nucleophilic aqueous/organic solvents used in these experiments. For example, it is not possible to obtain values of ks for reaction of secondary aliphatic carbocations in water and other nucleophilic solvents, because the chemical barrier to ks is smaller than that for a bond vibration.83 The vanishingly small barriers for reaction of secondary carbocations with nucleophilic solvents results in enforced concerted mechanisms2-3 for the nucleophilic substitution and elimination reactions of secondary derivatives in largely aqueous solvents.83-84... 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

Other terms that he invented include the system of classification for mechanisms of aromatic and aliphatic substitution and elimination reactions, designated SN1, SN2, El, and E2. "S" and "E" refer to substitution and elimination, respectively, "N" to nucleophilic, and "1" and "2" to "molecularity," or the number of molecules involved in a reaction step (not kinetic order, having to do with the equation for reaction rate and the concentration of reactants). Ingold first introduced some of these ideas in 1928 in a... 

You can express an addition reaction algebraically, using the equation a + b ab. Come up with similar equations for substitution and elimination reactions. 

In this section, you were introduced to some of the main types of organic reactions addition, substitution, and elimination reactions oxidation and reduction and condensation and hydrolysis reactions. In the next section, you will take a close look at each type of reaction. You will find out how organic compounds, such as alcohols and carboxylic acids, can react in several different ways. 

A methodology that can classify reactions by using similarity measures has recently been introduced and has now been extended to include a steric similarity index. Both substitution and elimination reactions are included. 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

We begin by bringing you up to speed on mechanisms and reminding you how to push electrons around with those curved arrows. We jog your memory with a discussion of substitution and elimination reactions and their mechanisms, in addition to free radical reactions. Next you review the structure, nomenclature, synthesis, and reactions of alcohols and ethers, and then you get to tackle conjugated unsaturated systems. Finally, we remind you of spectroscopic techniques, from the IR fingerprints to NMR shifts. The review in this part moves at a pretty fast pace, but we re sure you can keep up. 

Two of the most important ones you learned were substitution and elimination reactions 8, 1, 8, 2, El, and E2. We hope you learned them well, because you ll be seeing them again quite often. 

Understanding how functional groups behave in substitution and elimination reactions is key to understanding carbon chemistry. 

Alkyl halides undergo not only nucleophilic substitution but also elimination, and both reactions are carried out in basic reagents. Often substitution and elimination reactions occur in competition with each other. In general, most nucleophiles can also act as bases, therefore the preference for elimination or substitution is determined by the reaction conditions and the alkyl halide used. 

In another example710, the phosphonium salt 76 decomposes competitively by the two mechanisms of substitution and elimination (reaction 211). This salt behaves in a manner intermediate between salts 74 and 75 indeed, the SN(P) mechanism is not completely excluded as for the salt 74 but the EHfi mechanism is still favoured with regard to the salt 75 since the alkene formed is stabilized. 

Outline the basic chemistry of alkyl halides including nucleophilic attack at saturated carbon (substitution and elimination reactions S. 1, S 2, E1 and E2 mechanisms). A brush up with Cram chs 10 and 14, Norman ch 4, Roberts ch 11, Sykes chs 3 and 8, or Tedder vol 1... 

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