Eliminations substitution and

Substitution reactions of f-butyl halides, you will recall from Chapter 17, invariably follow the S l mechanism. In other words, the rate-determining step of their substitution reactions is unimolecu- 

You d also be wasting your alkyl halide. This is what actually happens if you try the substitution reaction with a concentrated solution of sodium hydroxide. 

The reaction stops being a substitution and an alkene is formed instead. Overall, HG has been lost from the alkyl halide, and the reaction is called an elimination. 

In this chapter we will talk about the mechanisms of elimination reactions—as in the case of substitutions, there is more than one mechanism for eliminations. We will compare eliminations with substitutions—either reaction can happen from almost identical starting materials, and you will learn how to predict which is the more likely. Much of the mechanistic discussion relates very closely to Chapter 17, and we suggest that you should make sure you understand all of the points in that chapter before tackling this one. This chapter will also tell you about uses for elimination reactions. Apart from a brief look at the Wittig reaction in Chapter 14, this is the first time you have met a way of making alkenes. 

The correlation is best for attack at C=0. In Chapter 17, you met examples of nucleophiles that are good at substitution at saturated carbon (such as 1 Br, PhS-) but that are not strong bases. 

Substitution reactions of t-butyl halides, you will recall from Chapter 15, invariably follow the S l mechanism. In other words, the rate-determining step of their substitution reactions is unimolecular—it involves only the alkyl halide. This means that, no matter what the nucleophile is, the reaction goes at the same rate. You can t speed this S l reaction up, for example, by using hydroxide instead of water, or even by increasing the concentration of hydroxide. You d be wasting your time, we said (see p. 332). 

Elimination happens when the nucleophile attacks hydrogen instead of carbon 

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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... 

Section 8 13 When nucleophilic substitution is used for synthesis the competition between substitution and elimination must be favorable However the normal reaction of a secondary alkyl halide with a base as strong or stronger than hydroxide is elimination (E2) Substitution by the Sn2 mechanism predominates only when the base is weaker than hydroxide or the alkyl halide is primary Elimination predominates when tertiary alkyl halides react with any anion... 

The stereochemistry of the most fundamental reaction types such as addition, substitution, and elimination are described by terms which specify the stereochemical relationship between the reactants and products. Addition and elimination reactions are classified as syn or anti, depending on whether the covalent bonds which are made or broken are on the same face or opposite faces of the plane of the double bond. 

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. 

CHAPTER 11 Reactions of Alkyl Halides Nucleophilic Substitutions and Eliminations... 

El eliminations begin with the same uni molecular dissociation we saw in the Sfsjl reaction, but the dissociation is followed by loss of H+ from the adjacent carbon rather than by substitution. In fact, the El and SN1 reactions normally occur together whenever an alkyl halide is treated in a protic solvent with a non-basic nucleophile. Thus, the best El substrates are also the best SN1 substrates, and mixtures of substitution and elimination products are usually obtained. For example, when 2-chloro-2-methylpropane is warmed to 65 °C in 80% aqueous ethanol, a 64 36 mixture of 2-methyl-2-propanol (Sjql) and 2-methylpropene (El) results. 

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