Alcohols substitution and elimination

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. 

It s this ability to drive otherwise unfavorable phosphorylation reactions that makes ATP so useful. The resultant phosphates are much more reactive as leaving groups in nucleophilic substitutions and eliminations than the corresponding alcohols they re derived from and are therefore more likely to be chemically useful. 

Now that we have seen how to make alcohols, we will focus our attention on reactions of alcohols. Let s start by reviewing reactions that we have already seen substitution and elimination. Let s begin our review with elimination reactions first. We have seen two types of elimination reactions El an E2. 

In this section, we have started looking at reactions of alcohols. So far in this section, we have focused on the details of familiar reactions (substitution and elimination). Before we leam some new reactions, let s practice the ones that we just reviewed ... 

Dehydration and RX formation from alcohols furnish another example of the competition between nucleophilic substitution and elimination. 

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... 

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. 

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. 

The reaction occurs via a carbocation intermediate. Therefore, it is possible to form both substitution and elimination products. Secondary alcohols with branching on the [f-carbon give rearranged products. The temperature must be kept low to avoid the formation of El product. 

In acidic solution, an alcohol is in equilibrium with its protonated form Protonation converts the hydroxyl group from a poor leaving group (-OH) to a good leaving group (H20). Once the alcohol is protonated, all the usual substitution and elimination reactions are feasible, depending on the structure (1°, 2°, 3°) of the alcohol. 

Eliminations. Heating an alcohol in a concentrated acid such as HC1 or HBr often leads to elimination. Once the hydroxyl group of the alcohol has been pro-tonated and converted to a good leaving group, it becomes a candidate for both substitution and elimination. 

The least expensive method for synthesizing simple symmetrical ethers is the acid-catalyzed bimolecular condensation (joining of two molecules, often with loss of a small molecule like water), discussed in Section 11-10B. Unimolecular dehydration (to give an alkene) competes with bimolecular condensation. To form an ether, the alcohol must have an unhindered primary alkyl group, and the temperature must not be allowed to rise too high. If the alcohol is hindered or the temperature is too high, the delicate balance between substitution and elimination shifts in favor of elimination, and very little ether is formed. Bimolecular condensation is used in industry to make symmetrical ethers from primary alcohols. Because the condensation is so limited in its scope, it finds little use in the laboratory synthesis of ethers. 

Alcohols also undergo substitution and elimination, but can only do so when OH is made into a good leaving group. 

Conversion of alcohols to sulfonate esters is a way of labilising them to nucleophilic substitution and elimination. The common leaving groups are arenesulfonates, particularly p-toluenesulfonate (tosylate), methanesulfonates (mesylate) and trifluromethanesulfonate (triflate) they are introduced by reaction of the acid chlorides (or, in the case of the trifluoromethanesulfonates, acid anhydrides) in a basic solvent such as pyridine. Traditionally, the reactions are carried out in pyridine as solvent, but both this solvent and the liberated... 

Amines caimot undergo the substitution and elimination reactions that alkyl halides undergo, because the leaving groups of amines are too basic. Protonated amines also cannot undergo the reactions that protonated alcohols and protonated ethers undergo. Amines are easily oxidized. Saturated heterocycles containing five or more atoms have physical and chemical properties typical of acyclic compounds that contain the same heteroatom. 

Alkyloxonium Ions Substitution and Elimination Reactions of Alcohols... 

This section concludes the coverage of alcohol chemistry for now. A summary chart concerning the various conditions for substitution and elimination reactions follows. 

Indeed, any of five bonds may be involved in alcohol chemistry, and if we treat substitution and elimination separately, a total of four types of reactions are possible, as shown. A related class of compounds—ethers—is also presented. Bccau.se the.se lack an oxygen-hydrogen bonu, the two reactions of alconols that involve the 0-H bond are not available to ethers. In fact, only substitution reactions turn out to be important in the chemistry of ethers, and those occur only under certain. sets of conditions, depending on the nature of the ether. By and large, ethers, in contrast with alcohols, have been found to be very unreactive molecules, a property that results in their usefulness as solvents for a wide variety of reactions in organic chemistry. 

Substitution and Elimination.—Cholesteryl trimethylsilyl ether (61) reacts smoothly with phenyltetrafluorophosphorane to give 3 -fluorocholest-5-ene (62) in 90 % yield. No information is yet available on the behaviour of other steroidal compounds with this novel reagent, but several model alcohols gave mixtures of fluorinated products and olefins. Free alcohols lead to lower conversion, and more elimination, compared with their trimethylsilyl ethers. 

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