Competition Between Substitution and Elimination

Elimination reactions to produce alkenes may compete in reactions in which nucleophilic aliphatic substitution is the desired process. Unimolecular elimination reactions, El, compete with substitutions, and bimolecular elimination processes, E2 (E stands for elimination and 2 for bimolecular), compete with S[ j2 transformations. These competitions are shown in Equations 14.6 and 14.7. The nature of El reactions is discussed in detail in Section 10.3 and that of E2 processes in Section 10.2. 

The mixture of hydrobromic acid and sulfuric acid may be prepared by either adding concentrated sulfuric acid to concentrated hydrobromic add or by generating the hydrobromic acid in situ by adding concentrated sulfuric acid to aqueous sodium bromide (Eq. 14.12). Both of these methods work well and give good yields of the alkyl bromide from low-molar-mass alcohols. The method of generating H-Br in situ is not effective with higher-molar-mass alcohols because of their low solubility in concentrated salt solutions, so concentrated (48%) hydrobromic acid is used instead. 

Although the presence of concentrated sulfuric acid promotes the formation of the alkyl bromide, several side reactions involving sulfuric acid and the alcohol can occur. One of these is esterification of the alcohol by sulfuric acid to form an alkyl hydrogen sulfate 2 (Eq. 14.13). 

This reaction is reversible, and the position of the equilibrium is shifted to the left, regenerating the alcohol from 2 as the alkyl bromide is produced. The formation of 2 itself does not directly decrease the yield of alkyl bromide rather, 2 undergoes other reactions to give undesired by-products. For example, it suffers elimination on heating to give a mixture of alkenes (Eq. 14.14). It may also react with another molecule of alcohol to give a dialkyl ether by an S]s[2 reaction in which the nucleophile is ROH (Eq. 14.15). Both of these side reactions consume alcohol, so the yield of alkyl bromide is decreased. Fortunately, these side reactions may be minimized for primary alcohols by controlling the temperature of the reaction and the concentration of sulfuric acid. 

It is necessary to use different procedures to prepare secondary alkyl bromides from secondary alcohols because such alcohols are easily dehydrated by concentrated sulfuric acid to give alkenes by way of Equations 14.13 and 14.14. In fact, the acid-catalyzed dehydration of secondary and tertiary alcohols is a common method for synthesizing alkenes (Sec. 10.3). This problem may be circumvented by using concentrated hydrobromic acid however, it is better to prepare secondary alkyl bromides by the reaction of secondary alcohols with phosphorus tribromide, PBrg (Eq. 14.16). 

The Nobel Prize is generally considered the highest honor a scientist can receive. These awards were established by Alfred Bernhard Nobel (1833-1896) and were first conferred in 1901. 

Nobel was bom in Stockholm, Sweden. When he was nine, he moved with his parents to St. Petersburg, 

Nobel invented dynamite. He also invented blasting gelatin and smokeless powder. Although he was the inventor of the explosives used by the military, he was a strong supporter of peace movements. 

Nobel s instructions said that the prizes for chemistry and physics were to be awarded by the Royal Swedish Academy of Sciences, the prizes for physiology or medicine by the Karolinska Institute in Stockholm, the prize for literature by the Swedish Academy, and the prize for peace by a five-person committee appointed by the Norwegian Parliament. The deliberations are secret, and the decisions cannot be appealed. In 1969, the Swedish Central Bank established a prize in economics in Nobel s honor. The recipient of this prize is selected by the Royal Swedish Academy of Sdaices. On December 10—the anniversary of Nobel s death— the prizes are awarded in Stockholm, except for the peace prize, which is awarded in Oslo. 

The Golden Hall inside the City Hall in Stockholm, where the Nobel prize winners have a celebratory dinner. 

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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... 

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. 

Nakai, T. Tanaka, K. Ishikawa, N. The reaction of 2,2,2-trifluoroethyl iodide with sodium phenolate. A novel competition between substitution and elimination reactions. /. Fluorine Chem. 1977, 9, 89-93. 

S. Gronert, Gas-Phase Studies of the Competition between Substitution and Elimination Reactions, Ace, Chem. Res. 2003, 36, 848-857. 

Elimination reactions are a useful method for the preparation of alkenes, provided that certain limitations are recognized. One problem is the competition between substitution and elimination. The majority of eliminations are done under conditions that favor the E2 mechanism. In these cases, steric hindrance can be used to slow the competing SN2 pathway. Tertiary substrates and most secondary substrates give good yields of the elimination product when treated with strong bases. Sterically hindered bases can be employed with primary substrates to minimize substitution. 

Katritzky and coworkers have extensively developed the activation of amines by reaction with pyry-lium salts to provide (V-alkyl (or N-aryl) pyridinium compounds. When buttressing substituents were present to discourage attack on the pyridine ring, the N-alkyl substituent was subject to displacement and elimination processes. In general, primary alkyl substituents reacted with most nucleophiles in a normal 5n2 process as shown in Scheme 12, whereas competition between substitution and elimination took place with the secondary analogs, with elimination dominating the reactions starting from cycloalkyl-amines. 

Let us return to a problem we encountered before, in the reaction between acetyiides and alkyl halides (Sec. 8.12) competition between substitution and elimination. Both reactions result from attack by the same nucleophilic reagent attack at carbon causes substitution, attack at hydrogen causes elimination. 

The product of a substitution reaction that follows the limiting Sf 2 mechanism is determined by the identity of the nucleophile. The nucleophile replaces the leaving group and product mixtures are obtained only if there is competition from several nucleophiles. Product mixtures from ionization mechanisms are often more complex. For many carbocations there are two competing processes that lead to other products elimination and rearrangement. We discuss rearrangements in the next section. Here we consider the competition between substitution and elimination under solvolysis conditions. We return to another aspect of this competition in Section 5.10, when base-mediated elimination is considered. 

Elimination Reactions of Alkyl Halides Competition Between Substitution and Elimination... 

In the next section we shall discuss the competition between substitution and elimination reactions. To understand this competition it is important that you have acquired a good grasp of the mechanisms of substitution and elimination reactions. 

Part Z The Mechanism of Substitution and Part 3 Elimination and Addition Pathways and Products are concerned with organic reaction mechanisms. Curly arrows are introduced and the key features of the two common mechanisms of nucleophilic substitution are reviewed. Including kinetic features, stereochemical outcome and reaction coordinate diagrams. This leads to a discussion of the features of El and E2 elimination reactions. The book finishes with a discussion of the factors that affect the competition between substitution and elimination reactions. Much of the teaching of substitution mechanisms Is carried out via interactive CD-ROM activities. 

To what extent does the competition between substitution and elimination influence the reactions In reactions of tertiary halides, unimolecular processes dominate in protic solvents (especially water and aqueous solvents), and substitution is usually faster than elimination. In aprotic solvents, bimolecular substitution is not observed for tertiary halides due to the high energy required to form the pentacoordinate transition state (see Chapter 11, Section 11.2). Under conditions that favor bimolecular reactions and in the presence of a suitable base, elimination is the dominant process. 

Analysis of Several Competitions Between Substitutions and Eliminations... 

Although in principle any base can be made to induce an E2 reaction under appropriate experimental conditions, chemists commonly employ particularly strong bases such as hydroxide, alkoxides, and amide anions (NR ). These bases have conjugate acids with above 11. When we use other bases whose conjugate acid p. s are near or below 11 (e.g., carboxylates, thiolates, and cyanide, the intention is to effect a substitution reaction via using these reactants as nucleophiles. Therefore, one simplifying aspect of the competition between substitution and elimination is to consider an E2 pathway only when hydroxide, alkoxides, acetylides, and amide anions are used. 

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