Elimination reactions competition between substitution

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. 

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. 

In practice, the scope and limitations of this reaction are governed by the limitations of the S 2 reaction and by competition between substitution and jS-elimination (Section 9.8). The reaction is most useful for preparation of thiols from primary halides. Yields are lower from secondary halides because of the competing jS-elimination (E2) reaction. V Sth tertiary halides, jS-elimination (E2) predominates, and the alkene formed by dehydrohalogenation is the major product. 

---