Elimination reactions and competing substitution

Reagents with basic properties (affinity for protons) often also possess nucleophilic character (affinity to form bonds to carbon). Consequently, bi-molecular elimination reactions occur frequently in competition with substitution reactions, viz- 

The kinetic order of both E2 and 8 2 reactions requires that during the course of the reaction, the product ratio remains constant. Any deviation from this behavior indicates a departure from kinetic control due to product instability under the reaction conditions or more frequently, the incursion of competing 

Within the bimolecular category, the proportion of reaction leading to elimination increases with alkyl-substitution at the alpha carbon independently of the leaving group s zsz variation increases the steric interaction in 

THE influence OF REACTION CONDITIONS ON THE YIELD OF OLEFINS ARISING FROM SOME ALKYL HALIDES AND SULPHONIUM 

80% aqueous ethanol contain Ethanol containing ethoxide. 

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EFFECTS OF STRUCTURE ON COMPETING SUBSTITUTION AND ELIMINATION REACTIONS... 

Competing Substitution and Elimination Reactions of Tertiary Haloalkanes... 

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. 

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). 

Of course, no attacking species acts purely as a hard or soft base, but rather each has a character that is somewhere along a continuum between one extreme and the other. Accordingly, in any given reaction mixture, substitution and elimination reactions usually compete with each other. 

Now we can consider how substitution and elimination reactions compete with one another. Let us consider the options for each class of alkyl halide. 

Substitution and elimination reactions compete with one another, giving rise to a wide variety of product mixtures. The reactions depend to a large extent on the structure of the substrate. We will consider the reactions of haloalkanes and consider them by type primary, secondary, and tertiary. 

Nucleophileic substitution reactions and elimination reactions often compete with each other. 

Since eliminations occur best by an E2 path when carried out with a high concentration of a strong base (and thus a high concentration of a strong nucleophile), substitution reactions by an SN2 path often compete with the elimination reaction. 

Aliphatic substrates that contain a good leaving group and an a-CH group may undergo reactions by competing nucleophilic substitution and elimination pathways (Scheme l).1 A stepwise mechanism is favored for these reactions... 

If the elimination takes place by the El mechanism, the reaction is more likely to compete with SN1 reaction and we get a mixture of substitution and elimination products. 

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. 

An elimination involves the loss of two atoms or groups from the substrate, usually with formation of a pi bond. Elimination reactions frequently accompany and compete with substitutions. By varying the reagents and conditions, we can often modify a reaction to favor substitution or to favor elimination. First we will discuss eliminations by themselves. Then we consider substitutions and eliminations together, trying to predict what products and what mechanisms are likely with a given set of reactants and conditions. 

The acetylide ion, then, can react with an alkyl halide in two ways by attack at carbon to give substitution, or by attack at hydrogen to give elimination. We have seen that the order of reactivity of alkyl halides toward elimination (Sec. 5.14) is 3 > 2° > 1°. In subslitution (of the present kind), we shall find (Sec. 14.11) the order of reactivity is just the opposite 1° > 2° > 3°. It is to be expected, then, that where substitution and elimination are competing reactions, the proportion of elimination increases as the structure of an alkyl halide is changed from primary to... 

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