Elimination substitution versus

Alkyl halides undergo competitive substitution and elimination reactions. The ratio of products derived from substitution and elimination depends on the nature of the alkyl halide, the base/nucleophile, the solvent and the temperature. SN2 reactions are normally in competition with E2 reactions, while SN1 reactions are normally in competition with El reactions. 

Primary and secondary alkyl halides can undergo SN2 reactions, while primary, secondary and tertiary alkyl halides can undergo E2 reactions. 

In general, large nucleophiles are good bases and promote elimination. This is because the bulky anion cannot attack the hindered carbon atom in an SN2 reaction. It is much easier for the anion to attack a (3-hydrogen atom because this is more accessible. 

With a large anion, this is more likely to act as a base and react with the hydrogen on the P-carbon because this is more exposed than the a-carbon atom 

E2 reactions are favoured over SN2 reactions by using high temperatures. In general, increasing the reaction temperature leads to more elimination. This is because the elimination reaction has a higher activation energy (than the substitution reaction), as more bonds need to be broken in order to form the alkene product. 

Factors that influence the strength of nucleophiles and bases are discussed in Sections 4.2 and 1.7.2 

AjG is the standard Gibbs free-energy change of reaction 

Reaction mechanisms are influenced by many factors. Different combination of factors result in different outcomes. Even though this is the case, let s boil down some of the factors that we can rely on for reasonably judging the outcome of reactions. We will consider some conditions that favor substitution over elimination, and vice versa. The two key factors that we look for are the type of substrate that is undergoing the reaction, and the extent of nucleophilicity or basicity of the anion involved in the reaction. 

1) Higher temperatures usually favor elimination over substitution. To be more precise, we can say that elimination is more favored than substitution reactions if the reaction occurs at a high temperature. The latter is more accurate because 

2) Strong bases guide or dictate elimination over substitution in most cases. In general, E2 type of elimination is favored vmder such conditions. 

4) Alkyl halides (tertiary), because of their bulky substituents, mostly prefer elimination rather than substitution provided that a strong base is present. Mild bases can make substitution to predominate even in tertiary alkyl halides. Can you think of a reason why tertiary alkyl halides prefer to undergo elimination The reason is steric hindrance to the nucleophilic approach. 

Note We have been discussing a number of reactions, both substitution and elimination in terms of alkyl halides. This surely doesn t mean that only alkyl halides imdergo these types of reactions. 

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C-L Leaving group on sp carbon Sulfonates R-OSO2R Alkyl iodides R-I Protonated alcohols ROH2 Alkyl bromides R-Br Alkyl chlorides R-Cl The better L is more reactive Substitution versus Elimination Decision Section 9.5 2 Ls on C R2CL2 3 Ls on C RCLj... 

Figure 9.5 A three-dimensional correlation matrix for substitution versus elimination.

The fundamental nature of the substitution-versus-elimination competition is illustrated in Figure 4.10, which is applicable to carbocations such as tertiary alkyl and secondary benzylic that have lifetimes on the order of 10 s in hydroxylic solvents (SOH). The carbocation is at a relatively high energy, with very small barriers to either solvent capture k, substitution product) or proton loss k, elimination product.) The... 

Secondary halides are borderline, and substitution or elimination may be favored, depending on the particular base/nucleophile, solvent, and temperature at which the reaction is carried out. Elimination is favored with strong bases/good nucleophiles—for example, hydroxide ion and ethoxide ion. Substitution is favored with weak bases/poor nucleophiles—for example, acetate ion. Table 7.7 summarizes these generalizations about substitution versus elimination reactions of haloalkanes. 

Relative Rates of Substitution versus Elimination for Alkyl Bromides... 

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