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Elimination Reactions

Introduction

In our discussion of the Sn1 mechanism we considered the formation of 2-methyl-2-propanol from the reaction of 2-chloro-2-methylpropane with hydroxide ion in aqueous ethanol. The fact that the rate of this reaction did not change when the concentration of hydoxide ion was changed was interpreted in terms of a rate limiting step that involved the formation of a carbocationic intermediate which reacted with hydroxide ion in a fast, product determing step. In fact, 2-methyl-2-propanol is not the only product formed in this reaction: 2-methylpropene is also formed. And like the formation of 2-methyl-2-propanol, the rate of formation of 2-methylpropene does not depend upon the concentration of hydroxide ion. It is a unimolecular reaction.

The E1 Mechanism

We have seen that 3o alkyl halides are prone to solvolysis reactions in polar-protic solvents. However, as Figure 1 indicates, nucleophilic substitution is often accompanied by the formation of an alkene, i.e. elimination.

Figure 1

Carbocations Go Both Ways

Exercise 1 Using curved arrows to depict the movement of electrons, show the bonding changes that occur when a molecule of water attacks one of the methyl hydrogen atoms of the intermediate carbocation in Figure 1.
The multiplicity of products shown in Figure 1 is typical of unimolecular processes. Substitution and elimination occur simultaneously.

Alkene Stability

Solvolysis of the tertiary alkyl halide 2-bromo-2-methylbutane in ethanol produces both substitution and elimination via competing Sn1 and E1 mechanisms. Equation 1 summarizes the product distribution for the two alkenes that are produced in this reaction.

The minor component, 2-methyl-1-butene, is classified as a disubstituted alkene, which is to say that two of the four substituents attached to the double bond are not hydrogens. By the same token, the major product, 2-methyl-2-butene, is a trisubstituted alkene. The scientists who performed reaction 1 established that it was thermodynamically controlled. Hence theproduct distribution is a reflection of the relative stabilities of the two alkenes. Similar studies have established the general rule that as the number on non-hydrogen substituents attached to a double bond increases, the stability of the pi bond increases.

Exercise 2 Classify each of the following alkenes as mono, di, tri, or tetra substituted. Enter the appropriate prefix in the text box.

CH3CH2CH=CHCH2CH3 (CH3)2C=CHCH2CH3 C6H5CH=CH2

(CH3CH2)2C=C(CH2CH3)2

Exercise 3 Rank the alkenes shown in Exercise 2 in order of increasing stability.

Exercise 4 Classify the cyclic alkenes a-d as mono, di, tri, or tetra substituted. Enter the appropriate prefix in the text box.

a. b. c. d.

Exercise 5 Select the correct order of stabilities for alkenes a, b, and c. a > b > c a > c > b b > a > c b > c > a c > b > a

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