Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

An Overview of the Reactions of Organic Chemistry
6.6 Elimination Reactions

Elimination reactions are the opposite of the addition reaction discussed in Section 6.3. Most of these reactions typically involve the removal of atoms or groups of atoms from a carbon—carbon single bond to form a double bond at the site of the removal of the groups. An example of an elimination reaction is shown in Reaction (6-24).

(6-24)

Note that for this reaction, water is the molecule that is eliminated to give only one organic product, which is an alkene. If on the other hand, the reactant was not a symmetrical alcohol, then there would have been two possible alkenes as shown in Reaction (6-25).

(6-25)

Most elimination reactions typically require more severe reaction conditions, compared to addition reactions. Addition reactions typically proceed in the presence of a small amount of a catalyst, such as dilute sulfuric acid. Hence, the reaction of the type shown in Reactions (6-24) and (6-25) is described as acid-catalyzed dehydration since it involves the removal of water.

An elimination reaction can also occur in which the molecule, HCl, is eliminated from a reactant in the presence of heat and base, such as KOH, to facilitate the removal; an example is shown in Reaction (6-26).

(6-26)

Note that for the reaction given in Reaction (6-26), an alkene is also formed as the product. Also, if the reactant is a symmetrical chloroalkane, there is only one alkene as product, but if the reactant chloroalkane is unsymmetrical, there will be two possible organic products. Elimination reactions of this type in which there is a removal of hydrogen and a halogen, such as the chloride from reactants, are known as dehydrohalogenation reactions. Also, note that if one of the adjacent carbons to the carbon that has the chlorine atom does not have a hydrogen, elimination will not occur at that site, as shown in Reaction (6-27) involving 3,3-dimethyl-4-chlorohexane.

(6-27)

Since one of the adjacent carbons to the carbon that has the chlorine atom has two methyl groups and no hydrogen, elimination does not occur at that carbon. Similarly, a reaction will not occur if there is not a hydrogen on either of carbons adjacent to the carbon that has the chlorine atom as shown for the reactant of 1,1,3,3-tetramethyl-2-chlorocyclohexane given in Reaction (6-28).

(6-28)

Problem 6.8

i. Give the elimination products for the following reactions.

ii. There are three possible elimination products for the following reaction, what are they?

It is possible for elimination reactions to occur from a carbon—heteroatom bond, as shown in Reaction (6-29).

(6-29)

The above reaction is very similar to the ones discussed earlier in this section where the elimination of water occurred to form an alkene. In this case, the elimination of water occurred, but in addition to the formation of an alkene, another organic product is also formed which has a carbon—nitrogen double bond and this functional group is called an imine.

Problem 6.9

Give the elimination products for the following reactions.

For elimination reactions, it is important to recognize the functional group, which is typically an alcohol or alkyl halide, and recognize that the elimination reaction will occur from either of the adjacent carbons where there is a hydrogen to form an alkene functional group in the organic product.