Reactions of alkenes, including addition, substitution, and elimination

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. Due to the presence of this double bond, alkenes exhibit unique reactivity and undergo a variety of reactions. In this lesson, we will discuss the three major types of reactions of alkenes: addition, substitution, and elimination.

Addition reactions:

Addition reactions are those in which a molecule is added across the carbon-carbon double bond of an alkene, resulting in the formation of a single product. The most common addition reactions of alkenes involve the addition of hydrogen, halogens, or hydrogen halides. For example, when hydrogen gas is added to ethene (C2H4), the product is ethane (C2H6), as shown below:

Substitution reactions:

In substitution reactions, a functional group or atom is substituted for a hydrogen atom in the alkene. One example of this is the halogenation reaction, in which a halogen atom replaces a hydrogen atom. For example, when chlorine gas is reacted with ethene (C2H4), the product is 1,2-dichloroethane (C2H4Cl2), as shown below:

Elimination reactions:

In elimination reactions, a molecule is eliminated from the alkene, resulting in the formation of a double bond. One example of this is the dehydrohalogenation reaction, in which a hydrogen halide molecule is removed from the alkene. For example, when 2-bromo-3-methylbutane (C5H11Br) is reacted with sodium hydroxide (NaOH), the product is 2-methylbut-2-ene (C5H10), as shown below:

For a complete list of alkene reactions used in Organic Chemistry 1, check out our Reaction Library!

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Summary

Alkenes are unsaturated hydrocarbons that exhibit unique reactivity due to the presence of a carbon-carbon double bond. Addition reactions involve the addition of a molecule across the double bond, substitution reactions involve the substitution of a functional group or atom for a hydrogen atom, and elimination reactions involve the removal of a molecule from the alkene, resulting in the formation of a double bond.

Test Your Knowledge:

What is the product of the reaction between hydrogen chloride and ethene?

What type of reaction involves the removal of a molecule from an alkene, resulting in the formation of a double bond?