7.2.2 Reactions of Alkenes

Alkenes, with their carbon-carbon double bond (C=C), exhibit unique reactivity due to the presence of this functional group. The reactions of organic compounds, including alkenes, are largely determined by the behaviour of their functional groupsAny group of atoms in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest of the molecule's composition..

Like other hydrocarbons, alkenes undergo combustion reactions with oxygen. However, due to incomplete combustion, alkenes tend to burn in air with smoky flames, resulting in the production of soot.

One important class of reactions for alkenes is addition reactions. Alkenes readily undergo addition reactions with hydrogen, water, and halogens (such as chlorine and bromine). In these reactions, atoms are added across the carbon-carbon double bond, converting the double bond into a single carbon-carbon bond. This process is known as "breaking" the double bond.

When alkenes react with hydrogen in the presence of a catalyst (such as platinum or palladium), they undergo hydrogenation. The double bond is broken, and hydrogen atoms are added to the carbons involved in the double bond, resulting in the formation of an alkane.

In the presence of an acid catalyst, alkenes can undergo hydration, where water molecules add across the double bond. This reaction results in the formation of an alcohol.

Alkenes also react with halogens, such as chlorine or bromine, in a process called halogenation. The halogen atoms add to the carbons involved in the double bond, leading to the formation of a dihaloalkane.

These addition reactions highlight the versatility of alkenes and their ability to undergo various transformations by breaking the carbon-carbon double bond and incorporating new atoms or groups.

Here are the reactions and conditions for the addition of hydrogen, water, and halogens to alkenes:

• Addition of Hydrogen: Alkenes can undergo hydrogenation, where hydrogen atoms are added across the carbon-carbon double bond. This reaction requires a catalyst, such as platinum (Pt) or palladium (Pd), and is typically carried out at high temperature and pressure. The result is the formation of an alkane, with the double bond converted to a single bond.
• Addition of Water (Hydration): Alkenes can undergo hydration, where water molecules add across the carbon-carbon double bond. This reaction is typically carried out in the presence of an acid catalyst, such as sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄). The product is an alcohol, with the double bond converted to a single bond and an -OH group attached to one of the carbons.
• Addition of Halogens: Alkenes can undergo halogenation, where halogen atoms (chlorine, bromine, or iodine) add across the carbon-carbon double bond. This reaction occurs readily at room temperature and does not require a catalyst. The result is the formation of a dihaloalkane, with the double bond converted to two single bonds and halogen atoms attached to each carbon.

You should be able to draw fully displayed structural formulae of the first four members of the alkenes (ethene, propene, butene, and pentene) and the products of their addition reactions with hydrogen, water, chlorine, bromine, and iodine. This includes showing the carbon-carbon double bond in the alkene and the addition of atoms or groups across the double bond in the products.

Ethene

Propene

Butene

Pentene