Question

What kinds of reactions are common to alkenes? List an example of each.

Short answer

Common reactions of alkenes include addition reactions (e.g., hydrogenation of ethene to ethane), polymerization (e.g., conversion of ethylene to polyethylene), oxidation (e.g., oxidation of ethene to ethylene glycol), and halogenation (e.g., addition of bromine to ethene to form dibromoethane).

Step-by-step solution

Identification of Alkene Reactions

The first step is to identify the types of reactions that are common to alkenes. Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond, which is reactive and can undergo several types of reactions. The common reactions are: 1) Addition reactions, 2) Polymerization, 3) Oxidation, and 4) Halogenation.

Listing Examples for Each Reaction Type

Now list examples for each reaction type: 1) Addition reaction: A typical example is the addition of hydrogen in the presence of a catalyst (hydrogenation), converting alkenes into alkanes. 2) Polymerization: Alkenes can polymerize to form polymers, like polyethylene from ethylene. 3) Oxidation: Alkenes can be oxidized to diols using reagents like potassium permanganate (KMnO4). 4) Halogenation: An example is the addition of bromine (Br2) to an alkene, resulting in a vicinal dibromide.

Key concepts

Addition Reactions

Alkenes are particularly reactive due to their double bonds which can easily be broken to allow additional atoms or molecules to attach, hence the term 'addition reactions'. This process includes the joining of two or more compounds to form a single product.

For instance, consider the addition of hydrogen (H₂) across the double bond of ethene (C₂H₄) in the presence of a platinum or palladium catalyst, known as hydrogenation. This reaction converts the ethene to ethane (C₂H₆), a saturated hydrocarbon. Other common addition reactions involve halogens, water (hydration), and hydrogen halides like HCl or HBr.

Polymerization

Polymerization is an essential chemical reaction where many small molecules, called monomers, link together to form long chain-like structures known as polymers. One well-known example of this process is the transformation of ethylene, a simple alkene, into polyethylene, a plastic used globally in myriad applications.

The mechanism that alkenes follow during polymerization can vary, leading to different types of polymers. For instance, during addition polymerization, the monomers add to each other without the loss of any small molecules. This leads to a wide array of plastics and synthetic rubbers that play a vital role in modern life.

Oxidation

Oxidation of alkenes involves the incorporation of oxygen into the molecule, often increasing the polarity and changing the properties of the initial compound. A common example is the use of a powerful oxidizing agent such as potassium permanganate (\( KMnO_4 \) to transform an alkene into a glycol - a molecule containing two hydroxyl groups (-OH) on adjacent carbons.

This reaction is of considerable importance in the synthesis of more complex organic molecules. Oxidation can also proceed under milder conditions, like with ozone (\( O_3 \) to form ozonides which, upon reduction, yield aldehydes or ketones depending on the substitution pattern of the original alkene.

Halogenation

The process of halogenation is characteristic of alkenes reacting with halogen elements such as chlorine (Cl₂) or bromine (Br₂). During this reaction, the double bond of the alkene opens up to allow each of the halogen atoms to attach to the carbons, forming a dihalide.

For instance, when ethene is treated with bromine, 1,2-dibromoethane is formed. This reaction is particularly useful for testing the presence of double bonds in unknown compounds, as the color of bromine disappears when it reacts with alkenes. This conversion provides a visually recognizable indicator, and halogenated products are often used in further chemical syntheses.