Alkenes

Structure of Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond is a key feature that distinguishes alkenes from alkanes, which only have single bonds.

The general formula for alkenes is C_nH_2n, meaning they have two fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms. This is because the double bond reduces the number of hydrogens attached.

Alkenes are called unsaturated hydrocarbons because they contain fewer hydrogen atoms than alkanes. The double bond means more atoms can be added in reactions, unlike saturated alkanes.

Alkenes can exhibit isomerism. This means molecules with the same molecular formula but different structures. For example, butene (C₄H₈) has two isomers:

• 1-butene, where the double bond is between the first and second carbon atoms
• 2-butene, where the double bond is between the second and third carbon atoms

These structural differences affect their chemical properties and physical properties like boiling points.

For instance, the molecular formula C₃H₆ corresponds to propene, which has a double bond between two of its carbon atoms:

The structure is CH₂=CH 6CH₃, showing the double bond between the first two carbons.

Example: Calculate the molecular formula of an alkene with 3 carbon atoms.

Using the general formula C_nH_2n, for n=3, the formula is C₃H₆.

Reactions of Alkenes

Alkenes are very reactive due to their carbon-carbon double bond. This double bond can open up to allow other atoms to add across it, called addition reactions. These reactions are important in both testing for alkenes and making useful products.

Addition reactions occur via an electrophilic addition mechanism, where the double bond electrons attract electrophiles, allowing new atoms to add across the bond.

Reaction with Bromine Water (Test for Alkenes)

Bromine water is orange-brown in colour. When bromine water is added to an alkene, the bromine adds across the double bond, turning the solution colourless. This is a simple test to identify alkenes.

Alkanes do not react with bromine water in this way, so the colour remains orange-brown.

The reaction can be represented as:

Alkene + Br₂ 16 Dibromoalkane (colour change from orange-brown to colourless)

For example, ethene reacts with bromine water:

CH₂=CH₂ + Br₂ 16 CH₂Br 6CH₂Br

This reaction is used in industry and labs to confirm the presence of a carbon-carbon double bond.

Hydrogenation

Hydrogenation is the addition of hydrogen (H₂) to an alkene, converting it into an alkane by breaking the double bond. This reaction requires a catalyst, usually nickel, and moderate heat.

The general reaction is:

Alkene + H₂ 16 Alkane (with nickel catalyst, heat)

For example, ethene hydrogenates to ethane:

CH₂=CH₂ + H₂ 16 CH₃ 6CH₃

Reaction with Steam (Hydration)

Alkenes react with steam (water vapour) in the presence of an acid catalyst (usually phosphoric acid) to form alcohols. This is called hydration.

The reaction is:

Alkene + H₂O (steam) 16 Alcohol (with acid catalyst and heat)

For example, ethene reacts with steam to form ethanol:

CH₂=CH₂ + H₂O 16 CH₃CH₂OH

This reaction is used industrially to produce ethanol, which is an important solvent and fuel.

For example, if 1 mole of ethene reacts with steam, 1 mole of ethanol is produced.

Uses of Alkenes

Alkenes are valuable in industry because of their reactivity and ability to form many useful products.

• Feedstock for polymers: Alkenes are the starting materials for making addition polymers (like polyethene and polypropene). The double bonds open up and link together to form long chains.
• Production of alcohols: Hydration of alkenes produces alcohols, such as ethanol, which are used as solvents, fuels, and in beverages.
• Starting materials for other chemicals: Alkenes can be converted into a wide range of chemicals including halogenoalkanes, diols, and other functionalised compounds used in pharmaceuticals and plastics.

It is important to consider the environmental impact of polymers made from alkenes, including issues with plastic waste and the need for recycling.