Alkanes
Structure: Alkanes are a group of hydrocarbons, which means they are made from only hydrogen and carbon atoms. They are called saturated because each carbon atom is bonded to as many hydrogen atoms as possible. All the bonds between the carbon atoms are single covalent bonds. The general formula for alkanes is CₙH₂ₙ₊₂, which means if you know the number of carbon atoms (n), you can calculate how many hydrogen atoms are needed.
Reactivity: Alkanes do not react easily with most chemicals because their single bonds are strong and stable. However, they can still take part in certain reactions, especially when there’s enough heat or light.
Combustion
Complete combustion: When alkanes burn in plenty of oxygen, they go through complete combustion. This means they produce carbon dioxide (CO₂) and water (H₂O) as products, and release a lot of heat energy. This makes them good fuels.
Example reaction: A simple example is when methane (CH₄), the smallest alkane, burns with oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O.
Incomplete combustion: If there is not enough oxygen, alkanes cannot burn completely. Instead of forming carbon dioxide, they produce carbon monoxide (CO) or even solid carbon (soot), along with water. This is called incomplete combustion.
Example reaction: One example of incomplete combustion is: 2CH₄ + 3O₂ → 2CO + 4H₂O. Another possible reaction is CH₄ + O₂ → C + 2H₂O.
Hazard: Carbon monoxide (CO) is a poisonous gas. It is dangerous because it has no smell or colour and can prevent your blood from carrying oxygen properly.
Substitution
Reaction with halogens: Alkanes can react with halogens like chlorine (Cl₂) or bromine (Br₂) when exposed to ultraviolet (UV) light. In this reaction, one hydrogen atom in the alkane is replaced by a halogen atom. This process is called a substitution reaction.
Example reaction: For example, when methane reacts with chlorine in the presence of UV light, the reaction is: CH₄ + Cl₂ → CH₃Cl + HCl. Here, one hydrogen in methane is replaced by a chlorine atom.
Further substitution: If there is more chlorine present, more hydrogen atoms can be replaced, resulting in compounds like CH₂Cl₂, CHCl₃, and CCl₄. This creates a variety of different chlorinated products.
Alkenes
Structure: Alkenes are a type of hydrocarbon, which means they are made only from carbon and hydrogen atoms. What makes them special is that they are unsaturated, meaning they have at least one double bond between two carbon atoms (C=C). The general formula for alkenes is CᵒH₂ᵒ, where “n” is the number of carbon atoms.
Reactivity: The double bond in alkenes makes them more reactive than alkanes (which only have single bonds). This double bond can easily be broken during chemical reactions, allowing other atoms to add to the molecule.
Combustion
Complete combustion: When alkenes burn in a large amount of oxygen, they undergo complete combustion. This means they produce carbon dioxide (CO₂) and water (H₂O), and they release energy in the form of heat and light. This is similar to how alkanes burn.
Incomplete combustion: If there is not enough oxygen available, alkenes burn incompletely. This can produce carbon monoxide (CO), carbon (soot or smoke), and water. The flames produced are usually smokier compared to those from alkanes.
Example reaction: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O (This is complete combustion of ethene.)
Addition Reactions
Double bond reactivity: Alkenes are well-known for undergoing addition reactions. This means that the double bond between the carbon atoms is broken and new atoms are added to the molecule. This is the main type of reaction for alkenes.
Hydrogenation: When alkenes react with hydrogen gas (H₂) in the presence of a nickel catalyst at about 180°C, they turn into alkanes. The double bond is broken and each carbon gets a hydrogen atom.
Example: C₂H₄ + H₂ → C₂H₆ (Ethene becomes ethane.)
Halogenation: Alkenes react quickly with halogen elements like bromine (Br₂). This reaction adds two halogen atoms to the molecule, creating a dihalogenated compound.
Bromine test: This reaction is used as a test for alkenes. If you add bromine water (orange) to an alkene, the solution loses its color and becomes colorless, showing the double bond is present.
Hydration: When alkenes react with steam in the presence of a phosphoric acid catalyst at 300°C and 60 atmospheres, they form alcohols. Water is added across the double bond.
Example: C₂H₄ + H₂O → C₂H₅OH (Ethene becomes ethanol.)
Oxidation: Alkenes can be oxidised by chemicals like potassium manganate(VII) (KMnO₄). This reaction forms diols (molecules with two -OH groups), and the purple solution becomes colorless.
Polymerisation: Under heat and pressure, many alkene molecules can join together to form long chains called polymers. For example, ethene becomes polythene (a common plastic).
Alcohols
Structure: Alcohols are organic compounds that contain one or more hydroxyl (-OH) groups. Their general formula is CᵒH₂ᵒ₁OH.
Combustion
Complete combustion: Alcohols burn in oxygen to produce carbon dioxide (CO₂) and water (H₂O). This reaction gives off a clean flame.
Example: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (This is ethanol burning.)
Incomplete combustion: When there isn’t enough oxygen, alcohols may produce carbon monoxide and black soot (carbon), just like alkenes.
Dehydration
To alkenes: Alcohols can be turned back into alkenes by removing water (H₂O). This process is called dehydration and requires heating with a catalyst such as aluminum oxide (Al₂O₃) or concentrated sulfuric acid (H₂SO₄).
Example: C₂H₅OH → C₂H₄ + H₂O (Ethanol turns into ethene.)
Oxidation
To acids: Alcohols can be oxidised to form carboxylic acids. This is done by using oxidising agents like acidified potassium dichromate (K₂Cr₂O₇) or potassium manganate(VII) (KMnO₄).
Example: C₂H₅OH + 2[O] → CH₃COOH + H₂O (Ethanol becomes ethanoic acid.)
Colour change: During this reaction, the orange dichromate solution turns green, and the purple manganate solution becomes colorless.
Other products: Depending on the conditions, oxidation can also produce aldehydes (with one -CHO group) or ketones (with a C=O group inside the chain).
Esterification
Reaction with acids: When alcohols react with carboxylic acids in the presence of an acid catalyst (like concentrated sulfuric acid), they form esters and water. This is a reversible reaction.
Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (Ethanoic acid + ethanol makes ethyl ethanoate.)
Carboxylic Acids
Structure: Carboxylic acids contain the carboxyl functional group (-COOH). Their general formula is CᵒH₂ᵒ₁COOH.
Neutralisation
With oxides: Carboxylic acids react with metal oxides to produce a salt and water.
With metals: Carboxylic acids can also react with metals to form a salt and release hydrogen gas.
With carbonates: They also react with metal carbonates to form a salt, water, and carbon dioxide gas.
Esterification
With alcohols: As mentioned above, carboxylic acids can react with alcohols to form esters, using a strong acid catalyst.
Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Interchangeability Between Homologous Series
Interchangeability Between Homologous Series:
- Alkanes to alkenes: Through a process called cracking, large alkane molecules are broken into smaller alkenes using heat and catalysts.
- Alkenes to alkanes: By hydrogenation, hydrogen is added to alkenes to turn them into alkanes.
- Alkenes to alcohols: Hydration adds water to the double bond in alkenes to make alcohols.
- Alcohols to alkenes: Dehydration removes water from alcohols to make alkenes.
- Alcohols to acids: Alcohols are oxidised by strong agents to become carboxylic acids.
- Alcohols + acids to esters: This is esterification, where an alcohol and acid combine to make an ester and water, usually with an acid catalyst.