Alkenes Revision Notes

Alkenes are a homologous series of hydrocarbons that contain at least one carbon–carbon double bond (C=C) within their molecular structure. This double bond consists of one sigma (σ) bond and one pi (π) bond formed by the sideways overlap of p-orbitals.

The presence of the double bond means that alkenes have fewer hydrogen atoms attached to the carbon chain compared to alkanes with the same number of carbon atoms.

The simplest alkene is ethene (C2H4), with the structural formula H2C=CH2, showing the double bond between the two carbon atoms.

• Carbon–carbon double bond (C=C) present in alkenes.
• Double bond consists of one sigma and one pi bond.
• Alkenes have fewer hydrogen atoms than alkanes with the same carbon number.

Hydrocarbons are classified as saturated or unsaturated based on the types of bonds between carbon atoms.

Saturated hydrocarbons, such as alkanes, contain only single bonds (C–C) and have the maximum number of hydrogen atoms bonded to carbon.

Unsaturated hydrocarbons contain one or more double or triple bonds, resulting in fewer hydrogen atoms than the corresponding saturated hydrocarbon. Alkenes are unsaturated because of their carbon–carbon double bond.

• Saturated hydrocarbons have only single bonds and maximum hydrogen.
• Unsaturated hydrocarbons have double or triple bonds and fewer hydrogens.
• Alkenes are unsaturated due to the C=C double bond.

The general formula for alkenes with one double bond is CnH2n, where n is the number of carbon atoms.

For example, ethene (C2H4), propene (C3H6), and butene (C4H8) all follow this formula.

In contrast, alkanes have the general formula CnH2n+2, reflecting their saturated nature.

• Alkenes general formula: CnH2n.
• Alkanes general formula: CnH2n+2.
• Example: Ethene (C2H4) vs Ethane (C2H6).

The carbon–carbon double bond is a region of high electron density, making alkenes more reactive than alkanes.

This double bond can undergo addition reactions where the π bond breaks and atoms add across the double bond, converting it into a single bond.

Because of this, alkenes readily react with bromine, hydrogen, and steam under suitable conditions, unlike alkanes which are less reactive.

• Double bond is electron-rich and reactive.
• Alkenes undergo addition reactions at the double bond.
• Reactivity distinguishes alkenes from alkanes.

Alkenes can be represented by molecular formulae (e.g., C2H4), structural formulae (showing the arrangement of atoms), or displayed formulae (showing all bonds).

For example, ethene's structural formula is H2C=CH2, indicating the double bond between the two carbon atoms.

The double bond restricts rotation, affecting the molecule's geometry and is important in stereochemistry.

• Molecular, structural, and displayed formulae represent alkenes.
• Double bond shown as '=' between carbon atoms.
• Double bond restricts rotation, influencing shape.

Cracking is a chemical process used to break down large alkane molecules, which are saturated hydrocarbons with single carbon-carbon bonds, into smaller, more useful molecules. This process is essential in the petrochemical industry to convert heavy fractions of crude oil into lighter hydrocarbons such as alkenes and hydrogen gas.

The main products of cracking are alkenes, which are unsaturated hydrocarbons containing at least one carbon-carbon double bond, and hydrogen gas (H₂). Alkenes are valuable as starting materials for making polymers and other chemicals, while hydrogen is used as a fuel and in various industrial processes.

• Cracking breaks large alkanes into smaller alkenes and hydrogen.
• Alkenes contain carbon-carbon double bonds (C=C).
• Hydrogen gas is a valuable by-product.

Cracking requires specific conditions to break the strong carbon-carbon bonds in alkanes. These conditions include:

High temperature, typically between 600°C and 700°C, to provide the energy needed to break bonds.

A catalyst, commonly a solid acid catalyst such as silica-alumina, to lower the activation energy and increase the reaction rate without being consumed in the process.

• Temperature range: 600–700°C.
• Catalyst: silica-alumina or similar solid acid catalyst.
• Catalyst increases reaction rate but is not consumed.

During cracking, the large alkane molecule undergoes homolytic cleavage of carbon-carbon bonds, producing smaller hydrocarbon fragments. One common outcome is the formation of an alkene and hydrogen gas.

For example, cracking decane (C₁₀H₂₂) can produce octene (C₈H₁₆) and ethane (C₂H₆), or alternatively, smaller alkenes and hydrogen. The exact products depend on the conditions and catalyst used.

A simplified example is the cracking of hexane (C₆H₁₄) to produce butene (C₄H₈) and ethane (C₂H₆): C₆H₁₄ → C₄H₈ + C₂H₆. However, hydrogen gas (H₂) is also produced in some cracking reactions, especially when the process is designed to maximise hydrogen yield.

• Carbon-carbon bonds break to form smaller hydrocarbons.
• Alkenes formed contain carbon-carbon double bonds.
• Hydrogen gas is released as a by-product.

A typical cracking reaction can be represented by the balanced equation:

C₁₀H₂₂ (l) → C₈H₁₆ (g) + C₂H₆ (g) (thermal cracking)

In catalytic cracking, hydrogen gas may also be produced: C₁₀H₂₂ (l) → C₆H₁₂ (g) + C₃H₆ (g) + H₂ (g) (catalytic cracking)

• Balanced equations show conservation of atoms.
• State symbols indicate physical states: (l) liquid, (g) gas.
• Hydrogen gas (H₂) is a product in catalytic cracking.

Cracking is vital in the petrochemical industry to convert less useful heavy alkanes into more valuable lighter alkenes and hydrogen. Alkenes serve as feedstock for polymers like polyethene and polypropene, while hydrogen is used in processes such as the Haber process and as a clean fuel.

The use of high temperatures and catalysts makes cracking efficient and economically viable. Understanding the chemical changes and conditions involved helps in optimising the process for desired products.

• Cracking produces alkenes for polymer manufacture.
• Hydrogen gas is used in industry and as fuel.
• High temperature and catalysts optimise product yield.

Cracking is a chemical process that breaks down large alkane molecules, typically found in crude oil fractions, into smaller hydrocarbons. This is achieved by breaking carbon-carbon single bonds in the long-chain alkanes.

The process is essential because many large alkanes have limited practical use due to their high boiling points and low volatility, making them less suitable as fuels or chemical feedstocks.

Cracking converts these less useful large molecules into smaller, more valuable hydrocarbons, including alkanes and alkenes.

• Large alkanes have high boiling points and are less useful as fuels.
• Cracking breaks C–C bonds to form smaller hydrocarbons.
• The process increases the yield of valuable products from crude oil.

One primary motivation is to produce smaller hydrocarbons that are more suitable as fuels, such as gasoline and diesel. These smaller molecules have lower boiling points and burn more efficiently.

Another motivation is to produce alkenes, which are unsaturated hydrocarbons containing carbon-carbon double bonds. Alkenes are highly reactive and serve as important starting materials in the manufacture of plastics, synthetic fibres, and other chemicals.

Cracking also helps balance supply and demand by converting excess heavy fractions into lighter, more marketable products.

• Smaller hydrocarbons are better fuels with lower boiling points.
• Alkenes produced are valuable chemical feedstocks.
• Cracking adjusts product distribution to meet market needs.

Cracking produces a mixture of smaller alkanes and alkenes. For example, cracking decane (C10H22) can produce octane (C8H18) and ethene (C2H4).

Alkanes produced are used as fuels, while alkenes are used in the chemical industry for polymerisation and synthesis of other compounds.

The general reaction can be represented as: C10H22 → C8H18 + C2H4 (simplified example).

• Mixture of smaller alkanes and alkenes formed.
• Alkanes mainly used as fuels.
• Alkenes used as chemical feedstocks for polymers and other products.

Cracking is carried out at high temperatures (around 600–700 °C) and sometimes in the presence of a catalyst such as silica or alumina to increase the rate and selectivity of the reaction.

Thermal cracking relies on heat alone, while catalytic cracking uses a catalyst to lower the temperature and produce more branched alkanes and alkenes.

These conditions ensure efficient breaking of C–C bonds and formation of desired products.

• High temperature (600–700 °C) required.
• Catalysts like silica or alumina may be used.
• Catalytic cracking produces more branched and reactive products.

Cracking transforms less useful large alkanes into smaller, more valuable hydrocarbons that meet fuel demands and chemical industry needs.

It increases the efficiency of crude oil usage by producing gasoline and alkenes for polymer production.

Understanding these motivations highlights the importance of cracking in modern chemistry and industry.

• Converts heavy fractions into valuable fuels and chemicals.
• Supports production of alkenes for polymer industry.
• Optimises crude oil resource utilisation.

Hydrocarbons are organic compounds composed solely of carbon and hydrogen atoms. They are classified as saturated or unsaturated based on the types of bonds between carbon atoms.

Saturated hydrocarbons, such as alkanes, contain only single covalent bonds (C–C) between carbon atoms. Unsaturated hydrocarbons, such as alkenes, contain one or more carbon–carbon double bonds (C=C). The presence of double bonds affects their chemical reactivity, particularly in addition reactions.

• Saturated hydrocarbons have only single bonds (C–C).
• Unsaturated hydrocarbons have at least one double bond (C=C).

Aqueous bromine (Br2 dissolved in water) is a reddish-brown solution used to test for unsaturation in hydrocarbons.

Alkenes undergo electrophilic addition reactions with bromine, breaking the double bond and forming a colourless dibromo-compound. This reaction decolourises the bromine solution.

Alkanes do not react with bromine water under normal conditions, so the reddish-brown colour remains unchanged.

• Bromine adds across the C=C double bond in alkenes.
• No reaction occurs with alkanes under standard conditions.
• Decolourisation of bromine water indicates unsaturation.

The reaction between an alkene and bromine is an electrophilic addition. The bromine molecule is polarised by the electron-rich double bond, leading to the formation of a bromonium ion intermediate and subsequent addition of bromide ion.

For example, ethene (C2H4) reacts with bromine water as follows:

C2H4 + Br2 → C2H4Br2 (1,2-dibromoethane) (balanced molecular equation)

• Ethene structural formula: H2C=CH2
• 1,2-dibromoethane structural formula: BrCH2–CH2Br
• Balanced equation: C2H4 (g) + Br2 (aq) → C2H4Br2 (aq)

1. Prepare two test tubes labelled 'Sample A' and 'Sample B' containing the hydrocarbon samples to be tested.

2. Add a few drops (about 2 cm3) of aqueous bromine to each test tube.

3. Shake or stir gently and observe any colour change over 1–2 minutes at room temperature (about 20–25 °C).

• If the bromine water remains reddish-brown, the hydrocarbon is saturated (alkane).
• If the bromine water decolourises (loses its brown colour), the hydrocarbon is unsaturated (alkene).
• Repeat the test to confirm results and ensure accuracy.
• Wear gloves and eye protection; bromine is toxic and corrosive.

Saturated hydrocarbon (alkane): No reaction with bromine water; solution remains reddish-brown.

Unsaturated hydrocarbon (alkene): Bromine water decolourises from reddish-brown to colourless due to addition across the double bond.

This test is qualitative and provides a quick identification method for unsaturation in hydrocarbons.

• Colour change from brown to colourless indicates unsaturation.
• No colour change indicates saturation.
• Use structural formulae to confirm the presence or absence of double bonds.

An addition reaction is a type of chemical reaction where two or more reactant molecules combine to form a single product molecule. This contrasts with substitution or elimination reactions, where multiple products may form.

In organic chemistry, addition reactions commonly occur with unsaturated hydrocarbons such as alkenes, which contain carbon–carbon double bonds (C=C). The double bond is reactive and can open up to allow atoms or groups to add across it, saturating the molecule.

The key feature of an addition reaction is that the product contains all the atoms of the reactants combined into one molecule, with no atoms lost or replaced.

• Addition reaction: multiple reactants → one product
• Common in alkenes due to C=C double bond
• Double bond breaks to allow addition of atoms
• Product is a saturated molecule

Alkenes have a carbon–carbon double bond consisting of one sigma (σ) bond and one pi (π) bond. The π bond is weaker and more reactive than the σ bond, making it the site for addition reactions.

During an addition reaction, the π bond breaks, allowing new atoms to bond to the two carbon atoms. This converts the alkene into an alkane or a saturated compound.

For example, when hydrogen (H₂) adds across the double bond, the alkene becomes an alkane. Similarly, bromine (Br₂) can add across the double bond, forming a dibromoalkane.

• C=C double bond contains a reactive π bond
• π bond breaks during addition
• New atoms add to carbons formerly double bonded
• Alkene converted to saturated compound

Addition reactions can be represented by balanced chemical equations showing reactants and the single product.

Example: Addition of bromine to ethene (an alkene)

C₂H₄ + Br₂ → C₂H₄Br₂ (ethene + bromine → 1,2-dibromoethane)

• Balanced equation shows all atoms conserved
• Structural formulae illustrate addition across C=C
• Example: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Addition reactions involving alkenes often occur at room temperature or with mild heating, sometimes requiring a catalyst depending on the reactants.

A common test for addition is the reaction of bromine water with an alkene: the bromine water (orange-brown) decolourises as it adds across the double bond, forming a colourless dibromo compound.

Hydrogen addition requires a nickel catalyst and elevated temperature (about 150°C) to convert alkenes to alkanes (hydrogenation).

• Bromine water decolourises (orange to colourless)
• Hydrogenation requires Ni catalyst and heat
• Addition reactions usually produce one colourless product

Propene (C₃H₆) reacts with hydrogen (H₂) in the presence of a nickel catalyst at 150°C.

The double bond breaks and hydrogen atoms add to the carbons, forming propane (C₃H₈).

Balanced equation: C₃H₆ + H₂ → C₃H₈ (with Ni catalyst, 150°C)

• Alkene + hydrogen → alkane
• Catalyst: nickel; temperature: 150°C
• Product contains all atoms from reactants
• Example: propene + hydrogen → propane

Alkenes contain a carbon–carbon double bond (C=C) which is electron-rich and reactive. This double bond undergoes addition reactions where reagents add across it, breaking the double bond and forming single bonds.

When bromine (Br₂) or aqueous bromine (Br₂ dissolved in water) is added to an alkene, the bromine molecules add across the double bond to form a dibromoalkane.

This reaction is used as a test for unsaturation: bromine water is orange-brown and decolourises when added to an alkene, indicating the addition reaction has occurred.

• General equation: CₙH₂ₙ + Br₂ → CₙH₂ₙBr₂
• Example: Ethene reacts with bromine to form 1,2-dibromoethane.
• C₂H₄ + Br₂ → C₂H₄Br₂
• Observation: The orange-brown colour of bromine water disappears, leaving a colourless solution.

Hydrogenation is the addition of hydrogen (H₂) across the carbon–carbon double bond of an alkene, converting it into an alkane.

This reaction requires a catalyst, typically finely divided nickel, and is carried out at elevated temperatures (around 150°C).

The double bond breaks and two hydrogen atoms add to the carbons, saturating the molecule.

• General equation: CₙH₂ₙ + H₂ → CₙH₂ₙ₊₂ (in presence of Ni catalyst, heat)
• Example: Ethene hydrogenation to ethane: C₂H₄ + H₂ → C₂H₆
• Conditions: Nickel catalyst, temperature about 150°C
• Observation: No colour change; the alkene is converted to a saturated alkane, which is less reactive.
• This reaction is important in the food industry for converting unsaturated vegetable oils to saturated fats.

Hydration is the addition of water (steam) across the double bond of an alkene to form an alcohol.

This reaction requires an acid catalyst, commonly concentrated phosphoric acid (H₃PO₄), and elevated temperature (around 300°C) and pressure (about 60 atm).

The double bond breaks, and an –OH group and a hydrogen atom add to the carbons of the former double bond.

• General equation: CₙH₂ₙ + H₂O → CₙH₂ₙ₊₁OH (in presence of acid catalyst)
• Example: Ethene hydration to ethanol: C₂H₄ + H₂O → C₂H₅OH
• Conditions: Acid catalyst (H₃PO₄), temperature ~300°C, pressure ~60 atm
• Observation: Formation of a colourless liquid alcohol.
• The reaction follows Markovnikov’s rule: the –OH group attaches to the carbon with more alkyl groups (more substituted carbon).

Drawing the structural or displayed formulae of products formed in these addition reactions helps visualise the changes.

In the bromine addition, the double bond is replaced by single bonds to two bromine atoms.

In hydrogenation, the double bond is replaced by single bonds to hydrogen atoms, saturating the molecule into an alkane structure.

• Ethene (C₂H₄) displayed formula: H₂C=CH₂
• After bromine addition: H₂C–CH₂ with each carbon bonded to one bromine atom (H₂CBr–CH₂Br)
• After hydrogenation: H₃C–CH₃ (ethane)
• After hydration: H₃C–CH₂OH (ethanol)

Each addition reaction has distinct conditions and observations that confirm the reaction type and product formed.

Bromine addition is a simple test for unsaturation, indicated by decolourisation of bromine water.

Hydrogenation requires a nickel catalyst and heat, converting alkenes to saturated alkanes, important in industry for producing margarine and other products from vegetable oils.

• Bromine addition: Room temperature, orange-brown bromine water decolourises.
• Hydrogenation: Nickel catalyst, ~150°C, no colour change, alkene to alkane.
• Hydration: Acid catalyst (H₃PO₄), ~300°C, ~60 atm, alkene to alcohol.
• Understanding these reactions aids in identifying alkenes and their chemical behaviour.