Alkenes

Q: What is the molecular geometry and bond angle around each double-bonded carbon atom in an alkene?

The shape is trigonal planar with a bond angle of approximately \( 120^\circ \) .

Q: What is the relative order of carbocation stability and why does stability increase with alkyl substitution?

Stability order: tertiary (\( 3^\circ \)) > secondary (\( 2^\circ \)) > primary (\( 1^\circ \)) , because alkyl groups donate electron density via the positive inductive effect.

Q: What are the reagents and conditions required for the catalytic hydrogenation of alkenes?

Hydrogen gas (\( \text{H}_2 \)) with a Nickel (\( \text{Ni} \)) catalyst at \( 150^\circ\text{C} \) (or Platinum at room temperature), forming an alkane.

Chemistry 1110 · CCEA AS Level · 10 min read · Updated: Aug 31, 2026

Welcome to Alkenes!

Welcome to one of the most exciting and dynamic topics in AS Chemistry: Alkenes! While alkanes are relatively unreactive and steady, alkenes are the lively "movers and shakers" of organic chemistry. From the plastics that make up your phone case to the synthesis of industrial alcohols, alkenes are essential starting materials.

Don't worry if organic mechanisms seem intimidating at first. We will break every single idea down into bite-sized, straightforward steps so you feel fully confident for your exam.

1. Structure and Bonding in Alkenes

What is an Alkene?

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double covalent bond (\( \text{C}=\text{C} \)).

• Hydrocarbon: A compound containing carbon and hydrogen atoms only.
• Unsaturated: Contains one or more double (or multiple) carbon-carbon bonds.
• General Formula for aliphatic alkenes with one double bond: \( \text{C}_n\text{H}_{2n} \)

The Nature of the Double Bond (\( \sigma \) and \( \pi \) Bonds)

The \( \text{C}=\text{C} \) double bond is not just two identical single bonds stacked together. It consists of two distinct types of covalent bonds:

1. The Sigma (\( \sigma \)) Bond: Formed by the direct, head-on overlap of atomic orbitals along the internuclear axis. This is a strong, stable covalent bond with high electron density directly between the two carbon nuclei.
2. The Pi (\( \pi \)) Bond: Formed by the sideways overlap of adjacent p-orbitals above and below the plane of the carbon atoms. The \( \pi \) bond has its electron density distributed in two clouds—one above and one below the plane of the \( \sigma \) bond.

Key Difference: The sideways overlap in a \( \pi \) bond is less effective than the head-on overlap of a \( \sigma \) bond. Therefore, the \( \pi \) bond is significantly weaker and easier to break than the \( \sigma \) bond!

Shape and Bond Angles

Around each carbon atom in the \( \text{C}=\text{C} \) double bond:

• There are three areas of electron density (three bonding regions: two single bonds and one double bond).
• These electron regions repel each other as far apart as possible to minimise repulsion.
• This gives a trigonal planar arrangement around each double-bonded carbon atom with a bond angle of approximately \( 120^\circ \).

Did You Know? The presence of the \( \pi \) bond prevents free rotation around the \( \text{C}=\text{C} \) bond. This rigidity is the root cause of E/Z (geometric) isomerism!

Key Takeaway: The \( \text{C}=\text{C} \) bond contains one strong \( \sigma \) bond and one weaker \( \pi \) bond. The shape around each double-bonded carbon is trigonal planar (\( 120^\circ \)).

2. Why Are Alkenes So Reactive?

Alkanes only have strong \( \sigma \) bonds and are non-polar, making them relatively inert. In contrast, alkenes are much more reactive because:

• The \( \pi \) bond contains a high concentration of electrons exposed above and below the molecular plane (a region of high electron density).
• This negative electron cloud readily attracts species that love electrons, known as electrophiles.
• Because the \( \pi \) bond is relatively weak, it easily breaks open to allow new atoms to bond to the carbon skeleton.

What is an Electrophile?

An electrophile is an electron-pair acceptor. It is an electron-deficient species (either carrying a full positive charge like \( \text{H}^+ \) or a partial positive charge \( \delta^+ \)) that is attracted to areas of high electron density.

Memory Trick: Think of electrophile as "electron-lover" (phile = lover). Since electrons are negative, an electrophile seeks negative charges!

3. Electrophilic Addition Mechanisms

The characteristic reaction of alkenes is electrophilic addition. In this reaction, the \( \pi \) bond breaks and two new \( \sigma \) bonds form, turning an unsaturated molecule into a saturated one.

Convention for Curly Arrows:

• A curly arrow shows the movement of an electron pair.
• It always starts at a bond or a lone pair of electrons and points directly to the atom or region where the new bond is being formed.

Mechanism 1: Reaction of Ethene with Hydrogen Bromide (\( \text{HBr} \))

Hydrogen bromide is a polar molecule because bromine is more electronegative than hydrogen: \( \text{H}^{\delta+}-\text{Br}^{\delta-} \).

• Step 1 (Attack of the \( \pi \) electrons): The electron-rich \( \pi \) bond attacks the electron-deficient \( \text{H}^{\delta+} \) atom. The \( \text{H}-\text{Br} \) bond breaks heterolytically, sending both electrons to the bromine atom to form a bromide ion (\( \text{Br}^- \)).
• Step 2 (Formation of the Carbocation): A new \( \text{C}-\text{H} \) single bond forms on one carbon. The other carbon is left with only 3 bonds and a positive charge, forming a carbocation intermediate (\( \text{CH}_3\text{CH}_2^+ \)).
• Step 3 (Nucleophilic attack): The lone pair on the bromide ion (\( :\text{Br}^- \)) attacks the positively charged carbon atom of the carbocation to form bromoethane (\( \text{CH}_3\text{CH}_2\text{Br} \)).

Overall Equation:
\( \text{CH}_2=\text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} \)

Mechanism 2: Reaction of Ethene with Bromine (\( \text{Br}_2 \))

You might wonder: Bromine is non-polar, so how can it act as an electrophile?

• Induced Dipole: As the non-polar \( \text{Br}_2 \) molecule approaches the electron-dense \( \pi \) bond of the alkene, the electrons in the \( \text{Br}-\text{Br} \) bond are repelled away from the closest bromine atom.
• This creates an induced dipole: \( \text{Br}^{\delta+}-\text{Br}^{\delta-} \).
• The \( \pi \) electrons attack the \( \text{Br}^{\delta+} \), releasing a bromide ion (\( \text{Br}^- \)) and producing a carbocation intermediate.
• The \( :\text{Br}^- \) ion then attacks the carbocation to form 1,2-dibromoethane.

Overall Equation:
\( \text{CH}_2=\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br} \)

The Test for Unsaturation: Adding bromine water (orange-brown) to an alkene results in a colour change from orange-brown to colourless (the solution is decolourised). Alkanes do not react under standard conditions.

4. Addition to Unsymmetrical Alkenes and Carbocation Stability

What is an Unsymmetrical Alkene?

An alkene is unsymmetrical if the two carbon atoms across the double bond are attached to different groups of atoms (e.g., propene, \( \text{CH}_3\text{CH}=\text{CH}_2 \)).

When an unsymmetrical reagent like \( \text{HBr} \) reacts with an unsymmetrical alkene like propene, two different isomeric products are possible:

1. 2-bromopropane (the major product)
2. 1-bromopropane (the minor product)

Carbocation Stability and Markovnikov's Rule

The product distribution depends entirely on the stability of the carbocation intermediate formed in Step 1:

• Primary (\( 1^\circ \)) Carbocation: The positive carbon is bonded to one alkyl group (\( \text{R}-\text{CH}_2^+ \)) — Least stable.
• Secondary (\( 2^\circ \)) Carbocation: The positive carbon is bonded to two alkyl groups (\( \text{R}_2\text{CH}^+ \)) — More stable.
• Tertiary (\( 3^\circ \)) Carbocation: The positive carbon is bonded to three alkyl groups (\( \text{R}_3\text{C}^+ \)) — Most stable.

Stability Order: Tertiary (\( 3^\circ \)) > Secondary (\( 2^\circ \)) > Primary (\( 1^\circ \))

Why are alkyl groups stabilising?

Alkyl groups (such as \( -\text{CH}_3 \)) are electron-releasing. They push electron density toward the positively charged carbon atom. This is called the positive inductive effect. By spreading out (dispersing) the positive charge, the carbocation becomes more stable.

Markovnikov's Rule: When a hydrogen halide (\( \text{HX} \)) adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon atom that already has the greater number of hydrogen atoms directly attached to it (forming the more stable carbocation intermediate).

• In propene (\( \text{CH}_3\text{CH}=\text{CH}_2 \)), the \( \text{C}1 \) carbon has 2 hydrogens, while \( \text{C}2 \) has 1 hydrogen.
• The \( \text{H}^+ \) adds to \( \text{C}1 \), creating a stable secondary carbocation (\( \text{CH}_3\text{CH}^+\text{CH}_3 \)) rather than a less stable primary carbocation (\( \text{CH}_3\text{CH}_2\text{CH}_2^+ \)).
• Attack by \( \text{Br}^- \) gives 2-bromopropane as the major product.

Key Takeaway: More alkyl groups on the carbocation \( \rightarrow \) greater positive inductive effect \( \rightarrow \) greater stability \( \rightarrow \) major product formed via this route.

5. Other Important Addition Reactions of Alkenes

1. Catalytic Hydrogenation (Addition of \( \text{H}_2 \))

• Reagent: Hydrogen gas (\( \text{H}_2 \))
• Conditions: Nickel (\( \text{Ni} \)) catalyst at \( 150^\circ\text{C} \) (or Platinum, \( \text{Pt} \), at room temperature)
• Product: Alkane
• Equation: \( \text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni, } 150^\circ\text{C}} \text{CH}_3-\text{CH}_3 \)
• Real-World Application: Used in the food industry to harden unsaturated vegetable oils to manufacture margarine.

2. Hydration (Addition of Steam, \( \text{H}_2\text{O}\text{(g)} \))

• Reagents: Steam (\( \text{H}_2\text{O}\text{(g)} \))
• Catalyst: Concentrated Phosphoric acid (\( \text{H}_3\text{PO}_4 \))
• Conditions: High temperature (approx. \( 300^\circ\text{C} \)) and high pressure (approx. \( 60-70\text{ atm} \))
• Product: Alcohol
• Equation: \( \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}\text{(g)} \xrightarrow{\text{H}_3\text{PO}_4} \text{CH}_3\text{CH}_2\text{OH} \)
• Industrial Importance: Essential industrial route for manufacturing pure ethanol.

6. Addition Polymerisation

What is Addition Polymerisation?

Addition polymerisation is the process where thousands of small unsaturated monomer molecules (alkenes) join together to form a very long saturated chain called a polymer, with no other product formed.

• The \( \pi \) bond in each monomer breaks.
• New \( \sigma \) bonds form between adjacent carbon atoms, linking them into a continuous backbone.

Representing Polymers and Repeating Units

A repeating unit is the shortest section of the polymer chain that, when repeated end-to-end, produces the complete polymer structure.

Step-by-step method to draw a repeating unit:
1. Draw the alkene monomer with all side groups pointing straight up or straight down (forming an "H" shape around the \( \text{C}=\text{C} \) bond).
2. Change the double bond to a single bond (\( \text{C}-\text{C} \)).
3. Extend the open single bonds out to the left and right beyond the square brackets.
4. Place square brackets around the unit and add the subscript \( n \).

Examples:
• Ethene (\( n\text{CH}_2=\text{CH}_2 \)) \( \rightarrow \) Poly(ethene): \( -[\text{CH}_2-\text{CH}_2]_n- \)
• Chloroethene (\( n\text{CH}_2=\text{CHCl} \)) \( \rightarrow \) Poly(chloroethene) (PVC): \( -[\text{CH}_2-\text{CH(Cl)}]_n- \)
• Propene (\( n\text{CH}_2=\text{CH(CH}_3) \)) \( \rightarrow \) Poly(propene): \( -[\text{CH}_2-\text{CH(CH}_3)]_n- \)

Environmental Concerns and Waste Disposal of Polymers

Addition polymers are extremely durable because they contain strong, non-polar \( \text{C}-\text{C} \) and \( \text{C}-\text{H} \) single bonds. However, this causes serious environmental challenges:

• Non-biodegradable: They are not broken down by microorganisms, leading to long-term buildup in landfill sites.
• Incineration (Combustion): Burning plastics releases energy, but can produce toxic gases. For example, burning poly(chloroethene) produces corrosive and toxic hydrogen chloride gas (\( \text{HCl} \)), which must be neutralised using basic scrubbers.
• Recycling: Polymers can be sorted, melted, and remoulded to conserve crude oil resources and reduce landfill volume.
• Feedstock Recycling: Waste polymers are broken down chemically into small hydrocarbons, which can be reused as raw materials for fuel or new chemical production.

Quick Summary Checklist

Before moving on to the next chapter, check that you can:

• Explain the formation and nature of \( \sigma \) and \( \pi \) bonds in a \( \text{C}=\text{C} \) double bond.
• State the trigonal planar shape and \( 120^\circ \) bond angle around double-bonded carbon atoms.
• Define an electrophile and draw the electrophilic addition mechanism for \( \text{Br}_2 \) and \( \text{HBr} \).
• Describe the test for unsaturation using bromine water (orange-brown to colourless).
• Use Markovnikov's rule and carbocation stability (\( 3^\circ > 2^\circ > 1^\circ \)) to predict major and minor products.
• Recall conditions and equations for hydrogenation (\( \text{Ni} / 150^\circ\text{C} \)) and hydration (\( \text{H}_3\text{PO}_4 / 300^\circ\text{C} / 60-70\text{ atm} \)).
• Draw monomers and repeating units for addition polymers and discuss their environmental impacts.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is an alkene and what is its general formula for aliphatic molecules containing one double bond?

An alkene is an unsaturated hydrocarbon containing at least one \( \text{C}=\text{C} \) double bond, with the general formula \( \text{C}_n\text{H}_{2n} \).

How is a \( \text{C}=\text{C} \) double bond constructed from \( \sigma \) and \( \pi \) bonds?

It consists of one strong \( \sigma \) bond formed by head-on orbital overlap and one weaker \( \pi \) bond formed by the sideways overlap of adjacent p-orbitals.

What is the molecular geometry and bond angle around each double-bonded carbon atom in an alkene?

The shape is trigonal planar with a bond angle of approximately \( 120^\circ \).

What is an electrophile and why are alkenes susceptible to electrophilic attack?

An electrophile is an electron-pair acceptor; alkenes attract electrophiles because the exposed \( \pi \) bond creates a region of high electron density.

What occurs during the electrophilic addition of hydrogen bromide (\( \text{HBr} \)) to ethene?

The \( \pi \) electrons attack \( \text{H}^{\delta+} \) to form a carbocation intermediate and \( \text{Br}^- \), which then attacks the positive carbon to yield bromoethane.

How does non-polar bromine (\( \text{Br}_2 \)) react with an alkene and how is this used as a test for unsaturation?

The \( \pi \) electron cloud induces a dipole (\( \text{Br}^{\delta+}-\text{Br}^{\delta-} \)); reacting with bromine water causes a colour change from orange-brown to colourless.

What is the relative order of carbocation stability and why does stability increase with alkyl substitution?

Stability order: tertiary (\( 3^\circ \)) > secondary (\( 2^\circ \)) > primary (\( 1^\circ \)), because alkyl groups donate electron density via the positive inductive effect.

How does Markovnikov's rule predict the major product when a hydrogen halide adds to an unsymmetrical alkene?

The hydrogen atom attaches to the carbon with more hydrogens already attached, forming the more stable carbocation intermediate.

What are the reagents and conditions required for the catalytic hydrogenation of alkenes?

Hydrogen gas (\( \text{H}_2 \)) with a Nickel (\( \text{Ni} \)) catalyst at \( 150^\circ\text{C} \) (or Platinum at room temperature), forming an alkane.

What are the conditions required for the industrial hydration of ethene to produce ethanol?

Steam (\( \text{H}_2\text{O}\text{(g)} \)) with a concentrated \( \text{H}_3\text{PO}_4 \) catalyst at approximately \( 300^\circ\text{C} \) and \( 60-70\text{ atm} \).

What is addition polymerisation and what happens to the monomer's double bond during the process?

It is the joining of many unsaturated monomer molecules into a long saturated polymer chain as the \( \pi \) bonds break to form new \( \sigma \) bonds.

Why are addition polymers non-biodegradable and what is an environmental hazard of incinerating poly(chloroethene)?

They contain unreactive, non-polar \( \text{C}-\text{C} \) bonds resisting microbial breakdown; incinerating poly(chloroethene) produces toxic, corrosive \( \text{HCl} \) gas.

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