Alkenes (Edexcel IAL Chemistry Unit 1)

Chemistry· 5.1–5.8· 25 min read

1. Alkene Structure and Geometric Isomerism★★☆☆☆⏱ 5 min

📘 Definition

Alkene

Unsaturated hydrocarbon with general formula CₙH₂ₙ for non-cyclic alkenes, CₙH₂ₙ₋₂ for cycloalkenes, containing at least one C=C double bond.

Example:

Ethene (C₂H₄), propene (C₃H₆)

The C=C double bond consists of one strong σ bond (head-on sp² orbital overlap) and one weaker π bond (side-on p orbital overlap). The π bond restricts rotation around the C=C axis, leading to geometric isomerism.

📐 Worked Example

Draw and label the cis and trans isomers of but-2-ene, and explain why E-Z notation is not required here.

1
1. But-2-ene has the structure CH₃CH=CHCH₃: both double bond carbons are bonded to one CH₃ group and one H atom.
2
1. Cis isomer: both CH₃ groups are on the same side of the C=C double bond:
3
$CH_3\qquad CH_3\\diagdown\quad\diagup\\quad C=C\\diagup\quad\diagdown\H\qquad H$
4
1. Trans isomer: CH₃ groups are on opposite sides of the double bond:
5
$CH_3\qquad H\\diagdown\quad\diagup\\quad C=C\\diagup\quad\diagdown\H\qquad CH_3$
6
1. E-Z notation is not required because both double bond carbons have identical substituents, so cis-trans labels are unambiguous.

Exam tip:

When drawing geometric isomers, clearly show the planar arrangement of groups around the C=C double bond to score full marks.

2. Alkene Addition Reactions and Unsaturation Test★★★☆☆⏱ 6 min

Alkenes undergo addition reactions, where the weak π bond breaks and two new σ bonds form, adding atoms across the double bond to produce a saturated product. The key reactions for Unit 1 are listed below:

Hydrogenation: H₂ gas, Ni catalyst, 150°C → alkane
Halogenation: Halogen (e.g. Br₂), room temp, dark → dihalogenoalkane
Hydrohalogenation: Hydrogen halide (e.g. HBr), room temp → monohalogenoalkane
Hydration: Steam, concentrated H₃PO₄ catalyst, 300°C, 60 atm → alcohol
Oxidation: Cold dilute acidified KMnO₄ → diol (two adjacent -OH groups)

📘 Definition

Test for C=C unsaturation

Addition of orange bromine water to an alkene at room temperature results in immediate decolourisation (orange → colourless) as a 1,2-dibromoalkane forms, no UV light required.

📐 Worked Example

Predict the organic product of the reaction between propene and cold dilute acidified KMnO₄, and write its structural formula.

1
1. The reaction oxidises the C=C bond to form a diol, with no cleavage of the carbon chain.
2
1. Each carbon in the double bond gains one -OH group. Propene is CH₂=CHCH₃, so the product is propane-1,2-diol:
3
$HOCH_2CH(OH)CH_3$

Exam tip:

Only diol products are expected for cold dilute KMnO₄ reactions: cleavage products from hot concentrated KMnO₄ are out of scope for Unit 1.

3. Electrophilic Addition Mechanism★★★★☆⏱ 7 min

Electrophilic addition proceeds via attack of the electron-rich C=C π bond on an electron-deficient electrophile. Curly double-barbed arrows show movement of a pair of electrons, starting from a bond or lone pair, not an atom.

📘 Definition

Carbocation

Positively charged carbon intermediate formed during electrophilic addition. Stability increases with number of electron-donating alkyl groups: 3° > 2° > 1° > methyl.

Example:

CH₃CH⁺CH₃ is a 2° carbocation, more stable than 1° CH₃CH₂CH₂⁺

📐 Worked Example

Draw the complete electrophilic addition mechanism for the reaction between HBr and propene, explaining the major product with reference to carbocation stability.

1
1. HBr is polar: H is δ⁺ (electrophilic), Br is δ⁻. The C=C π bond attacks the δ⁺ H atom, breaking the H-Br bond to form a Br⁻ ion:
2
$CH_3CH=CH_2 + HBr \rightarrow CH_3CH^+CH_3 + Br^-$
3
1. Two carbocations can form: the more stable 2° carbocation (CH₃CH⁺CH₃, major) and less stable 1° carbocation (CH₃CH₂CH₂⁺, minor).
4
1. Br⁻ attacks the positively charged carbon of the 2° carbocation to form the major product 2-bromopropane:
5
$CH_3CH^+CH_3 + Br^- \rightarrow CH_3CHBrCH_3$
6
1. This follows Markovnikov's rule: H adds to the double bond carbon with more H atoms, forming the most stable carbocation.

Exam tip:

Always draw curly arrows starting from the center of the C=C bond (representing π electrons) or a lone pair to avoid losing marks.

4. Addition Polymerisation★★☆☆☆⏱ 3 min

Addition polymerisation occurs when alkene monomers add together to form long polymer chains, with no other products formed. The C=C double bond in each monomer breaks, and new single bonds form between adjacent monomer units.

📘 Definition

Repeat unit

The smallest repeating section of a polymer chain, drawn with trailing bonds extending outside square brackets, and a subscript 'n' to indicate a large number of repeats.

📐 Worked Example

Draw the repeat unit of poly(chloroethene) (PVC) from its monomer chloroethene (CH₂=CHCl).

1
1. Start with the monomer structure, showing the C=C double bond:
2
$H_2C=CHCl$
3
1. Break the C=C double bond, extend single bonds out on either side of the two carbons, enclose in brackets and add subscript 'n':
4
$\left[ -CH_2-CHCl- \right]_n$

Exam tip:

Ensure trailing bonds extend beyond the square brackets when drawing repeat units to score full marks.

5. Polymer Disposal★★★☆☆⏱ 4 min

Most addition polymers are non-biodegradable, as their strong non-polar C-C bonds are not broken down by environmental microbes. Common disposal methods are outlined below:

Landfill: Low cost but occupies large space and does not decompose for hundreds of years
Incineration: Releases heat energy for electricity generation but can produce toxic gases (e.g. HCl from PVC)
Recycling: Reduces fossil fuel demand but requires labour-intensive sorting of polymer types

📐 Worked Example

State one advantage and one disadvantage of incinerating PVC waste.

1
Advantage: Incineration releases heat energy that can be used to generate electricity, reducing reliance on fossil fuels.
2
Disadvantage: PVC contains chlorine, so incineration produces toxic HCl gas if untreated, contributing to acid rain and respiratory harm.

6. Common Pitfalls

Wrong move:

Drawing curly arrows from an atom instead of the C=C π bond or a lone pair in mechanisms

Why:

Curly arrows represent movement of electron pairs, which are located in bonds or lone pairs, not atomic nuclei

Correct move:

Start all curly arrows at the center of the C=C bond or a lone pair on a negative ion (e.g. Br⁻)

Wrong move:

Describing the bromine water test result as 'clear' instead of 'colourless'

Why:

Clear describes transparency, not colour: an orange solution can be clear. The positive test is a colour change

Correct move:

Always specify the colour change as orange → colourless for the unsaturation test

Wrong move:

Predicting 1-bromopropane as the major product of HBr + propene

Why:

The major product is determined by the most stable carbocation intermediate: 2° carbocations are more stable than 1°

Correct move:

Identify the most stable carbocation first, then attach the nucleophile to the positively charged carbon to get the major product

Wrong move:

Drawing polymer repeat units with a double bond between the monomer carbons

Why:

The C=C double bond in the monomer breaks completely during addition polymerisation

Correct move:

Draw repeat units with only single bonds between the two monomer carbons, with trailing bonds outside the square brackets

Wrong move:

Using cis-trans labels for alkenes with no common substituent on both double bond carbons

Why:

Cis-trans labels require at least one shared group to assign same/opposite positions

Correct move:

Use E-Z notation for these cases, assigning priority based on atomic number of attached atoms

7. Quick Reference Cheatsheet

Reaction	Reagents & Conditions	Product
Hydrogenation	H₂, Ni catalyst, 150°C	Alkane
Halogenation	Br₂, room temp, dark	Dihalogenoalkane
Hydrohalogenation	HBr, room temp	Monohalogenoalkane
Hydration	Steam, H₃PO₄, 300°C, 60 atm	Alcohol
KMnO₄ oxidation	Cold dilute acidified KMnO₄	Diol
Unsaturation test	Br₂(aq), room temp	Orange → colourless

8. Frequently Asked

What is the positive test for an alkene?

Add orange bromine water at room temperature (no UV light required). If a C=C double bond is present, the solution will decolourise from orange to colourless as a dihalogenoalkane forms via addition reaction.

Why do alkenes show geometric isomerism?

The π bond in the C=C double bond restricts rotation around the bond axis, so substituents attached to the double bond carbons are fixed in relative positions, creating distinct isomers that cannot interconvert at room temperature.

Going deeper

official_resourceEdexcel IAL Chemistry 2018 SpecificationFull Unit 1 specification and assessment guidance

What's Next

Now you have mastered core alkene content for Edexcel IAL Chemistry Unit 1, move on to halogenoalkane nucleophilic substitution reactions, which use identical curly arrow notation for mechanisms. Consolidate your organic synthesis skills by combining alkene reactions with other functional group transformations, and work through past paper questions to familiarise yourself with marking criteria for mechanism and isomerism questions. Prioritise revising carbocation stability and Markovnikov's rule, as these are frequently tested high-mark topics.