Alkenes

Full Notes:

Alkenes are unsaturated hydrocarbons that contain a carbon–carbon double bond (C=C). Their general formula is C_nH_2n.

Example Ethene (C₂H₄)

Sigma and Pi Bonding

The C=C double bond is made up of a strong sigma (σ) bond and a weaker pi (π) bond.

Sigma (σ) bond

• Formed by end-to-end overlap of orbitals.
• Allows free rotation around single C–C bonds (not across C=C).

Pi (π) bond

• Formed by sideways overlap of p orbitals above and below the plane.
• Prevents rotation around the C=C bond.
• Creates a region of high electron density that attracts electrophiles, making alkenes reactive.

Because the pi-bond is slightly weaker than the sigma bond, the double bond can break in reactions and ‘open up’, allowing other atoms or group to bond to the carbons. When this happens, an addition reaction occurs.

Addition Reactions of Alkenes

There are several key addition reactions of alkenes you need to know.

Reaction with hydrogen (H₂)

Alkene + H₂ → alkane
Ni catalyst, ~150 °C.
Industrially used to hydrogenate vegetable oils and make margarine.

Reaction with halogens (e.g. Br₂)

Alkene + Br₂ → dihalogenoalkane
This is useful as it forms the basis of the bromine water test for alkenes (bromine water turns from orange to colourless when mixed with an alkene).

Reaction with hydrogen halides (e.g. HBr)

Alkene + HBr → halogenoalkane
Unsymmetrical alkenes give major/minor products due to carbocation stability.

Reaction with steam

Alkene + H₂O(g) → alcohol
Phosphoric acid catalyst, high pressure (60 to 70 atm) and temperature (300°C).

Reaction with potassium manganate(VII)

In acid conditions, alkenes are oxidised to diols (compounds with two OH groups).
Note a colour change of purple to colourless occurs (as the MnO₄^- ions (purple) from KMnO₄ get reduced to Mn²⁺ (colourless)).

Electrophiles and Heterolytic Fission

An electrophile is an electron pair acceptor.

Alkenes react with electrophiles because the C=C is electron-rich.

In electrophilic addition reactions, heterolytic fission occurs and the C=C bond breaks unevenly, forming ions. One atom takes both electrons, leaving a carbocation (positively charged carbon) and an anion.

The more substituted the carbocation (i.e., the more alkyl groups attached), the more stable it is. Stability order: tertiary > secondary > primary.

Addition Reaction Mechanisms

The high electron density within a carbon-carbon double bond attracts electrophiles and the reaction mechanism follows three basic steps:

Step 1: Electrophile Attraction

• The C=C bond has high electron density and attracts electrophiles (electron pair acceptors).
• Electrophiles include H⁺, Br₂, and H₂SO₄ (see below).

Step 2: Formation of Carbocation Intermediate

• The double bond breaks as one carbon gains a new bond with the electrophile.
• The other carbon becomes positively charged (carbocation).

Step 3: Forming final product

• A negative species (e.g., Br⁻, HSO₄⁻) bonds to the carbocation, forming the final product.

You need to know for the following examples of addition mechanisms:

Electrophilic addition mechanism for Bromine + Ethene

Equation: C₂H₄ + Br₂ → C₂H₄Br₂

Mechanism:

1. Br₂ molecule approaches C=C (polarised by electron density)
2. Double bond breaks, Br⁺ forms bond → Carbocation intermediate
3. Br⁻ ion attacks carbocation → Forms CH₂Br–CH₂Br

HBr + Ethene

Equation: C₂H₄ + HBr → C₂H₅Br

Mechanism:

1. HBr is polar (Hδ⁺—Brδ⁻), and H⁺ acts as the electrophile.
2. H⁺ bonds to one carbon of the C=C bond, forming a carbocation.
3. Br⁻ attacks the carbocation, forming a halogenoalkane.

Major and Minor Products

When adding HX to an unsymmetrical alkene, two possible products can form.

The two possible products won’t be formed in equal amounts. The product formed most is called the major product and the one formed the least is the minor product.

We can predict the major product based on the carbocation intermediate formed in the reaction.

• The major product forms from the most stable carbocation.
• Alkyl (carbon) groups bonded to the positively charged carbon in the intermediate stabilise the positive charge by ‘giving’ electron density to the positively charged carbon. This is called the positive inductive effect. The more alkyl groups there are bonded to the positively charged carbon, the more stable the carbocation is and the more likely it is to form.
• Carbocation stability order: Tertiary > Secondary > Primary

More stable carbocation = major product. This explains Markovnikov’s rule (Major product will be the one where H from HX bonds to carbon in C=C that is bonded to the most hydrogens) .

Example Propene + HBr

Secondary carbocation → 2-bromopropane (major)
Primary carbocation → 1-bromopropane (minor)

Test for Alkenes

Alkenes decolourise bromine water. This is a qualitative test for the presence of a C=C double bond.

Observation: orange solution → colourless

Polymerisation of Alkenes

Alkenes undergo addition polymerisation to form long-chain molecules called addition polymers. Each monomer must contain a C=C bond.

Example ethene → poly(ethene)

Drawing Polymer Structures

An addition polymer’s repeating unit is based on the monomer structure.

It is possible to draw a repeating unit and polymer from a monomer:

1. Redraw the monomer with the C=C double bond in the middle of the structure.
2. Break the C=C double bond.
3. Extend single bonds from the two carbons to show the continuation of the polymer chain.

To find the monomer from a polymer chain:

1. Identify the repeating unit.
2. Restore the C=C double bond.

Disposal and Sustainability of Polymers

Polymers are not biodegradable and cause environmental issues. Chemists have developed ways to reduce impact:

• Mechanical recycling: sorting and remoulding
• Incineration: burning polymers for energy
• Feedstock recycling: cracking polymers back into monomers

Concerns with incineration include toxic gases (e.g. HCl from PVC). These can be neutralised by chemical scrubbers.

Sustainable Polymers

Chemists work to improve sustainability by:

• Developing biodegradable polymers that break down naturally
• Designing polymers with lower environmental impact
• Using life cycle assessments to evaluate environmental costs

Summary

• Alkenes are unsaturated hydrocarbons with a C=C double bond and general formula C_nH_2n.
• The double bond has a sigma bond and a pi bond and the pi bond gives high electron density and reactivity.
• Alkenes undergo electrophilic addition with hydrogen halogens hydrogen halides and steam and carbocation stability explains major and minor products.
• Bromine water turns from orange to colourless in the presence of an alkene.
• Alkenes form addition polymers and polymer disposal involves recycling incineration or feedstock recycling with sustainability improvements being developed.