Alkenes

Describe the bonding in the C=C double bond

One sigma (σ) bond and one pi (π) bond

What is the bond angle around each carbon in the C=C double bond?

Approximately 120°

What is the shape around each carbon in the C=C double bond?

Trigonal planar

Why are alkenes more reactive than alkanes?

The pi (π) bond is a region of high electron density above and below the plane, making it susceptible to attack by electrophiles

What is an electrophile?

An electron pair acceptor; a species that is attracted to areas of high electron density

What type of reactions do alkenes undergo?

Electrophilic addition reactions

What is stereoisomerism?

Compounds with the same structural formula but a different arrangement of atoms in space

2 things that causes E/Z isomerism in alkenes?

Restricted rotation around the C=C double bond and two different groups attached to each carbon of the double bond

What is the test for unsaturation (double bond) and what is the positive result?

Add bromine water (orange/brown) to the compound and it will go colourless if positive

Why does bromine water decolourise? →

Bromine adds across the C=C double bond in an electrophilic addition reaction, forming a colourless dibromoalkane

What is the electrophile in the reaction of an alkene with bromine Br₂ ?

The Br atom that becomes δ+ due to induced polarity from the high electron density of the double bond

Describe the mechanism for electrophilic addition of Br₂ to ethene.

Curly arrow from double bond to Br, Br-Br bond breaks heterolytically, forming a carbocation intermediate and Br⁻; curly arrow from Br⁻ to carbocation forms 1,2-dibromoethane

What is the product of ethene with bromine?

1,2-dibromoethane

What is the electrophile in the reaction with hydrogen bromide (HBr)?

The δ+ hydrogen atom in the polar H-Br bond

What is the product of ethene with HBr?

Bromoethane

What is the electrophile in the reaction with concentrated sulfuric acid?

The δ+ hydrogen atom in the polar O-H bond of H₂SO₄

Describe the first step of the reaction of an alkene with concentrated sulfuric acid.

Electrophilic addition: double bond attacks H of H₂SO₄, forming a carbocation and HSO₄⁻ ion

What is the product formed with ethene and concentrated sulfuric acid?

Ethyl hydrogensulfate

How is an alcohol produced from an alkene via sulfuric acid?

Alkene reacts with concentrated H₂SO₄ to form alkyl hydrogensulfate, which is then hydrolysed with water to form the alcohol (regenerating H₂SO₄)

Write the overall equation for hydration of ethene using sulfuric acid catalyst.

C₂H₄ + H₂O → C₂H₅OH (with H₂SO₄ catalyst)

What determines which product is the major product during an electrophilic addition reaction with an alkene?

The stability of the carbocation intermediate formed during the reaction

How are carbocations stabilised?

By electron-donating alkyl groups through positive inductive effect (+I effect) the more alkyl groups, the more stable it is

Rank carbocation stability in order.

Tertiary > secondary > primary

What is the major product when HBr adds to propene?

2-bromopropane (via the more stable secondary carbocation)

What is the minor product when HBr adds to propene?

1-bromopropane (via the less stable primary carbocation)

What is addition polymerisation?

The process where many alkene monomers open their double bonds and join together to form long-chain saturated molecules

What is the only product of addition polymerisation?

The polymer itself (100% atom economy)

Name the polymer formed from ethene.

Poly(ethene) the original alkene must be in brackets

Name the polymer formed from chloroethene.

Poly(chloroethene) (also known as PVC)

What are the properties of addition polymers?

Chemically unreactive (saturated, non-polar C-C and C-H bonds), strong due to many van der Waals forces between chains

Why are addition polymers non-biodegradable?

They are unreactive and resistant to attack by nucleophiles, so microorganisms cannot break them down

What are the environmental issues with polymers?

They persist in landfill, release toxic gases when burned, and can harm wildlife if ingested

What is a plasticiser?

A substance added to a polymer to make it more flexible

How does a plasticiser work?

It gets between polymer chains, pushing them apart and reducing the strength of van der Waals forces, allowing chains to slide past each other

What is PVC used for without a plasticiser?

Rigid applications like drainpipes and window frames

What is PVC used for with a plasticiser?

Flexible applications like waterproof clothing, pool liners, and cable insulation

What are the two methods of producing ethanol?

Fermentation of sugars and hydration of ethene

What are the conditions for hydration of ethene into ethanol?

300°C, 60-70 atm pressure, conc. phosphoric acid catalyst or conc. sulfuric acid catalyst