Alkenes (Edexcel A Level Chemistry): Flashcards

True.

The C=C double bond is made up of one σ bond (end-to-end overlap) and one π bond (sideways overlap of p orbitals).

Pi bonds (π) are covalent bonds formed by the sideways overlap of adjacent p orbitals. The two electron clouds lie above and below the plane of the σ bond, and together represent one bond containing two electrons.

In ethene, each carbon atom forms three sigma bonds and one pi bond(s), using three of its four outer-shell electrons for σ bonding.

The lobes lie above and below the plane to maximise sideways overlap of the p orbitals. This orientation allows the greatest interaction between the two p orbitals on adjacent carbon atoms.

P orbitals are dumbbell-shaped atomic orbitals with two lobes. In alkenes, the p orbital on each carbon contains one spare electron that overlaps sideways with the adjacent carbon's p orbital to form a π bond.

False.

The σ bond lies directly between the nuclei (end-to-end), while the π bond lies above and below this axis as two electron clouds formed by sideways overlap of p orbitals.

An electrophile is an electron-loving (electron-accepting) species that is attracted to areas of high electron density and accepts a lone pair of electrons to form a new bond.

The four types of electrophilic addition reactions alkenes undergo are addition of hydrogen, steam (g), hydrogen halide, and halogens.

True.

Unsaturated vegetable oils contain C=C double bonds. Hydrogenation converts some of these to C-C single bonds, changing the oil into a solid: margarine.

The C=C double bond contains four electrons (one σ bond and one π bond), making it an area of high electron density that attracts electron-loving electrophilic species.

The pale purple solution of acidified KMnO₄ turns colourless. The product is a diol, formed by oxidation and addition of two OH groups across the C=C double bond.

Electrophilic addition is a reaction in which an electrophile adds across a C=C double bond, breaking the double bond and forming two new single bonds — one from each carbon atom.

False.

In heterolytic fission, the more electronegative atom takes both electrons from the bond, forming a negative ion, while the other atom is left as a positive ion.

When acidified KMnO₄ reacts with an alkene, it acts as an oxidising agent, and the product formed across the double bond is a diol compound.

Heterolytic fission is the breaking of a covalent bond in which both electrons are taken by the more electronegative atom, forming a negative ion and leaving a positive ion.

A carbocation is a highly reactive intermediate in which a carbon atom carries a positive charge, formed during electrophilic addition when heterolytic fission leaves the carbon atom without both bonding electrons.

The H atom (H⁺) acts as the electrophile, accepting electrons from the C=C bond. The Br^- ion formed by heterolytic fission of the H-Br bond acts as the nucleophile, attacking the carbocation intermediate.

When Br₂ approaches the C=C double bond, the high electron density repels the electrons in Br-Br, making the closer Br atom positively charged.

False.

Water is a weak electrophile and does not react with alkenes unless a strong acid catalyst is present. H₃O⁺ acts as the electrophile in the reaction.

A nucleophile is an electron-pair donor that is attracted to areas of positive charge or electron deficiency, and donates a lone pair of electrons to form a new bond.

Step 1: The π electrons in the C=C are attracted to H₃O⁺; heterolytic fission occurs and a carbocation intermediate forms.

Step 2: Water acts as a nucleophile, donating a lone pair to the carbocation to form a C-O bond; the H₃O⁺ catalyst is regenerated, and the product is an alcohol.

True.

Carbocation stability follows the order: tertiary > secondary > primary. More alkyl groups donate electron density to the positive carbon, stabilising the carbocation. The major product forms from the most stable intermediate.

Markovnikov's rule states that when a hydrogen halide adds to an unsymmetrical alkene, the major product is the one formed via the most stable carbocation intermediate (tertiary > secondary > primary).

In electrophilic addition of HBr to an alkene, the H-Br bond breaks heterolytically to form H⁺ and Br^-, which then attacks the carbocation.

The saturation test is a chemical test used to determine whether a molecule is unsaturated (contains a C=C double bond). The unknown compound is shaken with bromine water and the colour change observed.

Before: orange or yellow.

After: colourless. The decolourisation occurs because Br₂ is consumed in an addition reaction across the C=C double bond of the unsaturated compound.

True.

An unsaturated compound contains a C=C double bond and undergoes an addition reaction with Br₂ (aq), consuming the bromine and causing the orange/yellow solution to decolourise.

In the saturation test, the unknown compound is shaken with bromine water. If the compound is unsaturated, the orange solution will decolourise due to an addition reaction.

Bromine water is an aqueous solution of bromine, Br₂ (aq), that is orange or yellow in colour. It is the reagent most commonly used in the saturation test to detect C=C double bonds.

Halogens such as Br₂ (aq) undergo an addition reaction across the C=C double bond of unsaturated compounds. The halogen is incorporated into the product, so the coloured solution becomes colourless as Br₂ is consumed.

False.

Saturated hydrocarbons (e.g. alkanes) contain only C-C single bonds and cannot undergo addition reactions with Br₂ (aq). The bromine water remains orange/yellow.

Bromine water is orange or yellow in colour and is used to test for unsaturation by observing whether a colour change occurs.

Addition polymerisation is a reaction in which many monomers containing at least one C=C double bond join together to form a long-chain polymer as the only product.

Addition polymerisation gives only one product, which is why it differs from condensation polymerisation where a small molecule such as water is also released.

Identify the repeat unit (the smallest repeating group within the brackets), then replace the C-C bond in the backbone with a C=C double bond to give the monomer structure.

A monomer is a small, reactive molecule that joins together with many other monomers to form a polymer chain.

During addition polymerisation, the π bond in each C=C breaks and the monomers link together to form new C-C single bonds.

Poly(ethene) is formed from ethene monomers, and poly(chloroethene) (PVC) is formed from chloroethene monomers.

A repeat unit is the smallest group of atoms that, connected one after another, makes up the polymer chain. It is represented by square brackets in the displayed or general formula.

False.

The repeat unit has the same structure as the monomer except that the C=C double bond has been replaced by a C-C single bond.

Draw the bracketed repeat unit twice in a chain, omitting the square brackets and the n subscript. Each unit connects directly to the next, with a bond extending from each end to show the chain continues.

Many polymers are non-biodegradable, so they persist in landfill without breaking down, causing long-term environmental problems including harm to marine animals.

Feedstock recycling is the breakdown of waste polymers by chemical and thermal processes into monomers, gases and oils, which are then used as raw materials for producing new polymers and other organic chemicals.

True.

Polymers must be sorted before recycling because mixed polymers produce an unusable mix of materials. After sorting, they are chopped, washed, dried, melted and cast into pellets.

Feedstock recycling works with unsorted and unwashed polymers, making it more versatile than mechanical recycling, which requires careful sorting of polymer types.

During incineration of polymers, the energy released is used to boil water, and the water vapour turns turbines to generate electricity in a similar way to a coal-fired power station.

False.

PVC contains a large amount of toxic chlorine, which can be released during recycling. Modern techniques overcome this by dissolving the polymer and precipitating out the recycled material.

The waste gases are scrubbed with a base or carbonate, which neutralises the acidic HCl. For example: CaO (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l).

Biodegradable polymers are polymers that can be broken down over time by microorganisms, producing carbon dioxide, water and other organic compounds. Some polyesters (e.g. poly(lactic acid), PLA) are biodegradable as they contain ester linkages that can be hydrolysed. Most common polyesters (e.g. PET) and polyamides (e.g. nylon) are not readily biodegradable in the environment.

1. Use chemicals and solvents that are as safe and environmentally friendly as possible.

2. Use renewable feedstock chemicals where possible.

3. Reduce energy requirements and increase energy efficiency.