Alkenes (Edexcel International A Level (IAL) Chemistry): Flashcards

Geometric isomers are stereoisomers that arise when restricted rotation about a C=C double bond causes groups to be fixed in different spatial arrangements.

The pi bond is formed by sideways overlap of p orbitals above and below the sigma bond plane. Rotating the bond would break this pi overlap, so the groups attached to the C=C carbons remain fixed, creating distinct geometric arrangements.

A cis isomer is a geometric isomer in which two identical or similar groups are on the same side of the C=C double bond.

In cis / trans isomerism, cis isomers have the two groups on the same side of the double bond, while trans isomers have them on opposite sides.

False.

2-methylpropene cannot show cis / trans isomerism because one carbon of the C=C bond carries two identical methyl groups. For geometric isomers to exist, each carbon of the double bond must carry two different groups.

The E / Z naming system is a method of naming geometric isomers based on Cahn-Ingold-Prelog priority rules, used when the cis / trans system cannot be applied.

The atom with the higher atomic number attached directly to the C=C carbon is assigned the higher priority. If the first atoms are identical, the second atoms are compared, and so on.

In E / Z nomenclature, E stands for the German word entgegen, meaning opposite, and Z stands for zusammen, meaning together.

The cis / trans system fails when all four groups attached to the C=C carbons are different. In this case, CIP priority rules must be used to assign E or Z to each isomer.

True.

E comes from the German word "entgegen" meaning opposite. The two groups with the highest CIP priority on each carbon of the double bond are on opposite sides.

An electrophilic addition is a reaction in which an electrophile adds across the C=C double bond, breaking the double bond to form two new single bonds.

The C=C double bond contains four electrons and is an area of high electron density. This attracts electron-loving species (electrophiles) which accept the electrons to form a new bond.

The addition of steam (H₂O (g)) to an alkene produces an alcohol as the only product.

1. Hydrogen (H₂)

2. Steam (H₂O (g))

3. Hydrogen halide (HX)

4. Halogen

True.

In electrophilic addition, the C=C double bond breaks and a new single bond forms from each of the two carbon atoms to the added atoms.

Hydrogenation is the addition of hydrogen (H₂) across the C=C double bond of an alkene, converting it into an alkane.

In electrophilic addition, the electrophile accepts electrons from the C=C double bond.

The pale purple solution of KMnO₄ turns colourless. The alkene is oxidised and the product is a diol (two OH groups added across the double bond).

True.

Cold dilute acidified potassium manganate(VII) oxidises the alkene. Two OH groups are added across the double bond to give a diol.

When shaken with an alkene, the pale purple KMnO₄ solution turns colourless due to oxidation. With an alkane, no reaction occurs and the purple colour is retained.

Bromine water is an orange or yellow aqueous solution of bromine (Br₂(aq)) used to test for unsaturation in organic compounds.

The saturation test determines whether a compound is unsaturated (contains a C=C double bond) by observing whether it reacts with bromine water.

In the saturation test, the unknown compound is shaken with bromine water to test for the presence of a C=C double bond.

True.

Decolourisation of bromine water indicates that an addition reaction has taken place. This only occurs with unsaturated compounds that contain a C=C double bond.

The orange (or yellow) bromine water turns colourless. This indicates an addition reaction has occurred across the C=C double bond.

False.

Alkanes are saturated (no C=C double bond) and cannot undergo addition reactions. The bromine water remains orange when shaken with an alkane.

The C=C double bond of the unsaturated compound undergoes an addition reaction with bromine. The coloured Br₂ molecules are consumed, so the solution becomes colourless.

A carbocation is a highly reactive intermediate in which a carbon atom carries a positive charge due to the loss of an electron pair during heterolytic bond breaking.

Bromine has a greater electronegativity than hydrogen, so it pulls the bonding electrons closer to itself. This creates a partial negative charge (δ−) on Br and a partial positive charge (δ+) on H.

In the electrophilic addition of HBr to an alkene, the H-Br bond breaks heterolytically to form a Br^- ion.

Br^- acts as a nucleophile and donates a pair of electrons to the positively charged carbocation intermediate, forming a new C-Br bond to give the addition product.

False.

Br₂ is a non-polar molecule because both atoms have the same electronegativity. It only becomes polarised when the high electron density of the C=C double bond repels electrons in the Br-Br bond.

The high electron density of the C=C double bond repels the electrons in the Br-Br bond away from the nearer Br atom. This gives the closer Br atom a δ+ charge and the further Br atom a δ− charge.

Carbocation stability decreases in the order: tertiary > secondary > primary. A more substituted carbocation is stabilised by the electron-donating effect of alkyl groups.

When more than one carbocation intermediate is possible, the major product forms from the most stable carbocation.

True.

Addition of H to the terminal carbon gives a secondary carbocation at C-2, which is more stable than a primary carbocation at C-1. Br^- attacks C-2, giving 2-bromopropane as the major product.

The H-Br bond breaks heterolytically: both electrons in the bond go to one atom (Br), forming a Br^- ion and leaving the H as an electrophile.

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

In addition polymerisation, the pi bond in each monomer breaks and the monomers link together to form new C-C single bonds.

A monomer is a small reactive molecule containing a C=C double bond that can join with many others through addition polymerisation to form a polymer.

False.

Addition polymerisation gives only one product: the polymer. This is a defining feature of addition (as opposed to condensation) polymerisation.

A repeat unit is the smallest group of atoms that, when connected one after another, makes up the polymer chain. It is identical to the monomer except that the C=C double bond is replaced by a C-C single bond.

The repeat unit is the same as the monomer except that the C=C double bond is replaced by a C-C single bond. The repeat unit is enclosed in square brackets in the polymer formula.

The repeat unit of an addition polymer is represented by square brackets in its displayed or general formula.

Poly(chloroethene) is also known as PVC. Its monomer is chloroethene (CH₂=CHCl), which polymerises when its C=C bond opens to form repeating -CH₂-CHCl- units.

False.

The section inside the square brackets is the repeat unit, not the monomer. To obtain the monomer, replace the C-C single bond in the repeat unit with a C=C double bond.

Like all addition reactions, addition polymerisation gives only one product. The difference is that many thousands of monomers are added together to produce a high-molecular-mass polymer chain.

1. Landfill

2. Incineration / burning

3. Recycling

A biodegradable polymer is a polymer that can be broken down over time by microorganisms, producing carbon dioxide, water and other organic compounds as products.

Plastics are non-biodegradable, so microorganisms cannot break them down and they persist for hundreds of years. Landfill sites also take up valuable land that cannot be used for other purposes.

False.

Incineration produces the greenhouse gas carbon dioxide, contributing to climate change. Some plastics also release toxic fumes when burned, for example HCl from PVC.

When poly(vinylchloride) (PVC) is incinerated, it releases the toxic gas hydrogen chloride which must be scrubbed from the waste gases before emission.

The waste gases are scrubbed by reacting them with a base or carbonate. For example, CaO (s) reacts with HCl (g) to neutralise it, preventing the acidic gas from being released into the atmosphere.

True.

Polyesters and polyamides are condensation polymers that can be hydrolysed. This makes them biodegradable, which is a major advantage over polyalkenes produced from alkene monomers.

Recycling reduces the volume of plastic in landfill and avoids the toxic emissions of incineration. It also conserves crude oil reserves, as less energy and fewer raw materials are needed to recycle plastics than to manufacture new ones.

Compostable polymers are commonly plant-based and degrade naturally leaving no harmful residues.

A compostable polymer is a polymer that is typically plant-based and degrades naturally leaving no harmful residues, making it a sustainable alternative to conventional plastics.