Define a sigma (σ) bond .

Sigma (σ) bond is a covalent bond formed by the end-to-end (axial) overlap of atomic orbitals along the line joining two nuclei. The electron density is symmetrical about the bond axis.

In a C=C double bond, there is one .......... bond and one .......... bond formed by sideways overlap of p orbitals.

In a C=C double bond, there is one sigma bond and one pi bond formed by sideways overlap of p orbitals.

True or False? The three regions of electron density around each carbon in ethene give a trigonal planar shape with bond angles of 120°.

True. Each carbon in ethene forms three sigma bonds, whose electron pairs repel equally, giving a trigonal planar arrangement with 120° bond angles.

Where are the two regions of electron density in a pi (π) bond located?

Above and below the plane of the sigma bond, formed by the sideways overlap of parallel p orbitals on adjacent carbon atoms.

In ethene, each carbon uses .......... of its four outer electrons to form sigma bonds, leaving one electron in a .......... orbital perpendicular to the plane of the molecule.

In ethene, each carbon uses three of its four outer electrons to form sigma bonds, leaving one electron in a p orbital perpendicular to the plane of the molecule.

True or False? The greater the orbital overlap in a covalent bond, the weaker the bond formed.

False. Greater orbital overlap produces a stronger bond. More overlap increases electron density between nuclei, strengthening the electrostatic attraction.

Define a pi (π) bond .

Pi (π) bond is a covalent bond formed by the sideways overlap of parallel p orbitals on adjacent atoms. The electron density lies above and below the plane of the sigma bond and the bond cannot rotate freely.

Define geometrical isomerism in alkenes.

Geometrical isomerism is a type of stereoisomerism arising from restricted rotation about a C=C double bond. Different groups attached to each carbon are fixed in space, giving rise to E and Z (or cis and trans) isomers.

True or False? Free rotation about a C=C double bond is possible because the pi bond is above and below the molecular plane.

False. The pi bond restricts rotation about C=C. The sideways overlap of p orbitals would be broken if rotation occurred, so attached groups remain fixed in their positions.

A tertiary carbocation is more stable than a primary carbocation because alkyl groups have an .......... effect, donating electron density to reduce the .......... charge.

A tertiary carbocation is more stable than a primary carbocation because alkyl groups have an inductive effect, donating electron density to reduce the positive charge.

State Markovnikov's rule.

In the addition of a hydrogen halide (HX) to an alkene, the halogen bonds to the more substituted carbon atom (the one with more alkyl groups attached).

The stability order of carbocations is: .......... > secondary > .......... .

The stability order of carbocations is: tertiary > secondary > primary .

True or False? Geometrical isomerism can occur in cyclic compounds as well as in alkenes.

True. Cyclic compounds have restricted rotation about the C–C single bonds within the ring, so substituents remain fixed above or below the ring, giving rise to geometrical isomers.

Define the inductive effect of alkyl groups on carbocations.

Inductive effect is the electron-donating ability of alkyl groups attached to a positively charged carbon. Alkyl groups push electron density towards the carbocation, spreading the charge and stabilising it.

Define electrophilic addition .

Electrophilic addition is a reaction in which an electrophile adds across a C=C double bond. The pi bond is broken and new sigma bonds form with each carbon atom, giving a single saturated product.

True or False? When Br 2 approaches an alkene, the bromine molecule becomes polarised because the double bond repels electrons in the Br–Br bond.

True. The high electron density of the C=C repels electrons in the Br–Br bond, making the closer Br atom δ+ and the further one δ−. The δ+ Br acts as the electrophile.

In the electrophilic addition of HBr to ethene, the H–Br bond breaks .......... forming a Br⁻ ion and a reactive .......... intermediate.

In the electrophilic addition of HBr to ethene, the H–Br bond breaks heterolytically forming a Br⁻ ion and a reactive carbocation intermediate.

What three reagents can undergo electrophilic addition with alkenes?

Hydrogen halide (HX) Halogen (e.g. Br 2 ) Concentrated sulfuric acid (H 2 SO 4 )

Alkenes react with concentrated H 2 SO 4 in the cold. The δ+ hydrogen of H 2 SO 4 acts as an .......... and adds across the double bond, forming an .......... intermediate.

Alkenes react with concentrated H 2 SO 4 in the cold. The δ+ hydrogen of H 2 SO 4 acts as an electrophile and adds across the double bond, forming a carbocation intermediate.

True or False? Alkenes are less reactive than alkanes towards electrophiles.

False. Alkenes are more reactive than alkanes because the C=C double bond has high electron density, making it susceptible to electrophilic attack. Alkanes lack this reactive pi electron density.

What is a carbocation ?

Carbocation is a positively charged carbon species with only three covalent bonds. It forms as a reactive intermediate during electrophilic addition when the electrophile accepts electrons from the C=C bond.

What is bromine water and how is it used to test for unsaturation?

Bromine water is an orange or yellow aqueous solution of Br 2 used to test for unsaturation and certain functional groups. It is decolourised by alkenes (addition across the C=C double bond) and by aldehydes (oxidation of the aldehyde group), but remains orange with alkanes, ketones, and benzene under standard conditions.

True or False? A saturated compound will decolourise bromine water when shaken with it.

False. Saturated compounds do not contain a C=C double bond and cannot undergo addition with bromine. The orange colour of bromine water remains unchanged.

When an unsaturated compound is shaken with bromine water, the orange colour .......... because an .......... reaction takes place across the C=C bond.

When an unsaturated compound is shaken with bromine water, the orange colour disappears because an addition reaction takes place across the C=C bond.

What colour change is observed when an alkene reacts with bromine water?

The orange or yellow colour of bromine water disappears (decolourises) as the bromine adds across the double bond to form a colourless dibromo compound.

True or False? The decolourisation of bromine water by an alkene is an example of a substitution reaction.

False. The reaction is an addition reaction. Bromine adds across the C=C double bond, breaking it and forming new C–Br bonds without any atoms being eliminated.

The term .......... means a molecule contains at least one C=C double bond, which can be detected using .......... water.

The term unsaturated means a molecule contains at least one C=C double bond, which can be detected using bromine water.

Define addition polymerisation .

Addition polymerisation is a reaction in which many monomer molecules containing at least one C=C double bond join together to form a long-chain polymer. The pi bond breaks and new C–C single bonds form between monomers. No other product is formed.

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

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

True or False? The repeat unit of an addition polymer is identical to the monomer, including the C=C double bond.

False. The repeat unit is the same as the monomer except the C=C double bond is replaced by a C–C single bond. The repeat unit is shown inside square brackets in the structural formula.

What is a plasticiser and how does it affect PVC?

A substance added to PVC that prevents polymer chains from packing closely together. This weakens the van der Waals forces between chains, making the polymer more flexible.

Poly(chloroethene), also known as .......... (PVC), is made from the monomer .......... .

Poly(chloroethene), also known as polyvinyl chloride (PVC), is made from the monomer chloroethene .

True or False? Addition polymerisation produces two products: the polymer and a small molecule such as water.

False. Addition polymerisation produces only one product, the polymer. No small molecule is eliminated. This distinguishes it from condensation polymerisation.

Define monomer in the context of polymerisation.

Monomer is a small reactive molecule containing at least one C=C double bond that can join with many other monomers to form a polymer. The repeat unit of the polymer is derived from the monomer.

Define non-biodegradable in the context of poly(alkenes).

Non-biodegradable is a material that cannot be broken down by biological agents such as enzymes. Poly(alkenes) are non-biodegradable because their long, unreactive C–C chains resist chemical attack.

True or False? Poly(alkenes) are easily broken down in landfill sites because they are made of long-chain molecules.

False. Poly(alkenes) are unreactive and non-biodegradable. They resist chemical and biological attack, so they can persist in landfill sites for hundreds of years.

In feedstock recycling, polymers are heated until the .......... bonds break to reform .......... which can be used to make new plastics.

In feedstock recycling, polymers are heated until the polymer bonds break to reform monomers which can be used to make new plastics.

What are two problems with burning poly(alkenes) as a disposal method?

Harmful combustion products are released, causing pollution. Burning only transfers the pollution problem rather than solving it.

In mechanical recycling, plastics are separated, washed and ground into .......... then heated until they .......... and remoulded.

In mechanical recycling, plastics are separated, washed and ground into pellets then heated until they melt and remoulded.

True or False? Poly(propene) can be mechanically recycled an unlimited number of times without any change to its properties.

False. Each time poly(propene) is heated, some polymer chains break, degrading its properties over successive cycles of recycling.

What is feedstock recycling of polymers?

Feedstock recycling is a recycling process in which polymers are heated to high temperatures to break the polymer bonds and regenerate monomers, which are then used to produce new plastics.

A LevelChemistryAQAFlashcardsOrganic ChemistryAlkenes

Alkenes (AQA A Level Chemistry): Flashcards

Exam code: 7405

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FrontStructure, Bonding & Reactivity
Define a sigma (σ) bond.

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Define a sigma (σ) bond.
Sigma (σ) bond is a covalent bond formed by the end-to-end (axial) overlap of atomic orbitals along the line joining two nuclei. The electron density is symmetrical about the bond axis.
In a C=C double bond, there is one .......... bond and one .......... bond formed by sideways overlap of p orbitals.
In a C=C double bond, there is one sigma bond and one pi bond formed by sideways overlap of p orbitals.
True or False?
The three regions of electron density around each carbon in ethene give a trigonal planar shape with bond angles of 120°.
True.
Each carbon in ethene forms three sigma bonds, whose electron pairs repel equally, giving a trigonal planar arrangement with 120° bond angles.
Where are the two regions of electron density in a pi (π) bond located?
Above and below the plane of the sigma bond, formed by the sideways overlap of parallel p orbitals on adjacent carbon atoms.
In ethene, each carbon uses .......... of its four outer electrons to form sigma bonds, leaving one electron in a .......... orbital perpendicular to the plane of the molecule.
In ethene, each carbon uses three of its four outer electrons to form sigma bonds, leaving one electron in a p orbital perpendicular to the plane of the molecule.
True or False?
The greater the orbital overlap in a covalent bond, the weaker the bond formed.
False.
Greater orbital overlap produces a stronger bond. More overlap increases electron density between nuclei, strengthening the electrostatic attraction.
Define a pi (π) bond.
Pi (π) bond is a covalent bond formed by the sideways overlap of parallel p orbitals on adjacent atoms. The electron density lies above and below the plane of the sigma bond and the bond cannot rotate freely.
Define geometrical isomerism in alkenes.
Geometrical isomerism is a type of stereoisomerism arising from restricted rotation about a C=C double bond. Different groups attached to each carbon are fixed in space, giving rise to E and Z (or cis and trans) isomers.
True or False?
Free rotation about a C=C double bond is possible because the pi bond is above and below the molecular plane.
False.
The pi bond restricts rotation about C=C. The sideways overlap of p orbitals would be broken if rotation occurred, so attached groups remain fixed in their positions.
A tertiary carbocation is more stable than a primary carbocation because alkyl groups have an .......... effect, donating electron density to reduce the .......... charge.
A tertiary carbocation is more stable than a primary carbocation because alkyl groups have an inductive effect, donating electron density to reduce the positive charge.
State Markovnikov's rule.
In the addition of a hydrogen halide (HX) to an alkene, the halogen bonds to the more substituted carbon atom (the one with more alkyl groups attached).
The stability order of carbocations is: .......... > secondary > .......... .
The stability order of carbocations is: tertiary > secondary > primary.
True or False?
Geometrical isomerism can occur in cyclic compounds as well as in alkenes.
True.
Cyclic compounds have restricted rotation about the C–C single bonds within the ring, so substituents remain fixed above or below the ring, giving rise to geometrical isomers.
Define the inductive effect of alkyl groups on carbocations.
Inductive effect is the electron-donating ability of alkyl groups attached to a positively charged carbon. Alkyl groups push electron density towards the carbocation, spreading the charge and stabilising it.
Define electrophilic addition.
Electrophilic addition is a reaction in which an electrophile adds across a C=C double bond. The pi bond is broken and new sigma bonds form with each carbon atom, giving a single saturated product.
True or False?
When Br₂ approaches an alkene, the bromine molecule becomes polarised because the double bond repels electrons in the Br–Br bond.
True.
The high electron density of the C=C repels electrons in the Br–Br bond, making the closer Br atom δ+ and the further one δ−. The δ+ Br acts as the electrophile.
In the electrophilic addition of HBr to ethene, the H–Br bond breaks .......... forming a Br⁻ ion and a reactive .......... intermediate.
In the electrophilic addition of HBr to ethene, the H–Br bond breaks heterolytically forming a Br⁻ ion and a reactive carbocation intermediate.
What three reagents can undergo electrophilic addition with alkenes?
1. Hydrogen halide (HX)
2. Halogen (e.g. Br₂)
3. Concentrated sulfuric acid (H₂SO₄)
Alkenes react with concentrated H₂SO₄ in the cold. The δ+ hydrogen of H₂SO₄ acts as an .......... and adds across the double bond, forming an .......... intermediate.
Alkenes react with concentrated H₂SO₄ in the cold. The δ+ hydrogen of H₂SO₄ acts as an electrophile and adds across the double bond, forming a carbocation intermediate.
True or False?
Alkenes are less reactive than alkanes towards electrophiles.
False.
Alkenes are more reactive than alkanes because the C=C double bond has high electron density, making it susceptible to electrophilic attack. Alkanes lack this reactive pi electron density.
What is a carbocation?
Carbocation is a positively charged carbon species with only three covalent bonds. It forms as a reactive intermediate during electrophilic addition when the electrophile accepts electrons from the C=C bond.
What is bromine water and how is it used to test for unsaturation?
Bromine water is an orange or yellow aqueous solution of Br₂ used to test for unsaturation and certain functional groups.
It is decolourised by alkenes (addition across the C=C double bond) and by aldehydes (oxidation of the aldehyde group), but remains orange with alkanes, ketones, and benzene under standard conditions.
True or False?
A saturated compound will decolourise bromine water when shaken with it.
False.
Saturated compounds do not contain a C=C double bond and cannot undergo addition with bromine. The orange colour of bromine water remains unchanged.
When an unsaturated compound is shaken with bromine water, the orange colour .......... because an .......... reaction takes place across the C=C bond.
When an unsaturated compound is shaken with bromine water, the orange colour disappears because an addition reaction takes place across the C=C bond.
What colour change is observed when an alkene reacts with bromine water?
The orange or yellow colour of bromine water disappears (decolourises) as the bromine adds across the double bond to form a colourless dibromo compound.
True or False?
The decolourisation of bromine water by an alkene is an example of a substitution reaction.
False.
The reaction is an addition reaction. Bromine adds across the C=C double bond, breaking it and forming new C–Br bonds without any atoms being eliminated.
The term .......... means a molecule contains at least one C=C double bond, which can be detected using .......... water.
The term unsaturated means a molecule contains at least one C=C double bond, which can be detected using bromine water.
Define addition polymerisation.
Addition polymerisation is a reaction in which many monomer molecules containing at least one C=C double bond join together to form a long-chain polymer. The pi bond breaks and new C–C single bonds form between monomers. No other product is formed.
In addition polymerisation, the .......... bond in each C=C breaks and monomers link together to form new .......... single bonds.
In addition polymerisation, the pi bond in each C=C breaks and monomers link together to form new C–C single bonds.
True or False?
The repeat unit of an addition polymer is identical to the monomer, including the C=C double bond.
False.
The repeat unit is the same as the monomer except the C=C double bond is replaced by a C–C single bond. The repeat unit is shown inside square brackets in the structural formula.
What is a plasticiser and how does it affect PVC?
A substance added to PVC that prevents polymer chains from packing closely together. This weakens the van der Waals forces between chains, making the polymer more flexible.
Poly(chloroethene), also known as .......... (PVC), is made from the monomer ...........
Poly(chloroethene), also known as polyvinyl chloride (PVC), is made from the monomer chloroethene.
True or False?
Addition polymerisation produces two products: the polymer and a small molecule such as water.
False.
Addition polymerisation produces only one product, the polymer. No small molecule is eliminated. This distinguishes it from condensation polymerisation.
Define monomer in the context of polymerisation.
Monomer is a small reactive molecule containing at least one C=C double bond that can join with many other monomers to form a polymer. The repeat unit of the polymer is derived from the monomer.
Define non-biodegradable in the context of poly(alkenes).
Non-biodegradable is a material that cannot be broken down by biological agents such as enzymes. Poly(alkenes) are non-biodegradable because their long, unreactive C–C chains resist chemical attack.
True or False?
Poly(alkenes) are easily broken down in landfill sites because they are made of long-chain molecules.
False.
Poly(alkenes) are unreactive and non-biodegradable. They resist chemical and biological attack, so they can persist in landfill sites for hundreds of years.
In feedstock recycling, polymers are heated until the .......... bonds break to reform .......... which can be used to make new plastics.
In feedstock recycling, polymers are heated until the polymer bonds break to reform monomers which can be used to make new plastics.
What are two problems with burning poly(alkenes) as a disposal method?
1. Harmful combustion products are released, causing pollution.
2. Burning only transfers the pollution problem rather than solving it.
In mechanical recycling, plastics are separated, washed and ground into .......... then heated until they .......... and remoulded.
In mechanical recycling, plastics are separated, washed and ground into pellets then heated until they melt and remoulded.
True or False?
Poly(propene) can be mechanically recycled an unlimited number of times without any change to its properties.
False.
Each time poly(propene) is heated, some polymer chains break, degrading its properties over successive cycles of recycling.
What is feedstock recycling of polymers?
Feedstock recycling is a recycling process in which polymers are heated to high temperatures to break the polymer bonds and regenerate monomers, which are then used to produce new plastics.

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