Define a sigma (σ) bond .

A carboxylic acid is an organic compound containing the carboxyl group (–COOH). It is a weak acid that partially dissociates in water to give carboxylate ions and H + .

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 .

A condensation reaction is a reaction in which two molecules join together with the elimination of a small molecule — typically water or hydrogen chloride.

Define geometrical isomerism in alkenes.

A phenol is an organic compound in which a hydroxyl group (–OH) is bonded directly to a benzene ring. It is more acidic than an alcohol because the phenoxide ion formed is stabilised by delocalisation of the negative charge into the ring.

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.

An NMR spectrum is a spectrum that shows the chemical environment of atoms (typically 1 H or 13 C) in a molecule by measuring their absorption of radiofrequency radiation in a magnetic field. It provides information about molecular structure.

Define electrophilic addition .

An infrared spectrum is a graph showing the absorbance of infrared radiation by a compound at different wavenumbers. Specific functional groups absorb at characteristic wavenumbers, allowing identification of bonds present in the molecule.

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 ?

A colorimetric analysis is a technique that measures the concentration of a coloured solution by comparing its absorbance with that of solutions of known concentration, using a colorimeter or spectrophotometer.

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

A ligand substitution reaction is a reaction in which one ligand bonded to a central metal ion is replaced by a different ligand. The reaction can occur stepwise, with each ligand being replaced individually.

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 .

An optical rotation is the rotation of the plane of plane-polarised light by a chiral compound. A (+) or (–) sign indicates the direction (clockwise or anticlockwise) of rotation.

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.

A Lewis acid is a species that accepts a lone pair of electrons. Lewis acids include metal ions with empty d orbitals and molecules with an incomplete outer shell such as BF 3 .

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

A monodentate ligand is a ligand that donates one lone pair of electrons to the central metal ion via a single dative covalent bond. Examples include H 2 O, NH 3 , Cl - and CN - .

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?

A precipitation reaction is a reaction in which two aqueous solutions are mixed and an insoluble solid (precipitate) forms as a product.

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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Cards in this collection (41)

Define a sigma (σ) bond.
A carboxylic acid is an organic compound containing the carboxyl group (–COOH). It is a weak acid that partially dissociates in water to give carboxylate ions and H⁺.
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.
A condensation reaction is a reaction in which two molecules join together with the elimination of a small molecule — typically water or hydrogen chloride.
Define geometrical isomerism in alkenes.
A phenol is an organic compound in which a hydroxyl group (–OH) is bonded directly to a benzene ring. It is more acidic than an alcohol because the phenoxide ion formed is stabilised by delocalisation of the negative charge into the ring.
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.
An NMR spectrum is a spectrum that shows the chemical environment of atoms (typically ¹H or ¹³C) in a molecule by measuring their absorption of radiofrequency radiation in a magnetic field. It provides information about molecular structure.
Define electrophilic addition.
An infrared spectrum is a graph showing the absorbance of infrared radiation by a compound at different wavenumbers. Specific functional groups absorb at characteristic wavenumbers, allowing identification of bonds present in the molecule.
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?
A colorimetric analysis is a technique that measures the concentration of a coloured solution by comparing its absorbance with that of solutions of known concentration, using a colorimeter or spectrophotometer.
What is bromine water and how is it used to test for unsaturation?
A ligand substitution reaction is a reaction in which one ligand bonded to a central metal ion is replaced by a different ligand. The reaction can occur stepwise, with each ligand being replaced individually.
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.
An optical rotation is the rotation of the plane of plane-polarised light by a chiral compound. A (+) or (–) sign indicates the direction (clockwise or anticlockwise) of rotation.
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.
A Lewis acid is a species that accepts a lone pair of electrons. Lewis acids include metal ions with empty d orbitals and molecules with an incomplete outer shell such as BF₃.
Define non-biodegradable in the context of poly(alkenes).
A monodentate ligand is a ligand that donates one lone pair of electrons to the central metal ion via a single dative covalent bond. Examples include H₂O, NH₃, Cl^- and CN^-.
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?
A precipitation reaction is a reaction in which two aqueous solutions are mixed and an insoluble solid (precipitate) forms as a product.

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