Alkanes are a homologous series of saturated hydrocarbons with the general formula C n H 2n+2 , containing only C–C and C–H sigma bonds .

True or False? Each carbon atom in an alkane has a tetrahedral bond angle of 109.5°.

True. Each carbon in an alkane forms four sigma bonds with no lone pairs, giving equal bond pair–bond pair repulsion and a tetrahedral geometry of 109.5°.

Why are alkanes non-polar and unreactive towards ionic reagents?

Carbon and hydrogen have very similar electronegativities , so C–H bonds have very low polarity , meaning alkanes lack the electron-rich or electron-deficient sites needed to attract nucleophiles or electrophiles.

As the chain length of an alkane increases, the boiling point .......... because the .......... forces between molecules become stronger.

As the chain length of an alkane increases, the boiling point increases because the induced dipole–dipole (London dispersion) forces between molecules become stronger.

How does branching affect the boiling point of an alkane compared to its unbranched isomer?

Branched alkanes have lower boiling points because their compact shape reduces the surface area available for induced dipole–dipole interactions between molecules.

True or False? Alkanes exhibit permanent dipole–dipole forces as their only intermolecular force.

False. Alkanes are non-polar, so they only exhibit induced dipole–dipole (London dispersion) forces — they have no permanent dipoles.

Define complete combustion

A Complete combustion occurs when an alkane burns in excess oxygen , producing only carbon dioxide and water as products.

Give two reasons why alkanes have high bond enthalpies and are unreactive.

The C–C bond (346 kJ mol −1 ) and C–H bond (411 kJ mol −1 ) both have high bond enthalpies. The bonds have very low polarity due to the similar electronegativities of carbon and hydrogen.

True or False? Incomplete combustion of alkanes can produce carbon monoxide as a product.

True. When oxygen supply is limited, carbon is only partially oxidised , forming toxic carbon monoxide rather than carbon dioxide.

Carbon monoxide is toxic because it binds to .......... and prevents it from transporting .......... to organs.

Carbon monoxide is toxic because it binds to haemoglobin and prevents it from transporting oxygen to organs.

Why do alkanes not react with polar reagents such as nucleophiles?

Alkanes are non-polar molecules with no electron-deficient areas to attract nucleophiles and no electron-rich areas to attract electrophiles.

True or False? The C–H bond in an alkane is stronger than the C–C bond.

True. The C–H bond enthalpy (411 kJ mol −1 ) is greater than the C–C bond enthalpy (346 kJ mol −1 ) because the shorter bond length creates a greater force of attraction.

A free radical is a species with an unpaired electron , represented by a dot (e.g. Cl•), making it highly reactive.

What condition is required for free radical substitution of alkanes to occur?

Ultraviolet (UV) light is required to provide energy for the homolytic fission of the halogen–halogen bond.

List the three stages of free radical substitution in order: .......... .......... ..........

Initiation — UV light causes homolytic fission of Cl 2 , forming two Cl• radicals Propagation — chain reaction in which Cl• attacks CH 4 to regenerate radicals Termination — two radicals combine to form a single, stable molecule

True or False? In the initiation step, the Cl–Cl bond undergoes heterolytic fission.

False. The Cl–Cl bond undergoes homolytic fission , where each atom receives one electron from the shared pair, forming two Cl• free radicals .

Write the two propagation steps for the chlorination of methane.

Step 1: Cl• + CH 4 → HCl + CH 3 • Step 2: CH 3 • + Cl 2 → CH 3 Cl + Cl•

True or False? Free radical substitution is a reliable method for making a single pure halogenoalkane product.

False. A mixture of products is formed because further substitution and different termination reactions occur, making the reaction low in selectivity .

Give two reasons why free radical substitution of alkanes has poor selectivity.

Further substitution can replace additional hydrogen atoms to give di- and tri-substituted products. Multiple termination products form when any two radicals combine.

A LevelChemistryOCRFlashcards4. Core Organic ChemistryAlkanes

Alkanes (OCR A Level Chemistry A): Flashcards

Exam code: H432

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FrontIntroduction to Alkanes
What are alkanes?

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Cards in this collection (19)

What are alkanes?
Alkanes are a homologous series of saturated hydrocarbons with the general formula C_nH_2n+2, containing only C–C and C–H sigma bonds.
True or False?
Each carbon atom in an alkane has a tetrahedral bond angle of 109.5°.
True.
Each carbon in an alkane forms four sigma bonds with no lone pairs, giving equal bond pair–bond pair repulsion and a tetrahedral geometry of 109.5°.
Why are alkanes non-polar and unreactive towards ionic reagents?
Carbon and hydrogen have very similar electronegativities, so C–H bonds have very low polarity, meaning alkanes lack the electron-rich or electron-deficient sites needed to attract nucleophiles or electrophiles.
As the chain length of an alkane increases, the boiling point .......... because the .......... forces between molecules become stronger.
As the chain length of an alkane increases, the boiling point increases because the induced dipole–dipole (London dispersion) forces between molecules become stronger.
How does branching affect the boiling point of an alkane compared to its unbranched isomer?
Branched alkanes have lower boiling points because their compact shape reduces the surface area available for induced dipole–dipole interactions between molecules.
True or False?
Alkanes exhibit permanent dipole–dipole forces as their only intermolecular force.
False.
Alkanes are non-polar, so they only exhibit induced dipole–dipole (London dispersion) forces — they have no permanent dipoles.
Define complete combustion
A Complete combustion occurs when an alkane burns in excess oxygen, producing only carbon dioxide and water as products.
Give two reasons why alkanes have high bond enthalpies and are unreactive.
1. The C–C bond (346 kJ mol⁻¹) and C–H bond (411 kJ mol⁻¹) both have high bond enthalpies.
2. The bonds have very low polarity due to the similar electronegativities of carbon and hydrogen.
True or False?
Incomplete combustion of alkanes can produce carbon monoxide as a product.
True.
When oxygen supply is limited, carbon is only partially oxidised, forming toxic carbon monoxide rather than carbon dioxide.
Carbon monoxide is toxic because it binds to .......... and prevents it from transporting .......... to organs.
Carbon monoxide is toxic because it binds to haemoglobin and prevents it from transporting oxygen to organs.
Why do alkanes not react with polar reagents such as nucleophiles?
Alkanes are non-polar molecules with no electron-deficient areas to attract nucleophiles and no electron-rich areas to attract electrophiles.
True or False?
The C–H bond in an alkane is stronger than the C–C bond.
True.
The C–H bond enthalpy (411 kJ mol⁻¹) is greater than the C–C bond enthalpy (346 kJ mol⁻¹) because the shorter bond length creates a greater force of attraction.
Define free radical
A free radical is a species with an unpaired electron, represented by a dot (e.g. Cl•), making it highly reactive.
What condition is required for free radical substitution of alkanes to occur?
Ultraviolet (UV) light is required to provide energy for the homolytic fission of the halogen–halogen bond.
List the three stages of free radical substitution in order:
1. ..........
2. ..........
3. ..........
1. Initiation — UV light causes homolytic fission of Cl₂, forming two Cl• radicals
2. Propagation — chain reaction in which Cl• attacks CH₄ to regenerate radicals
3. Termination — two radicals combine to form a single, stable molecule
True or False?
In the initiation step, the Cl–Cl bond undergoes heterolytic fission.
False.
The Cl–Cl bond undergoes homolytic fission, where each atom receives one electron from the shared pair, forming two Cl• free radicals.
Write the two propagation steps for the chlorination of methane.
Step 1: Cl• + CH₄ → HCl + CH₃•
Step 2: CH₃• + Cl₂ → CH₃Cl + Cl•
True or False?
Free radical substitution is a reliable method for making a single pure halogenoalkane product.
False.
A mixture of products is formed because further substitution and different termination reactions occur, making the reaction low in selectivity.
Give two reasons why free radical substitution of alkanes has poor selectivity.
1. Further substitution can replace additional hydrogen atoms to give di- and tri-substituted products.
2. Multiple termination products form when any two radicals combine.

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