Reactions of Alkanes
Alkanes are relatively unreactive because C-C and C-H bonds are strong and essentially non-polar. Their key organic reaction in this section is radical substitution with halogens, which occurs through a chain mechanism involving initiation, propagation and termination steps.
Revision Notes
5 revision notes, in the order the specification teaches them.
Free Radical Substitution
Revise the overall reaction type, why radicals are formed and how halogenation replaces a hydrogen atom in an alkane.
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Initiation
Learn how ultraviolet light causes homolytic fission of a halogen molecule to form two reactive halogen radicals.
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Propagation
Review the two repeating steps that produce the organic product and regenerate a radical so the chain reaction continues.
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Termination
Revise how two radicals combine to form a stable molecule, removing radicals and stopping that part of the chain.
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Further Substitution
Learn how substitution can continue to form mixtures of products when more hydrogen atoms are replaced by halogen atoms.
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Alkanes Map
OCR A Level Chemistry A · 4.1.2 Alkanes
Every reaction on one A4 page: reagents, conditions and reaction type for each arrow, colour coded, with the key tests.
Alkanes Summary Map
OCR A Level Chemistry A · 4.1.2 Alkanes
The whole section on one A4 page: σ-bonds and free rotation, the tetrahedral carbon, boiling points, low reactivity, combustion and carbon monoxide, with radical substitution and its limits in a table.
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4.1.2
- a i
alkanes as saturated hydrocarbons containing single C–C and C–H bonds as σ-bonds (overlap of orbitals directly between the bonding atoms)
Revise - b i
explanation of the tetrahedral shape around each carbon atom in alkanes in terms of electron pair repulsion (see also 2.2.2 g–h)
Revise - b ii
explanation of the bond angle around each carbon atom in alkanes in terms of electron pair repulsion (see also 2.2.2 g–h)
Revise - c i
explanation of the variations in boiling points of alkanes with different carbon-chain length, in terms of induced dipole–dipole interactions (London forces) (see also 2.2.2 k)
Revise - c ii
explanation of the variations in boiling points of alkanes with different branching, in terms of induced dipole–dipole interactions (London forces) (see also 2.2.2 k)
Revise - d i
the low reactivity of alkanes with many reagents in terms of the high bond enthalpy of the σ-bonds present (see also 2.2.2 j)
Revise - d ii
the low reactivity of alkanes with many reagents in terms of the very low polarity of the σ-bonds present (see also 2.2.2 j)
Revise - f i
the reaction of alkanes with chlorine and bromine by radical substitution using ultraviolet radiation
Revise - f ii
the mechanism of radical substitution involving homolytic fission and radical reactions in terms of initiation (see also 4.1.1 f–g)
Revise - f iii
the mechanism of radical substitution involving homolytic fission and radical reactions in terms of propagation (see also 4.1.1 f–g)
Revise - f iv
the mechanism of radical substitution involving homolytic fission and radical reactions in terms of termination (see also 4.1.1 f–g)
Revise - g i
the limitations of radical substitution in synthesis by the formation of a mixture of organic products, in terms of further substitution
Revise - g ii
the limitations of radical substitution in synthesis by the formation of a mixture of organic products, in terms of reactions at different positions in a carbon chain
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