9701 · 14.1
Alkanes flashcards
Revision flashcards for Cambridge 9701 Alkanes (syllabus 14.1). Flip, recall, then mark a real past-paper question.
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What is an alkane?
A saturated hydrocarbon containing only single C-C and C-H bonds.
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What is the general formula for an alkane?
$C_n H_{2n+2}$, where n is an integer.
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What does 'saturated' mean in the context of hydrocarbons?
The molecule contains only single carbon-carbon bonds, meaning it has the maximum possible number of hydrogen atoms for its carbon skeleton.
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What is a $\sigma$ (sigma) bond?
A type of covalent bond formed by the direct, head-on overlap of atomic orbitals. All bonds in an alkane are sigma bonds.
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What is the geometry and bond angle around a carbon atom in an alkane?
Tetrahedral, with a bond angle of 109.5°. This is due to sp³ hybridisation.
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What is free-radical substitution?
A reaction mechanism in which an atom in a molecule (e.g., H in an alkane) is replaced by a free radical. It involves initiation, propagation, and termination steps.
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What is homolytic fission?
The breaking of a covalent bond where each of the two atoms involved gets one of the shared electrons, forming two free radicals. This happens in the initiation step.
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What is a free radical?
A species with an unpaired electron, making it highly reactive. For example, a chlorine radical, $Cl\cdot$.
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What are the three stages of a free-radical chain reaction?
1. Initiation: Radicals are formed (e.g., using UV light). 2. Propagation: A radical reacts to form a new molecule and another radical. 3. Termination: Two radicals combine to form a stable molecule.
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What are the products of incomplete combustion of an alkane?
Carbon monoxide (CO) and/or solid carbon (soot, C), along with water ($H_2O$). This occurs in a limited supply of oxygen.
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What is cracking?
The process of breaking down large, less useful hydrocarbon molecules into smaller, more useful ones, such as shorter-chain alkanes and alkenes.
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Why is UV light required for the chlorination of methane?
UV light provides the energy needed for the homolytic fission of the $Cl-Cl$ bond in the initiation step, creating chlorine radicals.