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QLDChemistryQuick questions
Unit 4: Structure, synthesis and design
Quick questions on Reactions of alkanes and alkenes (QCE Chemistry Unit 4)
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is reactions of alkanes?Show answer
Alkanes react with halogens (Cl2, Br2) in the presence of ultraviolet (UV) light or at high temperature. A hydrogen on the alkane is replaced by a halogen atom.
What is reactions of alkenes?Show answer
The C=C double bond consists of a sigma bond plus a pi bond. The pi bond is weaker and electron-rich, attracting electrophiles. Addition reactions break the pi bond and form two new single bonds (one to each carbon of the former C=C).
What is summary table?Show answer
For alkanes, the corresponding "Br2" reaction differs sharply: requires UV light, proceeds by substitution (not addition), produces HX as a by-product, and is non-selective. This is the cleanest discrimination test between alkanes and alkenes in QCAA stimulus.
What is common traps?Show answer
Writing alkane bromination as addition. Methane + Br2 does NOT give CH2Br2 by addition. Methane has no double bond. Always check saturation before choosing a mechanism.
What is markovnikov's rule?Show answer
When HX adds to an unsymmetrical alkene, the H attaches to the carbon already carrying more hydrogens; X attaches to the carbon with fewer hydrogens (the more substituted carbon).
What is writing alkane bromination as addition?Show answer
Methane + Br2 does NOT give CH2Br2 by addition. Methane has no double bond. Always check saturation before choosing a mechanism.
What is forgetting UV for alkane halogenation?Show answer
No UV, no reaction at room temperature. QCAA marks for the condition explicitly.
What is misapplying Markovnikov?Show answer
The rule applies to unsymmetrical alkenes only and to addition of HX or H2O (not Br2 or Cl2, since both ends of the reagent are identical).
What is treating "addition" as identical to "substitution"?Show answer
Addition preserves all atoms (no by-product); substitution releases HX (or similar). Atom counts diagnose which reaction occurred.
What is confusing catalytic hydrogenation with reduction by NaBH4 or LiAlH4?Show answer
H2 with Ni/Pt reduces C=C bonds but not C=O. Hydride reagents reduce C=O but not isolated C=C. QCAA Unit 4 uses catalytic hydrogenation only.