A student is given two compounds: 'butane' and 'but-1-yne'. (i) Draw their structural formulae and identify the type of bond(s) present in each. (ii) Predict the key differences in their chemical reactivity. (iii) Name the homologous series to which each compound belongs and state one characteristic property of a homologous series.
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
(i) Structural formulae:
Butane (C₄H₁₀): CH₃—CH₂—CH₂—CH₃
Bond type: Only single bonds (C–C and C–H). It is a saturated compound.
But-1-yne (C₄H₆): CH≡C—CH₂—CH₃
Bond type: Contains one triple bond (C≡C) and single bonds. It is unsaturated.
(ii) Chemical reactivity:
- Butane undergoes substitution reactions (e.g., with chlorine in sunlight).
- But-1-yne undergoes addition reactions (e.g., with H₂, Cl₂, HCl) due to the triple bond. Unsaturated compounds are more reactive than saturated ones.
(iii) Homologous series:
- Butane belongs to alkanes; but-1-yne belongs to alkynes.
- Characteristic property: Successive members of a homologous series differ by a –CH₂– unit (molecular mass difference of 14 u), and members show a gradation in physical properties but similar chemical properties.
Source: Chapter 4, Section 4.2.4 – Homologous Series; Section 4.2.2 – Chains, Branches and Rings
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Explanation
- Examiners expect the structural formula drawn as a condensed/chain structure — full electron dot structure is not required here unless asked.
- The key contrast for reactivity is substitution (saturated) vs. addition (unsaturated) — these exact terms score marks.
- For homologous series property, state both the –CH₂– difference AND similar chemical properties; either alone may lose half a mark.
- "Alkanes" and "alkynes" are the required series names — do not write just "hydrocarbons."