Draw two different possible structures of a saturated hydrocarbon having four carbon atoms in its molecule. What are these two structures of the hydrocarbon having same molecular formula called ? Write the molecular formula and the common name of this compound. Also write the molecular formula of its alkyne.
Generated by claude-sonnet-4-6 · 2026-06-15 06:48 · grounding rag
Model Answer
Two structures of saturated hydrocarbon with 4 carbon atoms:
Structure 1 (n-Butane – straight chain):
$$\text{H}_3\text{C} - \text{CH}_2 - \text{CH}_2 - \text{CH}_3$$
Structure 2 (Iso-butane – branched chain):
$$\begin{array}{c} \text{CH}_3 \\ | \\ \text{CH}_3 - \text{CH} - \text{CH}_3 \end{array}$$
These two structures with the same molecular formula but different arrangements are called structural isomers.
Molecular formula: $\text{C}_4\text{H}_{10}$
Common name: Butane
Molecular formula of its alkyne: $\text{C}_4\text{H}_6$ (But-1-yne / Butyne)
Source: Carbon and its Compounds, Section 4.2.2
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Explanation
- Examiners expect both structures drawn clearly (straight and branched chain) — sketching the carbon skeleton is enough.
- The term structural isomers is the key term; don't write "isomers" alone.
- Alkyne formula: for alkane $\text{C}_n\text{H}_{2n+2}$, alkyne is $\text{C}_n\text{H}_{2n-2}$, so for $n=4$: $\text{C}_4\text{H}_6$.
- Award-split: 1 mark for structures, 1 mark for "structural isomers" + molecular formula + common name, 1 mark for alkyne formula.