Chemistry-only: recognise the first four alkenes
| English | 中文 | Pinyin |
|---|---|---|
| alkene/ˈælkiːn/ | 烯烃 | xī tīng |
| unsaturated/ʌnˈsætʃəreɪtɪd/ | 不饱和的 | bù bǎo hé de |
What would explain this observation?
- Propane and propene both have three carbon atoms, but propene has two fewer hydrogen atoms. Its carbon–carbon double bond accounts for the difference in the displayed structure.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Alkenes 烯烃 are hydrocarbons containing a carbon–carbon double bond. The functional group C=C helps determine their characteristic reactions. The homologous series with one double bond in an open chain has general formula CnH2n. Its first four members are ethene C₂H₄, propene C₃H₆, butene C₄H₈ and pentene C₅H₁₀. They are unsaturated 不饱和的: each has two fewer hydrogens than the alkane with the same carbon number.
- alkene: An unsaturated hydrocarbon containing a carbon–carbon double bond; unsaturated: Containing a carbon–carbon multiple bond that can undergo addition in the stated hydrocarbon model.
Which displayed feature identifies the alkene functional group?
A displayed formula shows all atoms and all bond lines. Count a double bond as two of carbon’s four bonds. In the terminal-double-bond structures shown, the first carbon has two hydrogens and the next has one, except ethene where each has two. A longer chain finishes with CH₃. Butene and pentene can have different atom arrangements; the diagrams show one stated straight-chain arrangement, rather than claiming their molecular formula uniquely fixes a structure.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- A displayed formula shows all atoms and all bond lines. Count a double bond as two of carbon’s four bonds. In the terminal-double-bond structures shown, the first carbon has two hydrogens and the next has one, except ethene where each has two. A longer chain finishes with CH₃. Butene and pentene can have different atom arrangements; the diagrams show one stated straight-chain arrangement, rather than claiming their molecular formula uniquely fixes a structure.
- Build the four illustrated molecules using a model kit, then translate each into a displayed drawing and molecular formula. Check hydrogen valency one and carbon valency four. Explain what a flat drawing reveals about connectivity and what it omits about the real three-dimensional molecule. Recognise a supplied longer member from its double bond and formula without requiring a specific name beyond the four stated members.
Which two habits make the investigation or model in this case more defensible?
Build the four illustrated molecules using a model kit, then translate each into a displayed drawing and molecular formula. Check hydrogen valency one and carbon valency four. Explain what a flat drawing reveals about connectivity and what it omits about the real three-dimensional molecule. Recognise a supplied longer member from its double bond and formula without requiring a specific name beyond the four stated members.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: propane has C₃H₈ while propene has C₃H₆. Hydrogen difference=8−6=2 atoms. For a supplied open-chain member with six carbon atoms and one C=C, H=2n=2×6=12. Its corresponding alkane would have H=2n+2=14. These relations count composition; they do not locate the double bond by themselves.
A supplied open-chain alkene has nine carbon atoms and one C=C. Find its hydrogen count. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A supplied open-chain alkene has nine carbon atoms and one C=C. Find its hydrogen count.
The result is 18 atoms. Known: propane has C₃H₈ while propene has C₃H₆. Hydrogen difference=8−6=2 atoms. For a supplied open-chain member with six carbon atoms and one C=C, H=2n=2×6=12. Its corresponding alkane would have H=2n+2=14. These relations count composition; they do not locate the double bond by themselves.
Check the conclusion and its limits
- Do not draw an extra hydrogen on a carbon already making four bonds, or identify every CnH2n formula as an alkene without checking structure: other families can share a molecular formula. Unsaturated does not mean the sample contains no hydrogen. This is separate Chemistry content on both tiers, not a Higher-only extension.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
An alkene and its corresponding alkane have the same hydrogen count. This claim is false: Do not draw an extra hydrogen on a carbon already making four bonds, or identify every CnH2n formula as an alkene without checking structure: other families can share a molecular formula. Unsaturated does not mean the sample contains no hydrogen. This is separate Chemistry content on both tiers, not a Higher-only extension.
Chemistry-only: recognise the first four alkenes: A displayed formula shows all atoms and all bond lines. Count a double bond as two of carbon’s four bonds. In the terminal-double-bond structures shown, the first carbon has two hydrogens and the next has one, except ethene where each has two. A longer chain finishes with CH₃. Butene and pentene can have different atom arrangements; the diagrams show one stated straight-chain arrangement, rather than claiming their molecular formula uniquely fixes a structure.
An alkene and its corresponding alkane have the same hydrogen count.
Do not draw an extra hydrogen on a carbon already making four bonds, or identify every CnH2n formula as an alkene without checking structure: other families can share a molecular formula. Unsaturated does not mean the sample contains no hydrogen. This is separate Chemistry content on both tiers, not a Higher-only extension.
An unsaturated hydrocarbon containing a carbon–carbon double bond: write the technical term.
alkene means An unsaturated hydrocarbon containing a carbon–carbon double bond.