CDS Chemistry · Carbon and Its Compounds
Hydrocarbons and Fuels
The general formulas of alkanes, alkenes and alkynes, benzene's structure, why some fuels burn with a sooty flame, and what octane and cetane numbers measure.
Why this matters
Six CDS questions, three of them from the 2024 and 2025 papers. The fuel-rating questions (octane, cetane) are the newest and the hardest; the rest test one formula or one flame.
Concept 1 of 3: Alkanes, alkenes, alkynes and benzene
Definition
The series:
- Alkanes (saturated, single bonds only): CₙH₂ₙ₊₂ — methane CH₄, ethane C₂H₆.
- Alkenes (one double bond): CₙH₂ₙ — ethene C₂H₄.
- Alkynes (one triple bond): CₙH₂ₙ₋₂ — ethyne C₂H₂.
- Name stems give the number of carbons: meth 1, eth 2, prop 3, but 4, pent 5, hex 6, … hexadec 16.
- Benzene C₆H₆ is a flat ring: each carbon makes sigma bonds to two carbons at 120°, the pi electrons are delocalised above and below the ring, and every C–C bond is the same length, between single and double. Benzene itself has no isomers.
General formulas
Worked example
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 1 · Carbon and Its Compounds · Hydrocarbons and Fuels
CₙH₂ₙ₊₁ is a group, not a compound
Benzene has no isomers of its own
Concept 2 of 3: Clean and sooty flames
Definition
The rule:
- Saturated hydrocarbons burn with a clean blue flame when air is enough.
- Unsaturated hydrocarbons (and aromatics such as naphthalene) burn with a yellow, sooty flame.
- The soot is unburnt carbon: the cause is incomplete combustion, because the carbon-to-hydrogen ratio is high.
| Fuel | Flame | Why |
|---|---|---|
| Saturated (methane, LPG) | Blue, clean | Burns completely |
| Unsaturated hydrocarbons | Yellow, sooty | Incomplete combustionQ |
| Naphthalene (aromatic) | Yellow, sooty | High carbon to hydrogen ratio, so incomplete combustionQ CDS 2023 (I): the reason is incomplete combustion. 'The carbon to hydrogen ratio is low' states the reverse. |
Practice this concept3 quick reps
The same idea in a real exam question:
Example 2 · Carbon and Its Compounds · Hydrocarbons and Fuels
Sooty means carbon-rich, not carbon-poor
Concept 3 of 3: Octane and cetane numbers
Definition
The two ratings:
- Octane number (petrol): resistance to knocking. Iso-octane = 100, n-heptane = 0.
- Among straight-chain alkanes the octane number falls as the chain gets longer. Branching and rings raise it.
- So butane > cyclohexane > pentane > hexane: the ring puts cyclohexane above the five- and six-carbon chains, but still below butane.
- Cetane number (diesel): ease of ignition. Cetane is n-hexadecane (C₁₆H₃₄), rated 100.
Practice this concept4 quick reps
The same idea in a real exam question:
Example 3 · Carbon and Its Compounds · Hydrocarbons and Fuels
Longer chain, lower octane
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Alkanes, alkenes, alkynes and benzene
General formulas
Reference tables (2)
Clean and sooty flames3 rows
| Fuel | Flame | Why |
|---|---|---|
| Saturated (methane, LPG) | Blue, clean | Burns completely |
| Unsaturated hydrocarbons | Yellow, sooty | Incomplete combustionQ |
| Naphthalene (aromatic) | Yellow, sooty | High carbon to hydrogen ratio, so incomplete combustionQ CDS 2023 (I): the reason is incomplete combustion. 'The carbon to hydrogen ratio is low' states the reverse. |
Watch out for (4)
- CₙH₂ₙ₊₁ is a group, not a compound→ Alkanes, alkenes, alkynes and benzene
- Benzene has no isomers of its own→ Alkanes, alkenes, alkynes and benzene
- Sooty means carbon-rich, not carbon-poor→ Clean and sooty flames
- Longer chain, lower octane→ Octane and cetane numbers
Test yourself on Carbon and Its Compounds
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.