Chapter 9: Hydrocarbons

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Chapter Overview & SLOs

What are hydrocarbons? Hydrocarbons are organic compounds composed entirely of carbon and hydrogen atoms. They are classified into alkanes, alkenes, and alkynes based on the type of carbon-carbon bonds.

What is the difference between saturated and unsaturated hydrocarbons? Alkanes are saturated hydrocarbons with single C-C bonds (general formula CₙH₂ₙ₊₂). Alkenes and alkynes are unsaturated because they contain double (C=C) or triple (C≡C) bonds respectively. Alkenes have general formula CₙH₂ₙ, alkynes have CₙH₂ₙ₋₂.

How do we describe the bonding in alkenes? Alkenes contain a carbon-carbon double bond consisting of one sigma (σ) bond from the overlap of 2s subshells and one pi (π) bond from the overlap of 2p subshells. The π bond prevents rotation around the double bond.

Why is the cracking of large alkanes necessary? Cracking converts heavier, less valuable fractions of crude oil into lighter, high-demand products like gasoline, diesel, and jet fuel. Catalytic cracking uses a zeolite catalyst at moderate temperatures (around 450°C) to produce a higher yield of C₅ to C₁₀ gasoline molecules.

What is the principle behind fractional distillation of petroleum? The process uses a temperature gradient where crude oil vapors rise through a fractionating column. Heavier hydrocarbons with high boiling points condense at the bottom, while lighter ones with low boiling points condense near the top. Boiling point decreases from bottom to top of the column.

How do we distinguish between saturated and unsaturated hydrocarbons?

| Test | Reagent | Saturated (Alkane) | Unsaturated (Alkene/Alkyne) | |------|---------|-------------------|---------------------------| | Baeyer's Test | Dilute alkaline KMnO₄ | No change (purple remains) | Purple color discharged (colorless) | | Bromine water test | Bromine water (orange-brown) | No change (orange-brown remains) | Orange-brown decolorized (colorless) |

Unsaturated hydrocarbons undergo addition reactions across multiple bonds, while saturated alkanes undergo substitution reactions.

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we describe the bonding in alkenes? Alkenes contain a carbon-carbon double bond consisting of one sigma ($\sigma$) bond from the overlap of 2s subshells and one pi ($\pi$) bond from the overlap of 2p subshells.
• How do we distinguish between saturated and unsaturated hydrocarbons? Bromine water is added; it remains orange-brown with saturated alkanes but is decolorized to colorless by unsaturated hydrocarbons through an addition reaction across the multiple bonds. Similarly, Baeyer's test with dilute alkaline $KMnO_4$ discharges the pink color indicating unsaturation.
• What is the principle behind the fractional distillation of petroleum? The process uses a temperature gradient where crude oil vapors rise through a column; heavier hydrocarbons with high boiling points condense at the bottom, while lighter ones with low boiling points condense near the top.
• How does catalytic cracking improve refinery efficiency? It breaks long chains at moderate temperatures (around 450°C) using a zeolite catalyst (aluminum and silicon-based) to produce a higher yield of $C_5$ to $C_{10}$ gasoline molecules.

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