Definition
Petroleum is a natural mixture of many different hydrocarbons; the first step in refining it is fractional distillation, which separates it into useful fractions.
Fractional distillation of petroleum is based on the different boiling points of the hydrocarbons it contains — the crude oil is heated, its vapours rise through a fractionating column that's hotter at the bottom and progressively cooler towards the top, and each fraction condenses back to liquid and is drawn off at the height where the column temperature matches its own boiling range.
Physical properties of petroleum fractions depend on four factors: the size (number of carbon atoms/molecular mass) of the hydrocarbon molecules, their boiling point, their viscosity, and their volatility (flammability) — all of which are closely linked to chain length.
As you move from the bottom to the top of the fractionating column: molecular mass decreases, boiling point decreases, viscosity decreases, and volatility/flammability increases — heavier fractions with larger molecules stay lower (higher boiling point), while lighter, smaller molecules rise higher before condensing.
Remember
Petroleum fractions with lower molecular mass (shorter carbon chains) have higher volatility and lower boiling points because their molecules have weaker inter-molecular (van der Waals) forces between them — smaller molecules have less surface area for these forces to act over, so less energy is needed to separate them into the gas phase.
Definition
Kerosene oil is one of the middle fractions, with a boiling range of roughly –C; it's used as a fuel for jet engines/aircraft, heating, and lighting.
The gasoline (petrol) fraction contains hydrocarbons with roughly to carbon atoms per molecule.
Fuel oil (a heavier fraction) is the petroleum fraction commonly used to generate electricity in power plants.
Definition
Cracking breaks down large, less-useful, high-boiling-point hydrocarbon molecules from heavier petroleum fractions into smaller, more useful, more in-demand molecules (like petrol and useful alkenes) using heat and/or a catalyst — e.g. decane can be cracked into octane and ethene: . This is essential because the natural supply of lighter, high-demand fractions (like petrol) from distillation alone doesn't meet demand, while heavier fractions are comparatively over-supplied.
Definition
Aliphatic hydrocarbons come mainly from petroleum (and natural gas). Aromatic hydrocarbons come mainly from coal tar (obtained from the destructive distillation of coal) — coal tar and petroleum together yield a wide range of important compounds, including benzene and its derivatives (from coal tar) and alkanes/alkenes/alkynes (from petroleum).
Definition
An alkene can be identified through simple chemical tests, since it decolourizes both bromine water and a cold, dilute solution of potassium permanganate (), while a saturated compound (alkane) does neither — this contrast is exactly how you'd distinguish a saturated compound in test tube A from an unsaturated alkene in test tube B: shake each with bromine water (or cold dilute ) and see which one decolourizes it.
Ethene is oxidized by cold aqueous : the purple solution decolourizes as it oxidizes the double bond, adding two groups across it to form a diol (ethane-1,2-diol) — in this reaction, ethene is oxidized, while (which is itself reduced) acts as the oxidizing agent, meaning ethene acts as the reducing agent.
Since ethyne also contains a multiple bond (, even more unsaturated than ethene's ), it too decolourizes cold solution.
Definition
An addition reaction is one where atoms are added across a double or triple bond, converting an unsaturated compound into a more saturated one, with no atoms lost overall — e.g. bromine adding across ethene's double bond, or hydrogen adding to an alkene (hydrogenation).
An elimination reaction is the reverse idea: atoms/small molecules are removed from a saturated (or less unsaturated) compound, creating a new double bond — e.g. , where is effectively eliminated to form ethene.
Dehydration is elimination of a water molecule (e.g. from an alcohol, to form an alkene). Dehydrohalogenation is elimination of a hydrogen halide molecule (e.g. , as above, from a haloalkane) — both are elimination reactions, differing only in what small molecule is removed.
Definition
Two examples of addition reactions: (1) bromine () adding across a double bond, e.g. propene 1,2-dibromopropane; (2) hydrogen () adding across a double bond (hydrogenation) to convert an alkene into the corresponding alkane.
Example 1 — bromine addition to propene
Find the product formed when propene reacts with bromine.
- Propene's double bond opens, and one bromine atom adds to each carbon that was part of the double bond.
Answer: 1,2-dibromopropane ()
Example 2 — identifying an elimination product
In , which molecule is eliminated?
- Compare what's lost from the haloalkane and combined versus what appears as by-products alongside the alkene.
- A hydrogen (from the haloalkane) and bromine combine as , effectively removed as the alkene forms — appearing here as separate from the reaction with .
Answer: Effectively is eliminated (as once is accounted for)
Definition
Chlorine and hydrogen chloride both add across ethene's double bond in simple addition reactions: (1,2-dichloroethane), and (chloroethane) — in each case, the two atoms of the small molecule attach one to each of the carbons that made up the double bond.
Example 3 — identifying a reaction type
Identify the type of reaction: .
- A hydrocarbon reacting completely with oxygen to give carbon dioxide and water, releasing energy.
Answer: Combustion (complete combustion, an exothermic oxidation reaction)
Definition
Alkanes, and hydrocarbons generally, are widely used as fuels because their combustion releases large amounts of useful energy; a substance not itself a hydrocarbon or easily combustible (or one already fully oxidized) would not serve as a fuel in the same way.
Definition
The general formula of alkynes is .
Uses of acetylene (ethyne): as fuel gas for the oxyacetylene flame used in welding and metal cutting, and as a starting material for manufacturing various organic chemicals and plastics.
Definition
The oxyacetylene flame is generated by burning acetylene (ethyne) gas with pure oxygen. It's extremely hot because acetylene's triple bond stores a large amount of chemical energy, and burning it with pure oxygen (rather than air, which is only about oxygen and dilutes/cools the flame with unreactive nitrogen) allows the fuel to combust rapidly and completely, releasing that energy very intensely, reaching temperatures high enough to melt and cut through metal.
Example 4 — IUPAC name of an alkyne
Give the IUPAC name of .
- Four carbon atoms with a triple bond starting at carbon .
Answer: But-1-yne
Quick Recap
- ●Fractional distillation: separates petroleum by boiling point; molecular mass/boiling point/viscosity , volatility going up the column
- ●Cracking: breaks large, less useful molecules into smaller, more in-demand ones (petrol, alkenes), using heat/catalyst
- ●Aliphatic hydrocarbons petroleum; aromatic hydrocarbons coal tar
- ●Alkenes decolourize bromine water and cold dilute KMnO (alkanes don't) — the standard test for unsaturation
- ●Addition: atoms add across a double/triple bond (more saturated product). Elimination: a small molecule (HO dehydration, HX dehydrohalogenation) is removed, forming a double bond
- ●Combustion: hydrocarbon O CO HO, releases energy — why hydrocarbons are used as fuels
- ●Alkynes: ; oxyacetylene flame (ethyne pure O) is extremely hot, used in welding/cutting metal
