The gas in a kitchen cylinder, the petrol in a car, the white soft paraffin in an ointment and the plastic of a blister pack all start as the same thing: hydrocarbons from crude oil.
This lab covers how hydrocarbons are built, how to name and draw them by the IUPAC rules, how alkanes react, and how industry separates and converts crude oil into useful products.
Written for Pharmaceutical Chemistry II, Diploma in Pharmacy. Use Next to move through the pages, or Contents to jump around.
A hydrocarbon is a compound made of carbon and hydrogen atoms only. Carbon forms four bonds, so carbon atoms can link into chains, branches and rings. Hydrocarbons are sorted by the bonds they contain:
"Saturated" means the carbons hold the most hydrogen possible. Everything that is not aromatic is called aliphatic.
General formulas are for compounds with one ring or one multiple bond.
A carbon's hybridisation depends on how many atoms it bonds to, and it fixes the angles between those bonds. Tap each card.
A σ (sigma) bond sits directly between the two carbon nuclei. It is strong and hard to reach. Every single bond is a σ bond.
A π (pi) bond forms from side-on overlap of p orbitals, so its electrons sit above and below the bond, exposed to attack. It is weaker than a σ bond.
Alkanes have only strong σ bonds, so they react only under harsh conditions (heat, UV light, a flame). Alkenes and alkynes have exposed π electrons, so reagents add across the double or triple bond easily. You will see both on the reaction pages.
Each alkane has one more CH2 unit than the one before, and all fit CnH2n+2. Slide to explore. Phase is shown at 0 °C (the textbook standard) and at 30 °C, a typical Malaysian lab.
Data: OpenStax Chemistry 2e; C11–C20 standard reference values, rounded.
Alkanes are non-polar: carbon and hydrogen have similar electronegativities. Their molecules attract each other only by weak London dispersion forces, which grow with the size and surface area of the molecule.
So boiling point climbs with every carbon added. At 0 °C, C1–C4 are gases, C5–C13 are liquids, and C14 upwards are solids.
Structural (constitutional) isomers have the same molecular formula but their atoms are joined differently. Butane, C4H10, has two: a straight chain and a branched chain.
The number of possible isomers explodes as chains grow: C10H22 has 75 and C18H38 has 60,523. A molecular formula alone cannot tell you which compound you have. That is why we need structures and systematic names.
The three isomers of C5H12 have the same mass, yet boil at very different temperatures. A straight chain is long and can lie alongside its neighbours over a large surface. A branched molecule is more compact, like a ball, so it touches its neighbours less and the dispersion forces are weaker.
The same happens with C4H10: butane boils at −0.5 °C, branched 2-methylpropane at −11.7 °C.
Isomers can behave differently even with the same formula. In drugs, a change in shape can change how a molecule fits its target or how it is metabolised, which is why medicines are identified by full structure, not formula.
Alkanes do not dissolve in water and are less dense than it, so they float. They are also chemically quiet. Both properties make purified alkane mixtures, called paraffins, common pharmaceutical materials. Tap each card.
When the two ends of an alkane chain bond to each other, you get a cycloalkane. Closing the ring removes two hydrogens, so the formula becomes CnH2n, the same as an alkene. Name it by adding cyclo- to the alkane name. Tap each ring.
In skeletal drawings, each corner is a carbon and each carbon carries enough hydrogens to make four bonds.
We draw cyclohexane as a flat hexagon, but the real molecule is a puckered chair, which keeps every C–C–C angle near 109.5°.
Saturated rings form the framework of many drug molecules. The steroid skeleton, four fused rings, is shared by cholesterol, hydrocortisone, dexamethasone and the sex hormones. The ring core holds the functional groups in a fixed 3D arrangement that the body's receptors recognise.
An alkene contains at least one carbon–carbon double bond, C=C. With one double bond, the formula is CnH2n: two hydrogens fewer than the matching alkane.
The two double-bonded carbons and the four atoms attached to them all lie flat, at 120°. The simplest alkene, ethene (ethylene), is the most produced organic chemical in the world: it is the raw material for polyethylene.
Names end in -ene: ethene, propene, butene, and so on.
Rotating about a C=C would break the π bond, so the groups on each carbon are locked in place. When each carbon carries two different groups, two geometric isomers exist: cis (same side) and trans (opposite sides). They are different compounds: cis-but-2-ene boils at about 4 °C, trans-but-2-ene at about 1 °C.
Tretinoin (all-trans retinoic acid) and isotretinoin (13-cis retinoic acid) differ only in the geometry of one double bond. They are separate medicines with different uses and doses, used for acne by different routes.
A cycloalkene has a double bond inside a ring. The ring removes two hydrogens and the double bond removes two more, so the formula is CnH2n−2.
When naming, the two double-bond carbons are always numbered 1 and 2, so the simplest ones need no number: cyclopentene, cyclohexene.
Cycloalkene rings appear in plant chemistry: limonene, the fragrant oil in citrus peel used to flavour and scent medicines, is built on a cyclohexene ring.
Compare the number of H with 2n + 2. Each ring or double bond removes 2 H. Answer all three to continue.
The International Union of Pure and Applied Chemistry (IUPAC) sets the rules so that every structure has one systematic name, and every name can be drawn as only one structure. A name has three parts:
| C | Root | C | Root |
|---|---|---|---|
| 1 | meth- | 6 | hex- |
| 2 | eth- | 7 | hept- |
| 3 | prop- | 8 | oct- |
| 4 | but- | 9 | non- |
| 5 | pent- | 10 | dec- |
Memory aid for 1 to 4: Monkeys Eat Peeled Bananas. From 5 onwards the roots follow Greek and Latin numbers.
The way a structure is drawn can hide its longest chain. Count carbons along every path from one end to another, turning at branches.
Number from both ends and write down each set of locants. Compare them one term at a time; the set with the lower number at the first difference is correct. The sum of the numbers does not matter.
Branches are listed alphabetically, not by position. When both ends give the same locants, the branch that comes first alphabetically gets the lower number.
Each structure has been given a wrong name. Tap to see the true parent chain (green) and the correct name.
Quick check: a methyl on carbon 1, or an ethyl on carbon 2, always means you have not found the longest chain.
Answer all four to continue. After each answer, the parent chain turns green and is numbered.
Draw the parent chain first, number it, then hang the branches on. Answer all three to continue.
Remove one H from an alkane and you get an alkyl group (CnH2n+1, written R–): methyl –CH3, ethyl –CH2CH3, propyl, and the branched isopropyl –CH(CH3)2 and tert-butyl –C(CH3)3.
A carbon carrying a halogen or –OH is ranked by how many other carbons it is bonded to:
| Type | C attached | Haloalkane example | Alcohol example |
|---|---|---|---|
| Primary (1°) | 1 | 1-chloropropane | propan-1-ol |
| Secondary (2°) | 2 | 2-chloropropane | propan-2-ol (isopropyl alcohol) |
| Tertiary (3°) | 3 | 2-chloro-2-methylpropane | 2-methylpropan-2-ol |
For alkenes, the parent chain must contain the C=C, and the chain is numbered so the double bond gets the lowest possible number. The number of its first carbon goes just before -ene.
You will also see the older style "1-butene", "2-butene". Both mean the same compound; current IUPAC rules put the number next to -ene.
The old name for alkanes, paraffins, comes from Latin parum affinis, "little affinity". At room temperature, alkanes ignore acids, bases and most oxidising agents.
The reason: C–C and C–H bonds are strong σ bonds, and they are non-polar, so there is no partly positive or partly negative site for a reagent to attack. Breaking these bonds needs a lot of energy.
Alkanes have two important reactions, both needing energy to start:
With plenty of oxygen, an alkane burns completely to carbon dioxide and water, releasing a lot of heat. That heat is why alkanes are our main fuels. Pick an alkane and see the balanced equation.
General rule for complete combustion: CnH2n+2 + (3n+1)/2 O2 → n CO2 + (n+1) H2O. When the oxygen number is a fraction, double everything.
When oxygen is limited, carbon is only partly oxidised. The products include carbon monoxide (CO) and soot (carbon), as well as water. For pentane:
CO has no colour or smell. It binds to haemoglobin far more strongly than oxygen does, so the blood carries less oxygen. Early symptoms (headache, dizziness, nausea) are easily mistaken for flu.
In UV light, an alkane swaps a hydrogen for a halogen: CH4 + Cl2 → CH3Cl + HCl. This is a substitution reaction, and it runs through free radicals: atoms with an unpaired electron, shown as a dot. Open all three stages.
UV light splits the Cl–Cl bond evenly (homolytic fission): each chlorine atom keeps one electron.
Each step uses one radical and makes another. The Cl• made in the second step starts the cycle again, thousands of times.
Two radicals pair their electrons into a bond. No new radical is made, so the chain stops.
Tip: count the radicals on each side. Answer all three to continue.
The chain reaction does not stop at CH3Cl: radicals attack the product too. With excess chlorine, methane gives a mixture:
Dichloromethane and chloroform are useful solvents in drug manufacture, and chloroform was an early anaesthetic. But they are toxic, so traces left in a medicine are strictly limited. Carbon tetrachloride is a Class 1 solvent to avoid. (ICH Q3C(R5))
Alkenes undergo addition: the π bond opens and two new atoms attach, one to each carbon. No UV light is needed.
This gives a simple test. Orange bromine water turns colourless with an alkene but stays orange with an alkane (in the dark).
Hydrogenation (H2, nickel catalyst) turns liquid vegetable oils into solid fats; hydrogenated vegetable oil and hydrogenated castor oil are used as tablet lubricants. Hydration (steam, H3PO4 catalyst, about 300 °C and 60 atm) turns ethene into ethanol.
Crude oil is a mixture of hundreds of hydrocarbons, mostly alkanes and cycloalkanes. A refinery first heats it to about 400 °C, turning most of it to vapour, and feeds it into the bottom of a tall fractionating column.
The column is hot at the bottom and cool at the top. As vapour rises through trays, each compound condenses when it reaches a tray cooler than its boiling point. Small molecules with low boiling points rise to the top; large molecules condense near the bottom. Each tray collects a fraction: a mixture with a range of boiling points.
Tap each fraction. Carbon ranges are approximate; they overlap and vary between refineries.
Think about boiling point: larger molecules, stronger dispersion forces, higher boiling point. Answer both to continue.
Distillation produces more heavy fractions than people buy, and less petrol and ethene than industry needs. Cracking fixes this: long alkanes are heated, often over a catalyst (a zeolite at about 500 °C), so C–C bonds break. The products are a shorter alkane and an alkene.
In addition polymerisation, thousands of alkene molecules open their double bonds and link into one long chain: n CH2=CH2 → –[CH2–CH2]n–. The recycling code on a plastic tells you which polymer it is. Tap each code.
Packaging is part of a medicine's quality: it protects the product from moisture and light until use.
Petrol must burn smoothly, not explode early ("knock") in the engine. Its octane rating compares it with two reference alkanes: straight-chain heptane scores 0, and a highly branched octane, "isooctane", scores 100. RON95 petrol resists knocking like a 95 : 5 mixture of isooctane and heptane. Branched alkanes burn more smoothly, which is one reason refineries rearrange chains.
Made by Aufthority. Learning material for Pharmaceutical Chemistry II, Diploma in Pharmacy. Structures are drawn and named by the page itself from the IUPAC rules.
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