What Organic Chemistry Is

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.1 — A hydrocarbon as a compound of hydrogen and carbon only
• 4.2 — Representing organic molecules by empirical, molecular, general, structural and displayed formulae
• 4.3 — The terms homologous series, functional group and isomerism

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Around one hundred and twenty elements are known, and one of them has more compounds than all the rest put together. That element is carbon, and ORGANIC CHEMISTRY is the study of its compounds. The name is a historical accident: chemists once believed such substances could only be made by living things, and the word stuck long after that belief was abandoned.

The narrowest and most examined term comes first. A HYDROCARBON is a compound containing hydrogen and carbon ONLY — nothing else, not one oxygen atom, not one chlorine. Methane, CH₄, is a hydrocarbon. Ethanol, C₂H₅OH, is an organic compound but NOT a hydrocarbon, because of that oxygen. Marks are lost on this every year, and the fix is a single word: only.

Why carbon, and not any other element

1. Carbon has four electrons in its outer shell, so it forms FOUR covalent bonds — always four, in every stable compound it makes.
2. Carbon also bonds strongly TO OTHER CARBON ATOMS, which most elements do not do at any useful length.
3. So carbon atoms can be joined into chains of two, ten, or several thousand, and into branches and rings as well.
4. Every carbon in such a chain still has bonds left over, and those spare bonds are filled by hydrogen or by something else.
5. The result is that there is no limit in principle to the number of different carbon compounds, which is why the subject needs a filing system rather than a list.

Think of it like a train where the engine decides what it does

Picture a goods train. Every wagon is identical, and adding another wagon makes the train longer without changing what sort of train it is. What decides what the train is FOR is the engine at the front. Organic families work exactly this way: each extra CH₂ unit is another identical wagon, lengthening the molecule and slowly raising its boiling point, while the reactive group at the end — the engine — decides what the molecule actually does in a reaction. Two molecules with the same engine and different numbers of wagons behave chemically alike. Two molecules with different engines do not, however similar their length.

That reactive group is called the FUNCTIONAL GROUP, and a family of compounds sharing one is called a HOMOLOGOUS SERIES. The members of a homologous series differ from each other by CH₂, share one general formula, react in the same ways, and show a gradual change in physical properties such as boiling point as the chain gets longer.

One molecule, five formulae — butane

Butane is a four-carbon alkane. Write its empirical, molecular, general, structural and displayed formula, and say what each one is for.

1. MOLECULAR formula: count every atom in one molecule. Butane has four carbons and ten hydrogens, so C₄H₁₀. This is the one used in equations, because equations balance atoms.
2. EMPIRICAL formula: the simplest whole-number ratio of the atoms. Four to ten divides by two, giving C₂H₅. This is what an analysis of the compound gives you before you know its size.
3. GENERAL formula: the pattern for the whole family, written with n for the number of carbons. Alkanes are CₙH₂ₙ₊₂, and putting n = 4 gives 2×4+2 = 10 hydrogens, agreeing with C₄H₁₀.
4. STRUCTURAL formula: the arrangement written on one line, CH₃CH₂CH₂CH₃. It shows what is joined to what without drawing anything.
5. DISPLAYED formula: every atom and every bond drawn as a line — four carbons in a row, each bond shown, hydrogens attached to fill the four bonds on each carbon. This is the one an exam asks you to draw.

Answer: C₂H₅ · C₄H₁₀ · CₙH₂ₙ₊₂ · CH₃CH₂CH₂CH₃ · and the drawn structure. Five descriptions of one substance, at five levels of detail.

Why a molecular formula is not enough — isomerism

1. Take the molecular formula C₄H₁₀ and try to build it. Four carbons can be joined in a straight row, with hydrogens filling the spare bonds.
2. But they can also be joined as a row of THREE carbons with the fourth hanging off the middle one — and this also uses exactly four carbons and ten hydrogens.
3. Both arrangements obey every rule: four bonds on every carbon, one on every hydrogen. Both are real substances that can be bottled.
4. They are not the same substance. The straight one, butane, boils at about −1 °C; the branched one, methylpropane, boils at about −12 °C.
5. Compounds with the SAME molecular formula and DIFFERENT structural formulae are called ISOMERS, and the phenomenon is ISOMERISM.
6. So a molecular formula counts the atoms but cannot identify the compound. Only the arrangement can.

This is the deepest idea on the page, and it was discovered by accident. In the 1820s Wöhler and Liebig were studying two substances that behaved completely differently — one of them detonates — and found on analysis that they contained exactly the same atoms in exactly the same proportions. That result forced chemistry to accept that the properties of a substance depend on how its atoms are ARRANGED, not merely on which atoms are present. It is why organic chemistry is drawn rather than merely written down.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The four words the specification names

Hydrocarbon

A compound containing hydrogen and carbon ONLY.

Homologous series

A family of compounds with the same general formula and the same functional group, whose members differ by CH₂.

Functional group

The atom or group of atoms that gives a homologous series its characteristic chemical reactions.

General formula

The formula for a whole homologous series, written in terms of n, the number of carbon atoms.

Learn this definition · Isomers

Compounds with the same molecular formula but different structural formulae. They are different substances with different physical properties.

The five formulae, all describing butane

The four features of a homologous series — learn all four

• Same GENERAL FORMULA, so any member can be worked out from n
• Successive members differ by CH₂
• Same FUNCTIONAL GROUP, so they have similar chemical reactions
• A GRADUAL change in physical properties, such as a rising boiling point, as the chain lengthens

Getting a molecular formula from an empirical formula

1. Work out the relative formula mass of the EMPIRICAL formula.
2. Divide the compound's relative molecular mass by that number.
3. Multiply every subscript in the empirical formula by the whole number you obtain.
4. Check: the atoms in your answer should add up to the given relative molecular mass.

Empirical against molecular formula

A. EMPIRICAL: the simplest ratio only. Ethene, propene and butene all give CH₂ — which is not itself a molecule of anything.

B. MOLECULAR: the real count of atoms in one molecule. Ethene is C₂H₄ and butene is C₄H₈, and these are different substances.

Model answer [3 marks]

Explain why two different substances can both have the molecular formula C₄H₁₀. [3]

A molecular formula states only how many atoms of each element are present in one molecule, and the same atoms can be joined together in more than one arrangement. The four carbon atoms can form an unbranched chain, giving butane, or a chain of three carbons with the fourth attached to the middle one, giving methylpropane. Both use four carbons and ten hydrogens and both obey the rule that carbon forms four bonds, so both are real compounds. They are isomers: same molecular formula, different structural formulae, and therefore different physical properties such as boiling point.

Model answer [3 marks]

State what is meant by a homologous series, and give two features of one. [3]

A homologous series is a family of organic compounds that share the same functional group and the same general formula. Successive members differ from one another by CH₂. Because they share a functional group they undergo similar chemical reactions, and because the chain lengthens one carbon at a time their physical properties, such as boiling point, change gradually and predictably down the series.

Not this: "Organic" means the compound came from a living thing, or that it is natural.

This: It means a compound of carbon, wherever it came from. Urea is made by the kidney and was also made in a flask in 1828 from materials that had never lived. Plastics and medicines are organic compounds and none of them is natural.

Mark-losing trap. Hydrocarbon means hydrogen and carbon ONLY. Ethanol contains oxygen, so it is organic but not a hydrocarbon.

Mark-losing trap. Isomers share a MOLECULAR formula and differ in their STRUCTURAL formula. If both formulae match, it is one substance written twice.

Mark-losing trap. An empirical formula need not be a real molecule. CH₂ is the empirical formula of every alkene and is not a substance.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — Which one of these compounds is a hydrocarbon?
2. Grade 7 · Deduce [2 marks] — Hexane has the molecular formula C₆H₁₄. Give its empirical formula.
3. Grade 8 · Explain [3 marks] — Members of the same homologous series have similar chemical reactions but different boiling points. Explain why.
4. Grade 9 · Explain [4 marks] — Select every statement that belongs in a full explanation of why C₄H₁₀ can represent two different substances.
5. 9+ · Determine [5 marks] — A hydrocarbon has the empirical formula CH₂ and a relative molecular mass of 70. Determine its molecular formula. (Aᵣ: H = 1, C = 12)

The people behind this science

Two ways into the same idea — the one who supplied the two rules that generate every compound on this page, and the one who found by accident that the same atoms can make two different substances. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

August Kekulé — the one who supplied the two rules that generate every compound on this page

Everything this lesson organises rests on two claims Kekulé published in 1857 and 1858: that carbon is tetravalent, forming four bonds and never three or five, and that carbon atoms bond to one another so they can build chains and rings of any length. Without those two sentences there is no reason for homologous series to exist, no explanation of why adding CH₂ gives another real compound, and no way to draw an isomer at all. He went to university to be an architect, and he thought about molecules the way an architect thinks about a building.

• “Why does carbon form exactly four bonds and never three?”
• “What changes about chemistry once carbon can bond to carbon?”
• “How did you decide a molecule had a shape rather than just a composition?”
• “Why can a formula never identify a compound on its own?”
• “What did studying architecture teach you about molecules?”

Friedrich Wöhler — the one who found by accident that the same atoms can make two different substances

Isomerism, the last term this page defines, was not deduced from theory — it was forced on chemistry by a measurement nobody wanted. Wöhler and Liebig analysed silver cyanate and silver fulminate, found precisely the same atoms in precisely the same proportions, and could not deny that the two substances were nothing alike. Wöhler is also the man who made urea from materials that had never been alive, which is why the word organic no longer means what it once did. He is the right person to ask what it feels like when an analysis refuses to agree with the doctrine.

• “What did you find when you analysed the fulminate and the cyanate?”
• “How did chemists react to two substances with one formula?”
• “What were you actually trying to make when you got urea?”
• “Did making urea really overturn the idea of a vital force?”
• “Why should the arrangement of atoms matter so much?”

Carry on through the course

• Previous lesson: Dynamic Equilibrium
• Next lesson: Naming, Drawing And Classifying
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• Cracking — Breaking long molecules nobody wants into short ones everybody does — and the reason a refinery has to.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification