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IDHow to identify each familyThe 60-second method for any structure
In an exam question, look for the functional group first. It's the part of the molecule that tells you which family it belongs to.
C–C onlyending -aneC=Cending -eneC≡Cending -ynebenzene ringcontains C₆H₅–C–X (F, Cl, Br, I)prefix chloro-, bromo-…–OHending -olThe 60-second method
- Is there a ring? Then it's aromatic.
- Is there a halogen (F, Cl, Br, I)? Then it's a haloalkane.
- Count the oxygens. 0 means alkane, alkene or alkyne. 1 means alcohol, aldehyde or ketone. 2 means carboxylic acid or ester.
- One oxygen on the end carbon with an H beside it is an aldehyde. One in the middle of the chain is a ketone.
CₙH₂ₙ₊₂Naming rulesCarbon prefixes and the five-step method
Every organic name has a root (how many carbons) and a suffix (which family).
| C | Root | Alkane | Alkene | Alkyne | Alcohol |
|---|---|---|---|---|---|
| 1 | meth- | methane | — | — | methanol |
| 2 | eth- | ethane | ethene | ethyne | ethanol |
| 3 | prop- | propane | propene | propyne | propanol |
| 4 | but- | butane | butene | butyne | butanol |
| 5 | pent- | pentane | pentene | pentyne | pentanol |
| 6 | hex- | hexane | hexene | hexyne | hexanol |
Naming any compound
- Find the longest continuous carbon chain.
- Identify the functional group.
- Number the chain from the end nearest the functional group.
- Name each substituent with its position number.
- List substituents alphabetically before the parent name.
C–CAlkanesSaturated hydrocarbons
A saturated hydrocarbon with only C–C single bonds. General formula CₙH₂ₙ₊₂.
- Boiling point rises with molecular mass because the London dispersion forces get stronger.
- Branched alkanes boil lower than their straight-chain isomers.
- Non-polar, so they don't dissolve in water but do dissolve in non-polar solvents.
- Low reactivity because of strong sigma bonds and low polarity.
C=CAlkenesUnsaturated, with one double bond
An unsaturated hydrocarbon with one C=C double bond. General formula CₙH₂ₙ. The smallest is ethene.
Cis-trans isomerism
A C=C bond can't rotate. When each carbon of the double bond has two different groups, you get cis (matching groups on the same side) and trans (matching groups on opposite sides).
Reactivity
Alkenes are more reactive than alkanes. The electron-rich double bond attracts electrophiles, so alkenes undergo addition reactions.
C≡CAlkynesUnsaturated, with one triple bond
An unsaturated hydrocarbon with one C≡C triple bond. General formula CₙH₂ₙ₋₂. The smallest is ethyne.
- The two carbons and the atoms joined to them are linear (180°).
- Very reactive and undergo addition reactions, like alkenes.
- Insoluble in water, soluble in non-polar solvents.
- Ethyne is the fuel in the oxy-acetylene welding torch.
C₆H₆Aromatic compoundsBenzene and its delocalised electrons
Aromatic hydrocarbons contain a benzene ring, C₆H₆. Its six pi electrons are delocalised, which means they're spread evenly around the ring.
- Unexpectedly stable. Chemists expected three double bonds to make it very reactive, but it isn't.
- Equal bond lengths. Every C–C bond is between a single and a double bond in length.
- Electron clouds sit above and below the plane of the ring.
C–XHaloalkanesAn alkane with a halogen swapped in
One or more H atoms of an alkane replaced by a halogen (F, Cl, Br, I).
- Higher boiling points than the matching alkanes, because the C–X bond is polar.
- Insoluble in water, soluble in non-polar solvents.
- The polar C–X bond makes them open to attack by nucleophiles.
Naming: 2-bromopropane, 1-chlorobutane, 2,2-dichloropropane.
Uses: fire extinguishers, refrigerants (CFCs, now banned), solvents and pharmaceuticals.
–OHAlcoholsPrimary, secondary, tertiary
General formula CₙH₂ₙ₊₁OH. Functional group: –OH (hydroxyl).
| Type | The C–OH carbon is bonded to | Example |
|---|---|---|
| Primary 1° | at most one other carbon | ethanol, butan-1-ol |
| Secondary 2° | two other carbons | propan-2-ol, butan-2-ol |
| Tertiary 3° | three other carbons | 2-methylpropan-2-ol |
- Boiling points are much higher than alkanes because the polar O–H bond forms hydrogen bonds.
- Methanol, ethanol and propanol mix fully with water. Solubility falls as the chain gets longer.
O₂Decomposition of hydrogen peroxideRates · effect of a catalyst (MnO₂)
What happens: hydrogen peroxide breaks down into water and oxygen. Manganese dioxide speeds it up without being used up. You collect the oxygen and record its volume over time.
Why do it: to show that a catalyst increases the rate. Plot volume of O₂ against time. The steeper the curve, the faster the rate.
CO₂Effect of particle sizeRates · marble chips and hydrochloric acid
What happens: marble chips (CaCO₃) react with dilute HCl and give off CO₂. As the gas escapes, the mass on the balance drops. Repeat with smaller chips and keep everything else the same.
Why do it: smaller chips have a larger surface area, so more of the marble is exposed to acid at once and the rate is faster.
S↓Effect of concentrationRates · sodium thiosulfate and HCl
What happens: the reaction forms a cloudy yellow precipitate of sulfur. A cross drawn under the flask slowly disappears. Time how long it takes, then repeat with the thiosulfate diluted with water.
Why do it: to show that a higher concentration gives a faster rate. Rate is taken as 1/time.
C₂H₄Preparation of etheneOrganic · dehydration of ethanol over Al₂O₃
What happens: ethanol vapour passes over hot aluminium oxide, which removes water and leaves ethene gas. The ethene is collected over water. This is a dehydration (elimination) reaction.
Tests on ethene
- Bromine water: red-orange turns colourless, which shows unsaturation.
- Acidified KMnO₄: purple turns colourless, which also shows unsaturation.
- Combustion: burns with a yellow, slightly sooty flame. The CO₂ produced turns limewater milky.
X⁻Tests for anionsWhat you add, what you see, and why
Cl⁻Add AgNO₃: white precipitate that dissolves in dilute ammonia.
SO₄²⁻Add BaCl₂: white precipitate. Add dilute HCl and it stays.
SO₃²⁻Add BaCl₂: white precipitate. Add dilute HCl and it dissolves.
CO₃²⁻Add HCl: fizzing, and the gas turns limewater milky. MgSO₄ gives a white precipitate straight away.
HCO₃⁻Fizzes with HCl like carbonate. With MgSO₄ there's no precipitate until you boil it.
NO₃⁻Brown ring test: add FeSO₄, then carefully add concentrated H₂SO₄ down the side. A brown ring forms where the layers meet.
PO₄³⁻Add ammonium molybdate and concentrated nitric acid, then warm: yellow precipitate.
BaCl₂ + HClPrecipitate stays: sulfate. Precipitate disappears: sulfite.
Which family is this?
Use the 60-second method: count the oxygens, then check where they sit.
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The Nuclear, Bohr and Orbital models: the experiments behind them, their limits, and the exam definitions.
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