Organic Chemistry: Carbon's Family Tree
Spot the Group, Predict the Reaction 🧬
Introduction
1. Introduction
Okay, organic chemistry looks terrifying because of all the names, but here's the secret: it's basically one big family tree. Carbon makes four bonds and loves bonding to itself in chains, so all its compounds sort into a few families (homologous series). Each family has one special part, a functional group, and that part tells you exactly how it reacts.
The cheat code for the whole chapter: the functional group decides the chemistry. Spot the group (, , , or none) and you can predict the reactions. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact word the marker wants. Let's go 🧬
The cheat code for the whole chapter: the functional group decides the chemistry. Spot the group (, , , or none) and you can predict the reactions. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact word the marker wants. Let's go 🧬
2. Families and Functional Groups
A hydrocarbon is a compound made of carbon and hydrogen only (the second oxygen sneaks in and it stops being one). A functional group is an atom or group of atoms that determines the chemical properties of a homologous series. A homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group (learn that word for word). The functional groups to spot: C=C (alkenes), -OH (alcohols), -COOH (carboxylic acids), and none for alkanes (which is why alkanes are so chill and unreactive). Each series also has a general formula you can use to tell which series a formula belongs to: alkanes , alkenes . To get a molecular formula off a drawing, you literally just count every atom.
Key idea🔑 Key idea: Homologous series = similar compounds with similar chemical properties due to the same functional group (NOT the same number of carbons). Use the general formula to place a formula in its series. Hydrocarbon = C and H only. 🧩
Worked example
Worked Example: Same Family or Not?
Worked Example: Match the General Formula 🔍
Three compounds: 1 is , 2 is , 3 is . Which two are in the same family?
- 1Alkenes fit : (4 = 2×2) ✓ and (8 = 2×4) ✓.
- 2fits , so it's an alkane.
So 1 and 2 are the same family. Same number of carbons (2 and 3) is a trap! ✅
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3. Naming the Molecules
Every name is stem + ending. The stem counts carbons: meth- (1), eth- (2), prop- (3). The ending names the family: -ane (alkane), -ene (alkene), -ol (alcohol), -oic acid (carboxylic acid). So "ethanol" = eth (2 carbons) + ol (alcohol). Read the ending and you instantly know the family, the functional group, and whether it's saturated. You need methane through to ethanoic acid cold.
Key idea🔑 Key idea: Stem = carbons (meth/eth/prop = 1/2/3). Ending = family (-ane/-ene/-ol/-oic acid). The ending gives away the functional group. 🏷️
Worked example
Worked Example: Name That Acid
Worked Example: Count, Then Add the Ending 🧪
Name the carboxylic acid with 2 carbons, formula .
- 1Count carbons (including the one in ): 2 → stem eth-.
- 2It's a carboxylic acid → ending -oic acid.
So it's ethanoic acid. ✅
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4. Petroleum and the Big Tower
Petroleum (crude oil) is a mixture of hydrocarbons of all different chain lengths, and on its own it's basically useless. We split it by fractional distillation: heat it to a vapour and send it up a tower that's hot at the bottom, cool at the top. Each fraction rises until it's cool enough to condense at its own level, because they all have different boiling points. From the bottom to the top of the column the syllabus gives four trends: chain length decreases, volatility gets higher, boiling point gets lower and viscosity gets lower. Learn the eight fractions, top to bottom, with their uses: refinery gas (heating and cooking), gasoline/petrol (fuel for cars), naphtha (chemical feedstock), kerosene/paraffin (jet fuel), diesel oil/gas oil (fuel for diesel engines), fuel oil (ships and home heating), lubricating oil (lubricants, waxes and polishes), bitumen (making roads).
Key idea🔑 Key idea: Fractional distillation splits petroleum by boiling point. Bottom → top: shorter chains, higher volatility, lower boiling point, lower viscosity. 🏭
Worked example
Worked Example: Which Fraction?
Worked Example: Highest Boiling Point = Bottom 🌡️
Of gasoline, naphtha, kerosene, diesel oil, fuel oil and lubricating oil (lowest), which has the highest boiling point?
- 1Highest boiling point = biggest molecules = collected lowest in the tower.
- 2Lubricating oil sits lowest of these.
So it's lubricating oil. ✅
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5. Cracking and Burning
There are too many long-chain molecules and not enough short useful ones, so we do cracking: a high temperature + a catalyst snap a big alkane into a smaller alkane + an alkene, or an alkene + hydrogen (the atoms must add up). It's how alkenes and hydrogen are manufactured, and it makes shorter alkanes for petrol. Alkenes like ethene are gold because they make plastics. And when fuels burn: plenty of oxygen → complete combustion → carbon dioxide + water; not enough oxygen → incomplete combustion → carbon monoxide (toxic, colourless, no smell — sneaky and dangerous).
Key idea🔑 Key idea: Cracking = big alkane → small alkane + alkene (more useful). Lots of O₂ → CO₂ + water; little O₂ → toxic CO. 🔥
Worked example
Worked Example: What Comes Out of Cracking?
Worked Example: Make the Atoms Balance ⚖️
Cracking hexane, , gives which products?
- 1Cracking → a smaller alkane + an alkene, with atoms adding to 6 C and 14 H.
- 2: carbons 4+2 = 6 ✓, hydrogens 10+4 = 14 ✓.
So the products are and (ethene). ✅
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6. Alkanes
Alkanes are saturated (single bonds only), general formula — methane, ethane, propane. They have no functional group, so they're generally unreactive. They do two things worth knowing: they burn (complete → CO₂ + water), and they react with chlorine by substitution: a chlorine atom takes the place of a hydrogen atom. That's all Core asks about it.
Key idea🔑 Key idea: Alkanes = saturated (single covalent bonds only), , generally unreactive except for combustion and substitution by chlorine. 🧱
Worked example
Worked Example: What Reacts with Butane?
Worked Example: Substitution, Not Addition 🔄
Compound Z is . Which substance reacts with Z, and by which type of reaction: steam by addition, chlorine by addition, steam by substitution, or chlorine by substitution?
- 1fits , so Z is a saturated alkane. Alkanes react only by combustion and by substitution with chlorine; steam reacts with ethene, not alkanes.
- 2Nothing can add to a saturated molecule, because it has no double bond.
So the answer is chlorine, by substitution. ✅
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7. Alkenes
Alkenes are unsaturated: they've got a carbon-carbon double bond, , general formula — ethene, propene. That double bond is a reactive hotspot, so alkenes are way more reactive than alkanes. The classic exam move is the bromine water test: shake a compound with orange bromine water — an alkene turns it orange to colourless (the bromine adds to the C=C), an alkane leaves it orange. Alkenes also come from cracking.
Key idea🔑 Key idea: Alkene = C=C, , unsaturated, reactive. Bromine water test: alkene = orange → colourless; alkane = stays orange. 🌿
Worked example
Worked Example: The Bromine Test
Worked Example: Only the Double Bond Reacts 🟠
Methane and ethane (single bonds) and ethene (C=C) are each shaken with bromine water. What happens?
- 1Methane and ethane are saturated → bromine stays orange.
- 2Ethene is unsaturated → bromine goes orange to colourless.
So only ethene decolourises it. ✅
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8. Ethanol
Alcohols have the group; the star is ethanol . Two ways to make it: fermentation of aqueous glucose (yeast, 25 to 35 °C, absence of oxygen) or catalytic addition of steam to ethene (300 °C, 6000 kPa / 60 atm, acid catalyst). At Core you learn the two methods and their conditions; comparing their advantages is Extended work. Ethanol burns to give carbon dioxide and water, which is why it's a fuel; it also dissolves loads of stuff, so it's a solvent.
Key idea🔑 Key idea: Ethanol from fermentation (aqueous glucose, yeast, 25 to 35 °C, no oxygen) OR steam + ethene (300 °C, 6000 kPa, acid catalyst). Uses: fuel + solvent. 🍷
Worked example
Worked Example: Burning Ethanol
Worked Example: Don't Forget Ethanol's Oxygen ⚗️
Balance the complete combustion of ethanol, .
- 12 carbons → ; 6 hydrogens → (that's 7 oxygen atoms on the right).
- 2Ethanol gives 1 oxygen, so O₂ supplies the other 6 → 3 O₂.
So . ✅
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9. Ethanoic Acid
Carboxylic acids have the group; the one to know is ethanoic acid . It does the normal acid reactions, and its salts are called ethanoates: with a metal → salt + hydrogen; with a base → salt + water; with a carbonate → salt + water + carbon dioxide. Name the salt as metal + ethanoate (sodium ethanoate, magnesium ethanoate...).
Key idea🔑 Key idea: Ethanoic acid (-COOH): metal → salt + H₂; base → salt + water; carbonate → salt + water + CO₂. Salts = ethanoates. 🧂
Worked example
Worked Example: Acid Meets Carbonate
Worked Example: Fizz Means CO₂ 🫧
Ethanoic acid + sodium carbonate. Name the salt and the gas, and the test for the gas.
- 1Acid + carbonate → salt + water + carbon dioxide; the salt is sodium ethanoate.
- 2The carbon dioxide turns limewater milky.
So: sodium ethanoate + carbon dioxide (turns limewater milky). ✅
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10. Polymers
A polymer is a giant molecule made by joining loads of small monomers. The Core one is poly(ethene), made from ethene by addition polymerisation: each ethene's opens up and they join into one long chain, with no other product. Name it by sticking poly in front: poly(ethene). The catch: poly(ethene) is flexible, waterproof and unreactive (great for bags), but that means it's non-biodegradable — microbes can't break it, so it clogs landfill and oceans for years, and burning it makes toxic gases.
Key idea🔑 Key idea: ethene → poly(ethene) by addition polymerisation (C=C opens, no by-product). Non-biodegradable, so plastic waste lasts forever. 🔗
Worked example
Worked Example: Name the Polymer
Worked Example: Open the Double Bond 🧬
A repeat unit is . Name the polymer and its monomer.
- 1The monomer had a that opened up → it's ethene.
- 2Name the polymer: poly(ethene).
So poly(ethene), from ethene. (Not ethane — ethane has no double bond.) ✅
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