Organic Chemistry: Carbon, Families, and Functional Groups
Carbon Crew: Every Family, One Pattern 🧬
Introduction
1. Introduction
Okay, organic chemistry. It sounds like a whole new subject, but it's basically the chemistry of one element being extra: carbon. Carbon makes four bonds and happily bonds to itself, so it builds chains, branches and rings, which is why there are millions of carbon compounds.
The cheat code: chemists sort them all into families (homologous series), and each family has a functional group (its reactive bit). Learn the group, predict the chemistry. This is your fast refresh, not the full textbook: one key idea per family, one quick worked example, and the exact move the marker wants. A handful of families, one pattern. Let's go. 🧬
The cheat code: chemists sort them all into families (homologous series), and each family has a functional group (its reactive bit). Learn the group, predict the chemistry. This is your fast refresh, not the full textbook: one key idea per family, one quick worked example, and the exact move the marker wants. A handful of families, one pattern. Let's go. 🧬
2. Formulae and Functional Groups
A functional group is the reactive part of a molecule, and a homologous series is a family that all shares the same one ( for alkenes, for alcohols, for carboxylic acids). Big rule: the functional group decides the chemistry, the chain length decides the physical stuff (like boiling point). So ethanol and butanol react the same way (both have ) but boil at different temperatures. A structural formula shows the arrangement unambiguously ( for ethanol), and structural isomers have the same molecular formula but different structural formulae: butane and methylpropane (), or but-1-ene and but-2-ene ().
Key idea🔑 Key idea: Same homologous series = same functional group + same general formula + similar chemical properties. Members differ by a unit. Don't say "same boiling point", that's wrong. Isomers = same molecular formula, different structural formula. 🎯
Worked example
Worked Example: Same Family or Not
Worked Example: Spot the Family 👀
Compound 1 = ethane (alkane), compound 2 = ethene (alkene), compound 3 = ethanol (alcohol), compound 4 = propane (alkane). Which two are in the same homologous series?
- 1Family is decided by the functional group, not by how many carbons.
- 2Ethane and propane are both alkanes (single bonds only). Ethene is an alkene, ethanol is an alcohol.
So the same family is compounds 1 and 4. Don't get baited into pairing ethane and ethene just because both have two carbons. ✅
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3. Naming Organic Compounds
Every organic name is just a stem (counts the carbons: meth-1, eth-2, prop-3, but-4) plus an ending that names the group: alkane -ane, alkene -ene, alcohol -ol, acid -oic acid. If the group could sit in more than one place, a number (locant) tells you where, like but-2-ene with the double bond on carbon 2.
Key idea🔑 Key idea: Stem = number of carbons, ending = functional group. Ester names are sneaky: the first word is the alcohol, the second word (the -oate) is the acid. 🧠
Worked example
Worked Example: Naming an Alkene
Worked Example: Name That Chain 🔗
An alkene has a four-carbon chain with the double bond between carbons 2 and 3. Name it.
- 1Four carbons → stem but-; it's an alkene → ending -ene.
- 2The double bond starts on carbon 2 → drop a 2 in the middle.
So it's but-2-ene, . Clean. ✅
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4. Fuels and Fractional Distillation
Petroleum (crude oil) is a mixture of hydrocarbons, and we split it by fractional distillation. It works because longer chains have higher boiling points. The oil is vaporised in a column that's hot at the bottom, cool at the top, so each fraction condenses at its own level. The syllabus asks for four trends: going down the column the chain length increases, the volatility decreases, the boiling point increases and the viscosity increases (all reverse going up). The eight fractions, top to bottom: 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) and bitumen (making roads). (Natural gas, by the way, is mostly methane.)
Key idea🔑 Key idea: Fractional distillation separates by boiling point, not by reacting anything. Down the column: longer chains, lower volatility, higher boiling point, higher viscosity. Eight fractions, from refinery gas at the top to bitumen (roads) at the bottom. ⛽
Worked example
Worked Example: Incomplete Combustion
Worked Example: Not Enough Air 💨
Methane burns in a limited supply of oxygen to make carbon monoxide and water. Balance the equation.
- 1Limited oxygen → you get carbon monoxide, CO, not . Start with 2 methane: that's and .
- 2Count oxygen on the right: atoms, so you need .
"Limited oxygen" is your cue to write CO (the toxic gas). ✅
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Answer
5. Alkanes
Alkanes are saturated hydrocarbons (single bonds only), general formula . They're generally unreactive because they only have strong single bonds and no functional group. The one reaction to know: with chlorine in UV light they do substitution, where a Cl swaps in for an H and you also get HCl.
Key idea🔑 Key idea: Alkanes = saturated, unreactive. Substitution with needs UV light and makes a chloroalkane + HCl (not H₂). 🔆
Worked example
Worked Example: Methane Meets Chlorine
Worked Example: Swap an H 🔁
Methane reacts with chlorine in UV light, swapping one hydrogen. Write the equation and name the product.
- 1Swap one H of for a Cl → (chloromethane). The leftover H joins a Cl → HCl.
- 2Write and check it balances:
Substitution, UV light, and the second product is HCl. Three marks, locked. ✅
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Answer
6. Alkenes
Alkenes are unsaturated (they have a double bond), general formula . That double bond is the reactive bit, so alkenes do addition reactions: the double bond opens and a small molecule adds across it to give one product. The famous one is the bromine test: an alkene turns orange bromine water colourless, an alkane leaves it orange. Petroleum contains very little alkene, so alkenes and hydrogen are made by cracking larger alkanes with a high temperature and a catalyst (for example ). Cracking turns surplus long chains into shorter ones that are in demand, like petrol, and supplies alkenes for plastics.
Key idea🔑 Key idea: Alkene = = reactive. Addition gives one product. Bromine water goes orange → colourless with an alkene (that's the unsaturation test). Cracking (high temperature + catalyst) makes alkenes and hydrogen. 🧪
Worked example
Worked Example: Ethene Plus Bromine
Worked Example: Decolourise It 🟠➡️⚪
Ethene reacts with bromine. Write the equation and say why it decolourises bromine water but an alkane doesn't.
- 1The double bond opens and one Br adds to each carbon:
- 2The bromine is used up, so the orange colour disappears. An alkane has no double bond, so nothing reacts and it stays orange.
One product, two Br atoms in it. ✅
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7. Alcohols and Ethanol
Alcohols have the group, general formula . The star is ethanol, made two ways: fermentation (yeast on aqueous glucose at 25 to 35 °C, in the absence of oxygen; renewable and low-energy, but slow, a batch process, and it gives dilute impure ethanol) or catalytic addition of steam to ethene (acid catalyst, 300 °C and 6000 kPa; fast, continuous and pure, but it uses non-renewable ethene and a lot of energy). Ethanol burns cleanly to and is used as a fuel, a solvent, and in drinks.
Key idea🔑 Key idea: Ethanol = fermentation (25 to 35 °C, no oxygen; renewable, slow, impure) OR steam + ethene (acid catalyst, 300 °C, 6000 kPa; non-renewable, fast, pure). Pick the route that matches what the question wants. 🌱
Worked example
Worked Example: Fermentation
Worked Example: Sugar to Spirits 🧫
Ethanol is made from glucose, , by fermentation. Write the equation and the conditions.
- 1Yeast turns glucose into ethanol and carbon dioxide, one glucose → two ethanol + two CO₂:
- 2Conditions: yeast, 25 to 35 °C, in the absence of oxygen (below about 25 °C the enzymes work very slowly; above about 35 °C the rate falls again).
Check it: 6 C, 12 H, 6 O on both sides. Balanced. ✅
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8. Carboxylic Acids and Esters
Carboxylic acids have the group and are weak acids, so they react with metals (→ H₂), bases (→ water) and carbonates (→ CO₂), making ethanoate salts. Ethanoic acid is made by oxidising ethanol (purple manganate(VII) → colourless). And when an acid meets an alcohol with an acid catalyst, you get an ester + water (that's the fruity-smell reaction).
Key idea🔑 Key idea: Carboxylic acid + alcohol → ester + water (acid catalyst). Naming: first word = alcohol, the -oate = acid. So ethyl butanoate = ethanol + butanoic acid. 🧴
Worked example
Worked Example: Naming an Ester
Worked Example: Split the Linkage ✂️
An ester is . Name it.
- 1The bit with the C=O carbon has 4 carbons → from butanoic acid → butanoate.
- 2The bit on the single-bonded oxygen is an ethyl group → from ethanol → ethyl.
So it's ethyl butanoate (not butyl ethanoate — the acid gives the -oate). ✅
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9. Addition Polymers
A polymer is a giant molecule made from loads of small monomers. In addition polymerisation, alkene monomers (with ) join up: the double bond opens and they link end to end with nothing else made. You show the repeat unit (the monomer with the double bond opened and a bond out each end), like for poly(ethene).
Key idea🔑 Key idea: Addition polymerisation: alkene monomers, double bond opens, no small molecule lost. Number of repeat units . 🔗
Worked example
Worked Example: Counting Repeat Units
Worked Example: Divide It Out ➗
The repeat unit of poly(ethene), , has . One poly(ethene) molecule has . How many repeat units, ?
- 1Number of repeat units = whole-chain mass ÷ repeat-unit mass.
- 2Divide:
So 2500 ethene units joined up to make that molecule. ✅
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Answer
10. Condensation Polymers
In condensation polymerisation you use two monomers, each with two reactive groups, and you lose a small molecule (water) every time they link. A diol + dicarboxylic acid makes a polyester with ester linkages, (PET is one); a diamine + dicarboxylic acid makes a polyamide with amide linkages, (nylon is one). The only difference in the drawn structure is O in the polyester where the polyamide has N–H. Proteins are natural polyamides made from amino acids, . Plastics are non-biodegradable, so disposal is a problem: landfill, build-up in the oceans, and toxic gases when burned. PET can be converted back into its monomers and re-polymerised, which is one way to recycle it.
Key idea🔑 Key idea: Condensation = two monomers + water lost. Diol + diacid → polyester (ester linkage); diamine + diacid → polyamide (amide linkage); proteins are natural polyamides. The "di" lets the chain grow at both ends. 💧
Worked example
Worked Example: Building a Polyester
Worked Example: Link and Lose Water 🔗💧
Butane-1,4-diol (two groups) reacts with butanedioic acid (two groups). What linkage joins them, what type of polymer, and what's lost?
- 1An and a join to make an ester linkage, , losing water.
- 2Both monomers have two reactive groups, so the chain grows both ways → condensation polymerisation, making a polyester, with water lost each time.
Diol + diacid = polyester. Done. ✅
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