Chemistry of Carbon Compounds
FoundationCarbon compounds are sorted by functional groups: saturated alkanes undergo substitution, unsaturated alkenes addition. Primary alcohols oxidise to aldehydes and then carboxylic acids; alcohols and acids form fruity esters with concentrated sulfuric acid; fats give soap by alkaline hydrolysis; and alkenes polymerise into plastics.
- Functional groups decide properties; a homologous series shares structure and general formula, members differing by CH₂.
- Alkanes substitute (halogen plus sunlight); alkenes add, and bromine water decolourising is the unsaturation test.
- Primary alcohol oxidation ladder: alcohol → aldehyde → carboxylic acid; ethanol ends as ethanoic acid.
文體小知識：科學知識說明文——按官能基整理碳化合物家族，說明烷烴烯烴的反應、醇的氧化、酯化作用與肥皂製備，貫穿有機化學的合成主線。
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1A carbon atom has four outer electrons and can bond with hydrogen, oxygen, nitrogen and others to build chains, rings and branched molecules; carbon compounds number in the tens of millions, and their study is organic chemistry. Chemists group structurally similar compounds whose compositions differ by a fixed unit into a homologous series, and it is the functional group that decides chemical properties. Recognise the functional group and you recognise each family's character.
2Alkanes are saturated hydrocarbons with only single bonds, general formula CₙH₂ₙ₊₂; alkenes are unsaturated, each containing one carbon-carbon double bond, general formula CₙH₂ₙ. The typical reaction of a saturated hydrocarbon is substitution: under sunlight, halogen atoms take the places of hydrogen atoms. The typical reaction of an unsaturated hydrocarbon is addition: the double bond opens and hydrogen, halogen or hydrogen halide adds straight on. Adding brown bromine water to an unsaturated compound decolourises it — the standard test for unsaturation.
3Alcohols carry the hydroxyl group (—OH). When a primary alcohol is oxidised it first becomes an aldehyde and then, on further oxidation, a carboxylic acid — the 'alcohol, aldehyde, acid' ladder is the backbone of organic chemistry. For example, ethanol oxidised by acidified potassium permanganate solution gives ethanal first and ethanoic acid finally; wine left open to the air turning sour is nature's demonstration of the same change. Carboxylic acids carry the carboxyl group (—COOH); ethanoic acid is the main component of vinegar.
4An alcohol and a carboxylic acid, warmed with concentrated sulfuric acid as catalyst, condense into an ester and water — esterification, a reversible reaction. Esters are famously fruity: the aromas of pineapple, banana and orange all come from different esters, used in perfumes, food flavourings and paint solvents. Besides catalysing, concentrated sulfuric acid absorbs the water formed, shifting the equilibrium towards the ester and raising the yield.
5Carbon compounds also power two great everyday applications: fats boiled with sodium hydroxide undergo alkaline hydrolysis to give soap and glycerol — saponification; and alkene molecules open their double bonds to link up into addition polymers such as polythene, the stuff of plastics. Seen through functional-group interconversion, these reactions form a synthetic route map: alkene, alcohol, aldehyde, carboxylic acid and ester interlock and turn into one another, yielding useful materials.
1Summary 重點摘要
Carbon compounds are sorted by functional groups: saturated alkanes undergo substitution, unsaturated alkenes addition. Primary alcohols oxidise to aldehydes and then carboxylic acids; alcohols and acids form fruity esters with concentrated sulfuric acid; fats give soap by alkaline hydrolysis; and alkenes polymerise into plastics.
2Vocabulary cards Vocabulary
The atom group that decides the chemical properties of a family of carbon compounds, e.g. —OH, —COOH.
A series of compounds with similar structures, one general formula, and successive members differing by a fixed unit such as CH₂.
A hydrocarbon containing a carbon-carbon double or triple bond, such as an alkene; it decolourises bromine water.
A reaction in which an atom or group replaces another in a molecule, e.g. an alkane with a halogen under sunlight.
A reaction in which the unsaturated bond opens and atoms or groups add directly, e.g. an alkene with bromine.
An organic compound carrying the hydroxyl group —OH, e.g. ethanol; a primary alcohol oxidises to an aldehyde and then a carboxylic acid.
An organic acid carrying the carboxyl group —COOH, e.g. ethanoic acid, the main component of vinegar.
The product of condensing an alcohol with a carboxylic acid, usually fruity in smell, e.g. ethyl ethanoate.
The reversible reaction of an alcohol with a carboxylic acid to give an ester and water, catalysed by concentrated sulfuric acid.
The joining of unsaturated monomers by opening their double bonds into a macromolecule, e.g. ethene into polythene.
3Step by step Steps
- Step 1: Why carbon rules the organic worldA carbon atom has four valence electrons and bonds with hydrogen, oxygen, nitrogen and others to form straight chains, branches and rings; carbon compounds number in the tens of millions and belong to organic chemistry. Compounds with similar structures, one general formula and successive members differing by CH₂ form a homologous series, and the functional group decides chemical properties. For example, all alcohols carrying the hydroxyl group (—OH) — methanol, ethanol, propanol — behave alike, because it is the functional group that acts. Classify by functional group and organic chemistry becomes a regular map rather than scattered memorising.
- Step 2: Alkanes, alkenes and two kinds of reactionAlkanes are saturated, all single bonds, general formula CₙH₂ₙ₊₂; alkenes are unsaturated with one carbon-carbon double bond, formula CₙH₂ₙ. The typical alkane reaction is substitution: with a halogen under sunlight, halogen atoms progressively replace hydrogen atoms. The typical alkene reaction is addition: the double bond opens and hydrogen, halogen or hydrogen halide adds on. For example, ethene decolourises brown bromine water by addition — the unsaturation test; ethane with bromine in sunlight substitutes instead, fading far more slowly. One opens, one swaps — the key to telling the two families apart and to writing equations.
- Step 3: The oxidation ladder of alcoholsAlcohols carry the hydroxyl group (—OH). A primary alcohol oxidises first to an aldehyde and then to a carboxylic acid. For example, warming ethanol with acidified potassium permanganate solution decolourises the purple: ethanol becomes ethanal and, on further oxidation, ethanoic acid; with orange dichromate solution the colour turns from orange to green. Wine left open going sour is nature's version of the same change — ethanol oxidised by air to ethanoic acid, the acid of vinegar. This 'alcohol, aldehyde, acid' ladder is the backbone of the syllabus; questions often ask for conditions, products and colour changes at each stage.
- Step 4: Esterification and the smell of estersAn alcohol and a carboxylic acid, warmed with concentrated sulfuric acid as catalyst, form an ester and water; the reaction is reversible and written with the double arrow ⇌. For example, ethanol and ethanoic acid give ethyl ethanoate and water; ethyl ethanoate is a fragrant liquid. Concentrated sulfuric acid catalyses and also absorbs the water formed, shifting the equilibrium towards the ester and raising the yield. Esters smell of fruit — pineapple, banana, pear each owe their aroma to a different ester — and serve in perfumes, food flavourings and solvents. In answers, always show the reversible arrow and state concentrated sulfuric acid as catalyst with heating: these are the standard scoring points.
- Step 5: Hydrolysis of esters and the birth of soapEsters can be hydrolysed, and alkaline hydrolysis is the soap-maker's route: fats (triglycerides) boiled with sodium hydroxide solution give soap (sodium salts of long-chain fatty acids) and glycerol. For example, traditional soap-making boils fat with concentrated sodium hydroxide, then adds concentrated brine to salt out the soap, which floats up as its solubility drops, ready to be pressed into moulds. A soap molecule has a water-loving carboxylate end and a water-fearing long hydrocarbon tail; washing, the tails bury into grease while the heads face the water, wrapping the grease into droplets that rinse away — exactly why soap cleans.
- Step 6: Addition polymers and plasticsThe double bonds of alkenes can open and link up, hundreds of small molecules joining into a macromolecule — addition polymerisation; the small molecules are monomers. For example, ethene polymerises into polythene for carrier bags and cling film; vinyl chloride into PVC for water pipes; styrene into polystyrene foam. Polymers are light, mouldable and durable, but their durability also makes them hard to decompose, piling up as plastic waste — so recycling and degradable materials have become important directions of chemical research. When writing a polymerisation equation, draw the repeating unit and label the degree of polymerisation n.
- Step 7: The synthetic master-line of functional-group interconversionThe most important application of organic chemistry is synthesising useful carbon compounds by interconverting functional groups. The main line: an alkene plus water gives an alcohol; a primary alcohol oxidises to an aldehyde and then a carboxylic acid; alcohol and acid condense into an ester, and hydrolysing the ester returns alcohol and acid; alkenes polymerise, and alcohols dehydrate back to alkenes. For example, to make ethyl ethanoate from ethene: add water to ethene for ethanol, oxidise part of it to ethanoic acid, then esterify with the rest. Such route-planning questions appear every year in Paper 1 structured questions — write out reagents, conditions and intermediates step by step and the route speaks for itself.
4Diagrams Diagrams
5Worked example Example
A chemistry society activity: first oxidise ethanol to ethanoic acid with acidified potassium permanganate, then prepare an ester from the acid and ethanol, and finally discuss how soap is made. State the conditions, observations and products of each step.
- Warm ethanol with acidified potassium permanganate solution: the purple fades as ethanol is oxidised first to ethanal and then to ethanoic acid (CH₃CH₂OH → CH₃CHO → CH₃COOH).
- Distil the oxidised mixture to collect ethanoic acid — a sharp-smelling acid, soluble in water.
- Mix the acid with ethanol, add concentrated sulfuric acid as catalyst and heat under reflux: ethyl ethanoate and water form reversibly, and a fruity smell is noticed.
- The principle of soap-making: boil fats with sodium hydroxide solution (alkaline hydrolysis of esters) to give soap and glycerol, then add concentrated brine to salt out the soap.
- Answer: Ethanol is oxidised in two steps by acidified permanganate to ethanoic acid; the acid esterifies with ethanol under concentrated sulfuric acid and heating to give fruity ethyl ethanoate and water; fats boiled with sodium hydroxide (saponification) give soap and glycerol, separated by salting out with brine.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the typical reactions of alkanes and of alkenes?
Q2How can bromine water distinguish a saturated from an unsaturated hydrocarbon?
Q3What is the final organic product of completely oxidising ethanol with acidified potassium permanganate solution?
Q4Which catalyst is used for esterification, and what other role does it play?
Q5Which two products form when fats are boiled with sodium hydroxide solution, and what is the reaction called?
8Recap Recap
- Functional groups decide properties; a homologous series shares structure and general formula, members differing by CH₂.
- Alkanes substitute (halogen plus sunlight); alkenes add, and bromine water decolourising is the unsaturation test.
- Primary alcohol oxidation ladder: alcohol → aldehyde → carboxylic acid; ethanol ends as ethanoic acid.
- Esterification: alcohol + acid ⇌ ester + water, concentrated sulfuric acid catalyses and absorbs water; esters smell of fruit; saponification makes soap.
- Alkenes polymerise by addition; functional-group interconversion is the synthetic map — always state reagents and conditions.