Aldehydes, Ketones and Carboxylic Acids
Class 12 · Chemistry
4 ideas in Class 12 Chemistry, explained in plain words, with games and quizzes to practise.
Carbonyl Names, Shape and Behaviour
Part of: Meeting Carbonyl Compounds
An aldehyde carries its double-bonded oxygen at the end of a carbon chain, while a ketone carries it between two carbon groups. Everyday names come from the related acids, and systematic names end in al for aldehydes and one for ketones. The carbonyl carbon and its three neighbours lie flat, and oxygen pulls electrons towards itself, so the group is polar.
Why it matters. Names unlock every later reaction, and polarity explains why small carbonyl compounds dissolve in water while bigger ones carry perfume smells.
Formulas
- Aldehyde chains are numbered from the aldehyde carbon
- Ketone chains are numbered from the end nearer the carbonyl group
- An aldehyde on a ring takes the ending carbaldehyde
Key words
- Aldehyde
- : A carbonyl compound with the C=O group at a chain end, holding one hydrogen there.
- Ketone
- : A carbonyl compound with the C=O group joined to two carbon groups inside the chain.
- Carbonyl group
- : A carbon joined to oxygen by a double bond, flat around the carbon.
- Common name
- : A traditional name taken from the source acid, such as acetaldehyde.
- Carbaldehyde
- : The ending used when an aldehyde group hangs off a ring.
- Dipole-dipole attraction
- : The pull between the charged ends of neighbouring polar molecules.
Try it in two minutes
- Write the formulas of ethanal, propanone and butane on three slips.
- Ask the child to order them by expected boiling using polarity.
- Name each compound aloud using the al and one endings.
Try at home. Draw pentanal and pentan-2-one side by side and name which carbon carries the oxygen in each.
Making Aldehydes and Ketones
Part of: Meeting Carbonyl Compounds
Primary hydroxy compounds stop at aldehydes with gentle oxidants and volatile hydroxy compounds give the same pair over hot metal catalysts. Ozonolysis splits alkenes, water adds to alkynes, and special reagents turn acyl chlorides, nitriles and methylbenzene into aldehydes. Ketones come from secondary hydroxy compounds, acyl chlorides with cadmium reagents, nitriles with Grignard reagents, and benzene with acyl chlorides.
Why it matters. Factories and labs pick these routes daily, and board questions often ask which single reagent reaches a target in one step.
Formulas
- Primary hydroxy compound with PCC gives an aldehyde; strong oxidants push on to the acid
- Ethyne with water gives ethanal; all other alkynes give ketones
- Acyl chlorides with dialkylcadmium give ketones
Key words
- Rosenmund reduction
- : Hydrogenation of an acyl chloride over palladium on barium sulphate to an aldehyde.
- Stephen reaction
- : Reduction of a nitrile with stannous chloride and acid, then hydrolysis to an aldehyde.
- Etard reaction
- : Oxidation of toluene with chromyl chloride that halts at benzaldehyde.
- Gatterman-Koch reaction
- : Formylation of benzene with carbon monoxide and HCl to benzaldehyde.
- Friedel-Crafts acylation
- : Building an aromatic ketone from benzene and an acyl chloride with AlCl3.
- Ozonolysis
- : Splitting an alkene with ozone, then zinc and water, into carbonyl pieces.
Try it in two minutes
- Write five starting materials on cards and five reagents on others.
- Ask the child to pair each start with its reagent and product.
- Check pairs against the rule that gentle means stop and strong means go further.
Try at home. List three ways to make benzaldehyde from toluene and name the reagent in each.
Addition, Reduction and Oxidation Tests
Part of: How Carbonyl Compounds React
A nucleophile attacks the electron-poor carbonyl carbon, which turns from flat to tetrahedral and takes a hydrogen on oxygen. Aldehydes react faster than ketones because they are less crowded and more electron-poor. Hydrogen cyanide, bisulphite, hydroxy compounds and ammonia relatives all add this way. Gentle reduction gives hydroxy compounds, fierce reduction gives hydrocarbons, and mild oxidants colour-test aldehydes.
Why it matters. This one mechanism explains dozens of reactions, and the colour tests let chemists identify unknown liquids in seconds.
Formulas
- Aldehydes reduce to primary hydroxy compounds; ketones reduce to secondary hydroxy compounds
- Clemmensen or Wolff-Kishner reduction turns a carbonyl group into a CH2 group
- Tollens reagent gives a silver mirror; Fehling reagent gives a red-brown solid
Key words
- Cyanohydrin
- : The product of adding hydrogen cyanide across a carbonyl group.
- Tollens test
- : Warming with ammoniacal silver nitrate; aldehydes give a silver mirror.
- Fehling test
- : Heating with an alkaline copper mixture; aldehydes give a red-brown solid.
- Aldol condensation
- : Two carbonyl molecules with alpha hydrogen joining in dilute alkali, then losing water.
- Cannizzaro reaction
- : An aldehyde without alpha hydrogen splitting into hydroxy compound and salt in strong alkali.
- Haloform reaction
- : A methyl ketone with hypohalite giving a haloform plus a shorter acid salt.
Try it in two minutes
- Draw a carbonyl group and show the nucleophile flying in from above the plane.
- Rank methanal, ethanal, propanone and pentan-3-one by crowding.
- Match each test, Tollens, Fehling and iodoform, to the colour it gives.
Try at home. Write what each of Tollens, Fehling and alkaline iodine does to benzaldehyde and to acetophenone.
Carboxylic Acids: Names, Strength and Change
Part of: How Carbonyl Compounds React
A carboxyl group pairs a carbonyl with a hydroxyl on one carbon. Small acids are sharp liquids that pair up through hydrogen bonds and boil high. They beat phenols and hydroxy compounds in acidity because the leftover carboxylate ion spreads its charge over two oxygens. They fizz with carbonates, form esters with hydroxy compounds, swap OH for chlorine, and lose carbon dioxide on strong heating.
Why it matters. Vinegar, citrus, soaps and nylon all run on this family, and acidity ranking is a favourite board question.
Formulas
- A smaller pKa means a stronger acid
- Sodalime heating turns a sodium salt into a hydrocarbon with one carbon fewer
- Diborane or LiAlH4 reduces an acid to a primary hydroxy compound; NaBH4 cannot
Key words
- Carboxyl group
- : A carbon holding both a double-bonded oxygen and a hydroxyl group.
- pKa
- : A scale number for acid strength; a smaller value means a stronger acid.
- Electron-withdrawing group
- : A neighbour that pulls electrons and steadies the leftover ion, raising acidity.
- Esterification
- : An acid joining a hydroxy compound with acid catalyst to form an ester plus water.
- HVZ reaction
- : Chlorination or bromination at the alpha carbon using red phosphorus.
- Decarboxylation
- : Losing carbon dioxide on heating, usually from a sodium salt with sodalime.
Try it in two minutes
- Write methanoic, ethanoic and benzoic acid cards with their pKa values.
- Ask the child to order them from strongest to weakest acid.
- Add a fluorine card and ask which side of ethanoic acid it lands on.
Try at home. Explain to a family member why vinegar fizzes with baking soda but phenol does not.
What your child will practise
- Addition, Reduction and Oxidation Tests: Space Quick Fire (Quick Fire)
- Carbonyl Names, Shape and Behaviour: Ocean Puzzle (Clue Puzzle)
- Carbonyl Names, Shape and Behaviour: Ocean Quick Fire (Quick Fire)
- Carboxylic Acids: Names, Strength and Change: Jungle Quick Fire (Quick Fire)
- Making Aldehydes and Ketones: Candy Puzzle (Clue Puzzle)
- Quiz: Addition, Reduction and Oxidation Tests (8 questions)
- Quiz: Carbonyl Names, Shape and Behaviour (8 questions)
- Quiz: Carboxylic Acids: Names, Strength and Change (8 questions)
- Quiz: Making Aldehydes and Ketones (8 questions)