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Aldehydes & Ketones
AQA A-level chemistry year 13
| Term | Definition |
|---|---|
| Oxidation of alcohols and aldehydes | A chemical process in which these organic compounds undergo the addition of oxygen and/or the removal of hydrogen by an oxidising agent. Primary alcohols becomes aldehydes, secondary alcohols become ketones, and aldehydes become carboxylic acids |
| Reduction of aldehydes, ketones, and carboxylic acids | A chemical process in which these organic compounds undergo the removal of oxygen and/or the addition of hydrogen. Aldehydes become primary alcohols, ketones become secondary alcohols and carboxylic acids become aldehydes |
| Oxidation of ketones | Ketones can’t be easily oxidised, this can only be achieved with a strong oxidising agent such potassium manganate solution to break a C-C bond to turn the ketone into an aldehyde, this results in a shorter chain |
| Carbonyl group | C=O with 2 R groups bound to the carbon. Present in aldehydes, ketones, and carboxylic acids |
| Oxidation tests: Fehling’s | Ketones make the solution remain blue and aldehydes make the solution turn orange |
| Oxidation tests: Tollen’s (silver mirror) | Ketones make the solution remain clear and aldehydes make the solution turn black with a pure silver precipitate or “silver mirror” |
| Reduction of Aldehyde: formula and reagent | RCHO + 2[H] - RCH2OH Requires a source of hydride anions such as sodium borohydride (III) - NaBH4 |
| Boiling point of carbonyls | They have a higher boiling point than hydrocarbons since they can form hydrogen bonds with water but lower than alcohols which can form hydrogen bonds with each other |
| Solubility of carbonyls | Short chain aldehydes and ketones are soluble in water since they can form hydrogen bonds with water but decrease in solubility as chain length increases |
| Nucleophilic addition | A chemical reaction mechanism in which a nucleophile attacks an area of electron deficiency resulting in the addition of that nucleophile to the molecule |
| How aldehydes and ketones react with cyanides | The carbonyl is reduced to an alcohol and the cyanide group is added creating a hydroxynitrile |
| Why sulphuric acid is added to a carbonyl-cyanide reaction | Hydrogen cyanide is needed for this reaction, but hydrogen cyanide is also a deadly gas which is hard to contain so instead metal cyanides are used and reacted with a little sulphuric acid to form hydrogen cyanide in solution |
| Carbonyl-cyanide reaction: step 1 | The cyanide ion will attack the slightly positive carbon and add itself to the group creating a tetrahedral molecule with an unstable oxygen group |
| Carbonyl-cyanide reaction: step 2 | The unstable oxygen can then attack a hydrogen ion from the surrounding solution (i.e., a hydronium H3O+ ion) to form the hydroxynitrile |
| Why hydroxynitriles form racemates | Carbonyl groups are trigonal planar which means that there is an equal probability that the cyanide ion can attack from above and below which will create a roughly 50/50 mixture. All aldehydes and unsymmetrical ketones will react this way |
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