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Aldehydes
- Contain the carbonyl functional group C=O
- The carbon atom attached to the carbonylgroup contains 1 or 2 H atoms
- C=O is always found at the end of a Carbon chain
- In a structural formula, written as CHO
- Methanal is the simplest –HCHO
- Used in solution to preserve biological specimens
Naming Aldehydes:
- count the no. of C atoms- name as the parent alkane
- instead of ‘e’ add ‘al’ e.g.ethanal
Formation of Aldehydes
- Oxidation of primary alcohol with potassium dichromate(VI) (KCr2O7/ Cr2O72-/H+) and acidified with dilute sulfuric acid.
- Colour of solution changes from orange to green.
- Initially oxidised to aldehydes, distill off as it formed.
- Aldehyde is further oxidised under reflux with excess oxidising agent and heat to form carboxylic acid.
Carbonyls
- The double bond in a carbonyl is unsymmetrical so electronegative and polar ,unlike the double bond in alkenes (non-polar).
- Oxygen is more electronegative than Carbon, so the electron density in the double bond lies closer to the oxygen-slightly negative, than to the carbon-slightly positive
- The C in the C=O may attract nucleophiles, it attacks the C, resulting in addition across the C=O bond.
- Mechanism of carbonyls: nucleophilic addition
Pay attention to the mechanisms below, these always appear in AS/A-level Chemistry exams! 👩🏫
Reduction of aldehydes with NaBH4
- Reagents: aqueous NaBH4, sodium tetrahydridoborate(III)
- Condition: Room temperature
- Mechanism Nucleophilic addition
- Product: Primary alcohol
Nucleophilic addition reaction with HCN
- Reagent: Aqueous KCN followed by H2SO4 to form HCN (toxic)
- Condition: Room temperature
- Mechanism: Nucleophilic addition
- Product: Hydroxynitrile
Identifying Aldehydes
- Tollens' reagent: a solution of silver nitrate in aqueous ammonia
- aqueous silver nitrate (AgNO3 aq) and sodium hydroxide (NaOH) dissolve the brown precipitate with dilute NH3 until it is clear and colourless.
- In the presence of an aldehyde group, a silver mirror is produced.
- Ag+ ions act as an oxidising reagent in the presence of ammonia, so silver ions are reduced to silver and the aldehyde is oxidised to a carboxylic acid.
- Heat Potassium dichromate (VI) and dilute H2SO4 under reflux
- Colour changes from orange to green and forms carboxylic acids
- React with 2,4-dinitrophenylhydrazine, which has a specific, unique melting point that can be compared to a data book
- Yellow/orange precipitate formed
This is the end of the topic!
Drafted by Cherry (Chemistry)