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/ Cr₂O₇^2-/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

References:

• 3.5 Naming Aldehydes and Ketones | DAT Bootcamp
• How can you convert the ethanol to ethanoic acid? - Quora
• 19.4: Nucleophilic Addition Reactions of Aldehydes and Ketones - Chemistry LibreTexts

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