AS/A-level Chemistry - Carbonyls (Ketones)

Ketone

• Contain the carbonyl functional group C=O 
• The carbonyl functional group is joined to 2 Carbon atoms in the Carbon chain  
• In a structural formula, written as- CO 
• The simplest ketone is propanone – CH₃COCH₃ 
• Used as an important industrial solvent,also used in nail varnish removers  

Naming Ketones: 

• count the number of C atoms- name as the parent alkane
• instead of ‘e’ it ends in ‘one’
• number the lowest number carbon atom where the carbonyl group is present e.g. pentan-2-one 

Formation of Ketone

• Oxidation of secondary alcohol with potassium dichromate (VI) (K₂Cr₂O₇ / Cr₂O₇^2-/H⁺) acidified and dilute sulfuric acid 
• Propan-2-ol (secondary alcohol) to Propaone (ketone)

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 mechansims below, these always appear in AS/A-level Chemistry exams! 👩‍🏫

Reduction of Ketones

• Reagents: Aqueous NaBH4, followed by H2SO4
• Condition: Room temperature
• Mechanism: Nucleophilic Addition 
• Product: Secondary alcohol

Nucleophilic Addition reaction with HCN

• Reagent: Aqueous KCN followed by H2SO4 to form HCN (toxic)
• Condition: Room temperature
• Mechaism: Nucelophilic Addition 
• Product: Hydroxynitrile 

Identify Ketones

• NO REACTION with potassium dichromate (VI), diluteH₂SO₄ and heat because ketones cannot be oxidised as there is no H atom attached to the C=O bond  
• NO REACTION with ammoniacal silver nitrate solution and heat (Tollens’ reagent) 
• 2,4-dinitrophenylhydrazine forms a yellow/orange precipitate (2,4-dinitrophenylhydrazone)
• This is useful in determining identity of a ketone because it has a specific, unique melting point which can be compared to a data book  

References:

• 3.5 Naming Aldehydes and Ketones | DAT Bootcamp
• 19.4: Nucleophilic Addition Reactions of Aldehydes and Ketones - Chemistry LibreTexts

This is the end of the topic!

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