ALDEHYDES AND KETONES

• Carbonyl group (C=O)


  A double bond between carbon atom and oxygen atom
• Aldehydes


  Carbonyl group with 1 hydrogen atom and either another hydrogen atom or an alkyl/aryl group

  
• Ketones


  Carbonyl group with 2 alkyl or aryl group attached to it

  
• IUPAC nomenclature of aldehydes


  1. Select the longest chain containing the carbonyl group

  2. Name the parent chain by changing the "-e" to "-al"

  3. Number the parent chain by assigning the number 1 to carbonyl carbon atom

  4. Determine other substituents present and their corresponding locants

  
• Linear aldehydes


  • Pentanal
  • 2-ethylpentanal
  • 3-hydroxypentanal

  
• Cyclic aldehydes


  • The suffix is changed to "carbaldehyde"
  • -CHO is carbon 1 and other substituents would be given the lowest possible locants

  
• Common name prefixes for aldehydes


  • Meth-
  • Eth-
  • Prop-
  • But-
  • Pent-
  • Hex-
  • Hept-
  • Oct-
  • Non-
  • Dec-

  
• IUPAC nomenclature of ketones

  1. Select the longest chain containing the carbonyl group

  2. Name the parent chain by changing the "-e" to "-one"

  3. Number the chain such that the carbonyl carbon atom receives the lowest possible number

  4. Determine other substituents present and the corresponding locants

  
• Cyclic ketones


  • Assign the number 1 to the carbon atom bearing the carbonyl group
  • The ring is numbered as to give the lowest number/s to atom/s bearing the substituents

  
• Physical properties of aldehydes & ketones

  • Biochemistry: Aldoses are carbohydrates with -C=O, Ketoses are carbohydrates with -C=O-
  • Boiling point: Intermediate between ROH and alkanes, Cannot form hydrogen bonds with each other, Polar in nature due to electronegative oxygen atom
  • Solubility: Lower mass are soluble in water, ≥ 5 C atoms = solubility markedly decreases
  • Odor: Lower MM = disagreeable odor, Increased MM = odor becomes more fragrant

  
• Aldehydes in perfumes
• Oxidation of aldehydes & ketones

  1. Aldehydes take rapidly at RT, Reversible addition of water to carbonyl group → intermediate hydrates

  2. Ketones have no reaction due to absence of hydrogen

  
• Reduction of aldehydes & ketones

  1. Nucleophilic attack by hydride ion on the slightly positive carbon atom

  2. The electron lone pairs on the hydride ion forms a bond with carbon

  3. in 1 of the C-O bonds are repelled onto the oxygen → negative charge of oxygen

  4. Addition of acid: Negative ion formed picks up H+ → ROH

  5. Addition of water: Negative ion takes a H+ from H2O → ROH

  
• Nucleophilic addition reaction


  • A nucleophile is added to the electrophilic carbon of the carbonyl group
  • Occur under basic or acidic conditions

  
• Alcohol formation


  1. Reaction with hydride reagents

  2. Reaction with Grignard reagents

  
• Hydrate formation


  Addition of water to form carbonyl hydrate/germinal diols

  
• Acetal formation


  1. Hemiacetal: Compound where carbon atom is bonded to both –OH group and an –OR group

  2. Acetal: Compound with a carbon atom bonded to 2 –OR group

  
• Imine formation


  Addition of amines
• Fehling's test


  Fehling's reagent A: Blue in color, contains copper sulfate

  Fehling's reagent B: Clear and colorless, contains potassium sodium tartrate and a strong alkali

  Principle: Oxidation, Tartrate ions prevent formation of insoluble Cu(OH)2, Cu2+ is reduced to insoluble Cu2O ions

  Positive result: Presence of yellow or brick red precipitate due to Cu2O
• Tollen's test / Silver-mirror test


  Tollen's reagent: Silver nitrate mixed with ammonia in water

  Principle: Oxidation, Silver oxide reacts with ammonia to form [Ag(NH3)2]- complex which oxidizes the aldehyde group

  Positive result: Presence of dark grey precipitate or silver mirror due to reduction of silver ion to metallic silver
• Benedict's test


  Benedict's reagent: Copper sulfate, sodium carbonate, sodium citrate

  Principle: Oxidation, Reducing sugars form enediols which reduce Cu2+ to insoluble Cu2O

  Positive result: Presence of brick red precipitate due to Cu2O
• Reaction with potassium permanganate
• Glucose determination (POCT)
• Diabetic ketoacidosis