Module 8 Carbonyl Compounds

• Carbonyl compounds

  • contain a carbon bonded to oxygen with a double bond.
• The polarity of the carbonyl group

  • greater than compounds containing carbon atom linked to oxygen with a single bond.
• Carbonyl Compounds: Physical Properties

  • Although the aldehydes and ketones are highly polar molecules, they don't have any hydrogen atoms attached directly to the oxygen, and so they can't hydrogen bond with each other.
• Boiling Point

  • The boiling point of aldehydes and ketones is higher than that of non-polar compounds (hydrocarbons) but lower than those of corresponding alcohols and carboxylic acids as aldehydes and ketones do not form H-bonds with themselves.
• Water Solubility

  • The lower members (up to 4 carbons) of aldehydes and ketones are soluble in water due to H-bonding.
  • Increase in molecular weights or increase number of carbons, Decrease water solubility
• Physical Properties

  • Aldehydes and ketones are soluble in organic solvents and, in general, are less dense than water.
• Synthesis of Ketone from ROH

  • Heating secondary alcohol propan-2-ol with sodium or potassium dichromate(VI) solution acidified with dilute sulphuric acid.
• Chemical Reaction: Oxidation

  • Aldehydes can be easily oxidised to carboxylic acids due to the presence of a hydrogen atom on carbonyl group which can be easily converted to OH group.
  • Since ketones do not have any hydrogen atom attached to the carbonyl group, they cannot be oxidised easily.
• General Test for Carbonyl Group:

  • 2,4-dinitrophenylhydrazine Test
  • Sodium bisulfite Test
• 2,4-dinitrophenylhydrazine

  • often abbreviated to 2,4-DNP or 2,4-DNPH
• 2,4-dinitrophenylhydrazine

  • solution of mixture of methanol and sulfuric acid is known as Brady's reagent
  • can be used to qualitatively detect the carbonyl group of an eldehyde or ketone.
• 2,4-dinitrophenylhydrazine Test

  • A positive result is indicated by the formation of an yellow or orange-red precipitate of 2,4-dinitrophenyl hydrazone.
  • The color of the precipitate may give evidence for the amount of conjugation present in the original carbonyl.
• Sodium Bisulfite Test

  • Most aldehydes and ketones give bisulphate addition product with sodium bisulphate, which is white crystalline in nature

  Note: Acetone phenone and benzophenone do not give this test.
• Differentiating Tests for Aldehydes

  • Schiff’s Test
  • Tollens Test
• Schiff’s Test

  • Aldehydes give pink or magenta colour with Schiff’s reagent.
  • Schiff's reagent is made from pararosanilin treated with sulphurous acid

  Note: With benzaldehyde the pink colour developes slowly
• Tollen’s Test

  • ammoniacal silver nitrate.
  • Aldehydes react with Tollen’s reagent to form elemental silver, accumulated onto the inner surface of the reaction vessel, producing silver mirror on the inner surface of the vessel.
• Differentiating tests for Ketones

  • Sodium Nitroprusside Test
  • Iodoform Test
• Sodium Nitroprusside Test

  • The anion of the keton formed by a alkali reacts with nitroprusside ion to form a red coloured complex.
• Iodoform Test

  • A solution of iodine (I2)and iodide (I−) in NaOH can be used to test for methyl ketones or secondary alcohols adjacent to a methyl group.
  • This is a very specific test that will give a positive result (formation of a canary yellow precipitate) only for compounds with the structure RCH(OH)CH3 or RC=OCH3
• Drugs containing ketone substituents

  Griseofulvin

  • Antifungal and antibiotic

  Erythromycin

  • Antibiotic

  Tolcapone

  • A catechol-O-methyltransferase (COMT) inhibitor used as adjunct therapy in the symptomatic management of idiopathic Parkinson's disease.
• Aldehyde

  Glutaraldehyde

  • Disinfectant
  • Anti-Infective
  • Cell fixative