Lecture 6

• Aldehydes


  Compounds with general formula RCHO
• Ketones


  Compounds with general formula RCOR'
• Nomenclature of aldehydes and ketones

  • Select the longest continuous carbon chain
  • Start numbering the carbon from the end that has the functional group
  • Substituents are named in alphabetical orders
  • Whenever, there are more than one substituent of the same type, use di, tri, tetra or penta
• Polar molecules


  Molecules that have a net dipole moment
• Aldehydes and Ketones are polar molecules because there is charge separation
• Hydrogen bonding


  Aldehydes and ketones don't have an H atom attach to the carbonyl oxygen, hence they cannot function as H-bond donors to one another or other molecules such as H2O and alcohols. They can only function as H-bond acceptor.
• As the hydrocarbon chain length increases

  Hydrophobicity increases and hence aldehydes and ketones become less soluble in water
• Preparation of aldehydes and ketones from alcohols

  1. Oxidation of primary and secondary alcohols

  2. Use of mild oxidants like PCC to stop at aldehyde stage

  3. Swern oxidation to oxidise primary alcohols to aldehydes
• Preparation of aldehydes and ketones from alkenes

  Ozonolysis of alkenes to yield aldehydes and ketones
• Nucleophilic addition


  Addition of a nucleophile and proton across the C=O double bond
• Aldehydes are more reactive than ketones towards nucleophilic addition due to both steric and electronic effects
• Reaction of aldehydes and ketones with Grignard reagents

  1. RMgX + HCHO -> RCH2OH + MgBrOH

  2. RMgX + RCHO -> RCHR'OH + MgBrOH

  3. RMgX + R2C=O -> RCR'R''OH + MgBrOH
• Reaction of aldehydes with HCN

  1. Yields a b-hydroxycyano compound

  2. b-hydroxycyano compound can be hydrolysed to yield a b-hydroxycarboxylic acid
• Clemmensen reduction


  Reduces the carbonyl group of aldehydes and ketones to a methylene unit using Zn/Hg/HCl
• Wolff-Kishner reduction


  Reduces the carbonyl group of aldehydes and ketones to a methylene unit using hydrazine and a base
• Reaction of aldehydes and ketones with water


  Yields a dihydroxy compound
• Reaction of aldehydes with alcohols


  Forms hemi-acetals and acetals
• Reaction of aldehydes and ketones with ammonia and ammonia derivatives

  1. Forms a condensation product called a carbinolamine

  2. Carbinolamine on dehydration yields a Schiff's base type compound
• Oxime


  Condensation product of an aldehyde or ketone with hydroxylamine
• Hydrazone


  Condensation product of an aldehyde or ketone with hydrazine
• Phenylhydrazone


  Condensation product of an aldehyde or ketone with phenylhydrazine
• Reaction of aldehydes with Tollens reagent

  Silver ion, Ag+ oxidises aldehydes selectively in a convenient functional group test for aldehydes
• Formaldehyde


  A famous aldehyde that is a gas

  
• Formaldehyde


  • Makes formalin with the 40% solution in water
  • Used for preserving biological specimens
  • Used in the tanning, embalming, and preparing glues, and polymeric products such as insecticides, germicides, and fungicides for plants
  • Used in photography and drug testing
  • Reacts with phenol to form Bakelite used in coatings, adhesives, and plastics
• Acetaldehyde


  Used for the production of pyridine derivatives, and acetic acid
• Benzaldehyde


  • Used in cosmetic products, dyes, and perfumes
  • Added to food products to give almond flavor
  • Used as bee repellent
• Many aldehydes naturally occur as flavoring agents such as benzaldehyde, vanillin, and the oil of cinnamon
• Many aldehydes have various essential functions in the bodies of human beings and other living organisms
• Acetone


  • The most common ketone, an ideal solvent for many synthetic fibers and plastics
  • Used in households as paint thinner and nail polish remover
  • Used in medicine for acne treatment and chemical peeling
• Methyl ethyl ketone (butanone)


  • Commonly used solvent, used for production of plastics, varnishes, textiles, paraffin, wax, and paint remover
  • Used for welding of plastics due to its dissolving properties
• Cyclohexanone


  Primarily used for the production of nylon
• Ketones are produced on a massive scale as industrial solvents, pharmaceuticals, and polymer precursors
• Cyclohexanone and methylethylketone are commonly used in biochemistry
• Products of ketones are used in paints and perfumes, for stabilizing ingredients and to avoid degradation
• Pharmaceutical compounds


  • Tetracyclines
  • Aspirin
  • Praziquantel
  • Corticosteroids