chapter 16 Ochem

Organic Chemistry - Aldehydes and Ketones

Common Aldehydes and Ketones

• Formaldehyde (HCHO)
  

  

• Acetaldehyde (CH3CHO)
  

  

• Propanal (C3H6O)
  

  

• Benzaldehyde (C6H5CHO)
  

  

Formulae for Aldehydes and Ketones

• General Structure of Aldehydes:
  $RCHO$
  where R is an alkyl or aryl group.

• General Structure of Ketones:
  $RCOR'$
  where R and R' are alkyl or aryl groups.

Aldehyde Nomenclature

• Methanal (Formaldehyde) - HCHO

• Ethanal (Acetaldehyde) - CH3CHO

• Propanal (Propionaldehyde) - CH3CH2CHO

• 5-Chloropentanal - Cl attached at 5th position.

• Phenylethanal (Phenylacetaldehyde) - C6H5CH2CHO

• Benzenecarbaldehyde (Benzaldehyde) - C6H5CHO

Naming Aldehydes as a Group

• The prefix formyl is used to indicate the presence of an aldehyde group.

• Examples:

  • Cyclohexanecarbaldehyde

  • 2-Naphthalenecarbaldehyde

Ketone Nomenclature

• Butanone (Ethyl methyl ketone) - C4H8O

• 2-Pentanone (Methyl propyl ketone) - C5H10O

• Pent-4-en-2-one (Not 1-penten-4-one, allyl methyl ketone) - C5H8O

Ketones with Common Names

• Acetone (Propanone) - C3H6O

• Acetophenone (1-phenylethanone) - C8H8O

• Benzophenone (Diphenyl methanone) - C13H10O

Physical Properties

• 
  

• Table summarizing *physical properties of selected aldehydes and ketones, example:
  

  Compound	Formula	mp (°C)	bp (°C)	Solubility in Water
  Formaldehyde	HCHO	-92	-21	Very soluble
  Acetaldehyde	CH3CHO	-125	21	Very soluble
  Propanal	CH3CH2CHO	-81	49	Very soluble
  Acetone	CH3COCH3	-95	56.1	Very soluble
  Benzaldehyde	C6H5CHO	-26	178	Slightly soluble
  Aldehydes and Ketones Accept H-Bonds

  • Acetone can form hydrogen bonds with water.

  Oxidation of Primary Alcohols

  • Reaction Steps:

    1. (1) DMSO, (COCl)2

    2. (2) Et3N

  • Produces Aldehyde from Primary Alcohol (R-OH).

  Examples of Primary Alcohol Oxidation

  • Benzyl alcohol oxidation to Benzaldehyde

  • 1-Heptanol oxidation to Heptanal

  Alkene Ozonolysis to Aldehydes and Ketones

  • Reaction Steps:

    1. Ozone ($O3$)

    2. Dimethyl sulfide ($Me2S$)

  Mechanisms of Reduction

  • DIBAL-H for selective reduction of esters to aldehydes

  • Mechanism for acid chloride reduction

    • Stepwise reduction with lithium tri-tert-butoxy aluminum hydride is used.

  Nucleophilic Addition to Aldehydes and Ketones

  • General reaction example: $RCHO + Nu-H <br>ightarrow R-C(OH)-R'$

  • Formation of hemiacetals or cyanohydrins.

  Electronic Considerations

  • Aldehydes are more reactive than ketones due to less steric hindrance and more positive charge density at the carbonyl carbon.

  Acetal Formation Mechanism

  • Formation involves nucleophilic attack and subsequent loss of water, producing an acetal from an aldehyde or ketone with alcohol.

  Common Threads in Organic Synthesis

  • Reactions involving organometallic compounds, alcohols, amines, hydrides, etc., are essential for forming products such as imines, aldehydes, and ketones.

  Summary

  • Various reaction mechanisms emphasizing nucleophilic addition reactions and condensation reactions of aldehydes and ketones are fundamental in organic synthesis.

  • Several notable reactions include aldol condensation, Wittig reactions, and Baeyer-Villiger oxidations which showcase the versatility of these functional groups in synthetic organic chemistry.