Aldehydes and Ketones

• Aldehyde (formula + suffix)


  Suffix = -al e.g. propanal

  
• Ketone (formula + suffix)


  Suffix = -one e.g. propan-2-one

  
• What can aldehydes be oxidised to?


  carboxylic acids
• What can ketones be oxidised to?


  nothing, they cannot be oxidised
• How do you make Tollen's reagent?


  Silver nitrate + few drops of NaOH + few drops of ammonia until brown precipitate dissolves
• What is the formula of Tollen's reagent


  Tollen's reagent = [Ag(NH3)2]
• ve+ in Tollen's reagent?

  ve- in Tollen's reagent?


  ve+ = silver mirror

  ve- = no silver mirror
• Aldehyde + Tollen's reagent = ?

  Why?

  silver mirror

  Silver complex is reduced to silver

  Aldehyde is oxidised to carboxylic acid
• Ketone + Tollen's reagent = ?


  no reaction
• What are aldehydes reduced to?


  primary alcohols
• What are ketones reduced to?


  secondary alcohols
• What is the reducing agent in both these reactions:

  Aldehyde --> Primary Alcohol

  Ketone --> Secondary Alcohol

  NaBH4

  sodium tetrahydridoborate
• What is the name of the mechanism for this reaction?

  Nucleophillic addition
• Outline this mechanism (aldehyde --> primary alcohol)

  
• Outline this mechanism (ketone --> secondary alcohol)

  
• Aldehyde/Ketone + KCN (mechanism + product)

  Note: KCN has to be acidified

  hydroxynitrile

  
• Overall equation for aldehyde + acidified KCN


  CH3CH2CH2COH + KCN + H+ ---> CH3CH2CHC(CN)OH
• Other than safety reasons, why is KCN used over HCN (found this in an exam question and had no idea)


  KCN dissociates more than HCN
• outline Fehling's test


  Fehling = oxidising agent

  Will oxidise aldehydes but not ketones

  Fehlings contain Cu2+

  ve+ = brick red precipitate

  ve- = remains blue
• Issue with KCN


  Irritant (dangerous is inhaled/ingested)

  KCN + moisture can form very toxic gas HCN
• Enantiomer formation in this reaction


  - If we use an unsymmetrical ketone or an aldehyde (apart from methanol as it is symmetrical), enantiomers can be formed
• Confirm ethanal solution contains no ethanoic acid (aldehyde contains no carboxylic acid with steps)


  - Warm solution

  - Add Fehling's

  - Result: Brick Red solution

  - Confirm absence of acid by adding NaHCO3 (sodium hydrogen carbonate)

  - Result: No effervescence
• Describe experimental methods and conditions required to ensure acid is obtained in a high yield (propanoic acid from propan-1-ol)


  - Excess potassium dichromate

  - Heat under reflux

  (Draw apparatus)
• Describe the different experimental conditions necessary to produce propanol in high yield rather than propanoic acid


  Immediate distillation
• What functional group characterizes aldehydes?

  • CHO
• How are aldehydes produced?

  From oxidation and distillation of 1° alcohols
• What do aldehydes oxidize to in the presence of acidified potassium dichromate?

  Carboxylic acids
• Which reagents are used to test for aldehydes?

  Tollen’s reagent and Fehling’s solution
• What is the functional group that characterizes ketones?

  • C=O
• How are ketones produced?

  From oxidation of 2° alcohols

  
• What is the result of further oxidation of ketones?

  No further oxidation occurs
• What is the visible change of ketones with Tollen’s reagent and Fehling’s solution?

  No visible change occurs
• What can reverse the oxidation reactions of aldehydes and ketones?

  Reduction reactions

  
• What reducing agent is commonly used in the reduction of aldehydes and ketones?

  NaBH₄
• What type of reaction is the reduction of aldehydes and ketones?

  Nucleophilic addition

  
• What nucleophile does NaBH₄ provide in reduction reactions?

  H^- nucleophile
• What is required alongside the H^- nucleophile for the reaction to occur?

  H⁺ ion
• What is produced when nucleophilic addition occurs with the :CN^- nucleophile?

  Hydroxy-nitrile
• What does the reaction with the :CN^- nucleophile do to the carbon chain?

  Extends it by one carbon atom
• What is often used as the reagent to provide the nucleophile in hydroxy-nitrile synthesis?

  KCN