Carbonyls

• • The carbonyl group is a carbon double bonded to an oxygen ( c==o)
  • Due to the electronegativity of oxygen its a polar bond
• • Aldehydes and ketones are examples of carbonyl compounds
  • Aldehyde --> -al
  • Ketone --> -one
• • The double bond is formed by the overlap of p-orbital electrons to form a pi bond above the atoms
• Forming carbonyls:

  • Carbonyl compounds are formed as the result of partial or complete oxidation of alcohols
  • Acidified dichromate ions are used as the oxidising agent
• Primary alcohols:

  • Partial oxidation (distillation) forms aldehydes -CHO
  • Complete oxidation (reflux) forms carboxylic acids -COOH
• Secondary alcohols:

  • Complete oxidation (reflux) forms ketones -RCOR
• • When aldehydes are oxidised, the orange Cr2O7 2+ ions are reduced forming green Cr3+ ions
  • As ketones are difficult to oxidise there is no colour change
• Brady's reagent:

  • A positive result is a yellow/orange precipitate forming, confirming the presence if a carbonyl
• • Tollen's reagent is a weak oxidising agent so can only oxidise aldehydes to form a carboxylic acid and silver metal
  • Positive result is a silver mirror Ag+(aq) + e- --> Ag(s)
• Reduction:

  • Aldehydes and ketones can be reduced to their respective alcohols with a reducing agent
  • Ethanal to ethanol: CH3CHO + 2[H] --> CH3CH2OH
• Nucleophilic addition:

  • NaBH4 releases hydride ions (H-) that acts as nucleophiles & will attack the electron deficient carbon on the C=O bond
  • Donating a pair of electrons making a dative bond
  • At the same time the pi bond of the carbonyl breaks
  • The intermediate anion that forms donates a pair of electrons to a hydrogen on a water molecule creating a bond, whilst at the same time the OH bond breaks
  • The anion can also donate electrons to an H+ ion from an acid
  • The final product will then be the respective alcohol & a hydroxyl ion

  
• Cyanide:

  The cyanide ion ( :CN-) can also act as a nucleophile & reduce carbonyl groups so the carbon contains:

  • A hydroxyl group (-OH)
  • A nitrile group (-CN)
  • Hydrogen cyanide can be used to provide the nucleophile & the hydrogen ion for the hydroxyl group
  • The reaction rate is slow, so KCN and an acid is often used instead

  
• • The carbon atom in the C=O double bond is electron deficient & attracts nucleophiles
  • Aldehydes & ketones both react by nucleophilic addition to form alcohols
• What ion does NaBH4 contain that acts as a nucleophile?


  Hydride ion (:H-)
• How does the hydride ion (:H-) interact with the carbon atom in the aldehyde or ketone?


  The lone pair of electrons from the hydride ion is attracted to the delta positive carbon atom.
• What type of bond is formed between the hydride ion and the carbon atom of the C=O double bond?


  A dative covalent bond is formed.
• What happens to the pi bond in the C=O double bond during the reaction?


  The pi bond breaks by heterolytic fission.
• What is formed as a result of the heterolytic fission of the C=O double bond?


  A negatively charged intermediate is formed.
• What does the oxygen atom of the intermediate do during the reaction?


  The oxygen atom donates a lone pair of electrons to a hydrogen atom in a water molecule.
• What is the final product after the intermediate has been protonated?


  An alcohol is formed.
• What is the overall mechanism of the reaction with NaBH4?


  1. Hydride ion (:H-) acts as a nucleophile.
  2. Lone pair from :H- is attracted to delta positive carbon in C=O.
  3. Dative covalent bond forms between :H- and carbon.
  4. Pi bond in C=O breaks by heterolytic fission, forming a negatively charged intermediate.
  5. Oxygen in the intermediate donates electrons to a hydrogen in water.
  6. Intermediate is protonated to form an alcohol.
• Carboxylic acids:

  • Carboxylic acids contain the -COOH functional group and are prepared by the oxidation of primary alcohols
  • Given the suffix ' -oic acid'
• • Carboxyl groups don't act the same way as aldehydes & ketones due to the OH group attached to the C=O group
• Properties:

  • As the C=O & OH groups are both polar carboxylic acids experience dipole-dipole interactions, as well as being able to form hydrogen bonds, so have high boiling points
  • The ability to form hydrogen bonds allow them to dissolve in water, but their solubility decreases as the chain increases
  • In the absence of water they can form hydrogen bonds with other carboxylic acids to form dimers
• • Carboxylic acids are also able to partially dissociate the hydrogen from the OH bond, so can acts as weak acids
  • Proton donators
  • The remaining compound will form the negative ion that can accept a metal ion to form a salt (a carboxylate)
• Solubility of carboxylic acids:

  • Carboxylic acids with up to 4 carbon atoms are soluble in water
  • As the number of carbon atoms increase, the solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule
• Acid reactions of carboxylic acids:

  • Redox reactions with metals
  • Neutralisation reactions with bases (alkali, metal oxides, and carbonates)
  • In the reactions, carboxylic acids form carboxylate salt
  • The carboxylate ion in the salt is named by changing the -oic acid ending of the carboxylic acid to -ate
• Redox reactions of carboxylic acids with metals:

  • Aqueous solutions of carboxylic acids react with metal in a redox reaction to form hydrogen gas & the carboxylate salt
  • Would observe metal disappearing & effervescence as hydrogen gas is evolved
• • Carboxylic acids react with all bases - metal oxides, alkali and carbonates
  • These reactions are examples of neutralisation
  • Carboxylic acids react with metal oxides to form a salt and water
• • Carboxylic acids react with alkalis to form a salt
  • The 2 solutions react together to form an aqueous solution of the salt
  • Overall: CH3COOH + NaOH --> CH3COO-Na+ + H2O
  • Ionic: H+ + OH- --> H2O
• • When a carbonate is added to a carboxylic acid, carbon dioxide gas is evolved
  • If the carboxylic acid is in excess a solid carbonate would disappear
  • 2CH3COOH + Na2CO3 ---> 2CH3COO-Na+ + H2O + CO2
• Esters:

  • Contain functional group -COO (an ester linkage)
  • Formed by reacting an alcohol with a carboxylic acid in the presence of a conc H2SO4 acid --> known as esterfication ( a condensation reaction)
  • To name an ester the alcohol becomes the prefix and the carboxylic acid gains the -oate suffix
  • Propanoic acid + ethanol --> Ethyl propanoate + water
  • Ester often have pleasant aromas, so are often used as perfumes or as flavourings

  
• Acid anhydrides:

  • Acid anhydrides are dehydrated carboxylic acids
  • Acid anhydrides react with alcohols to form esters --> only gentle heating is required
  • Not a reversible reaction

  
• Ester hydrolysis:

  • Esters can be broken down into their constituent alcohols & carboxylic acids by hydrolysis --> splitting with water
  • This can be done by refluxing the ester under acidic conditions ( dilute HCl or H2SO4) to form an alcohol & a carboxylic acid
• • Ester hydrolysis can be done by refluxing the ester under alkaline conditions (NaOH) to form an alcohol & a carboxylate salt
  • The acid formed reacts with the hydroxide to form a salt

  
• Acyl chlorides:

  • An acyl chloride is named after the parent carboxylic acid from which it is derived
  • To name an acyl chloride, remove the -oic acid suffix from the parent carboxylic acid & replace with -oyl chloride

  
• • Acyl chlorides can be prepared directly from their parent carboxylic acid by reaction with thionyl chloride, SOCl2
  • The other products of this reaction, SO2 and HCl are evolved as gases, leaving just the acyl chloride
  • The reaction produces harmful products

  
• • Acyl chlorides are very reactive & very useful in organic synthesis as they can be easily converted into carboxylic acid derivatives, such as esters & amides, with good yields
  • Acyl chlorides react with nucleophiles by losing the chloride ion whilst retaining the c==o double bond
• • Acyl chlorides react with alcohols to form esters

  
• Reaction of acyl chlorides with phenols to form esters:

  • Carboxylic acids aren't relative enough to form esters with phenols
  • Acyl chlorides & acid anhydrides are both much more reactive than carboxylic acids & react with phenols to produce phenyl esters
  • Neither reaction needs an acid catalyst