Aldehyde and ketones

• If the c=o is at the end of the chain with a H attached, it is an aldehyde

  name ends in -al
• If the c=o is in the middle of the chain it is a ketone

  ends in -one
• Smaller carbonyls are soluble in water as they can form hydrogen bonds with water
• Carbonyl is functional group with formula c=o
• Pure carbonyls cannot hydrogen bond to themselves but are attracted by permanent dipole forces
• c=c bond isn’t as strong as c=o so doesn’t undergo addition reactions easily

  attracts electrophiles instead
• Potassium dichromate K2Cr2O7 is oxidising agent for aldehydes and alcohols
• Primary alcohol oxidised to aldehyde—> carboxylic acid
• Secondary alcohol oxidised to ketone
• tertiary alcohol does not oxidise
• oxidation of aldehyde

  reaction: aldehyde --> carboxylic acid

  reagent: potassium dichromate acidified with dilute sulfuric acid

  conditions: heat under reflux
• aldehyde oxidation equation

  RCHO + [O] --> RCO2H
• in oxidisation reaction you will observe orange dichromate ion reduce to green Cr3+ ion
• Tollens reagent is made from aqueous ammonia and silver nitrate

  produces complex ion [Ag(NH3)2]+
• Tollens reagent test

  add reagent and heat gently

  only aldehydes oxidised into carboxylic acid, see silver mirror coating when silver ions reduced to silver atom

  no change for ketones
• Fehlings solution test

  Solution contains blue Cu2+ ions, heat gently

  only aldehydes oxidised, copper ions reduced to copper oxide, producing brick red precipitate

  no change for ketones
• reduction of carbonyls with NaBH4

  mechanism: nucleophilic addition

  reaction: aldehyde --> primary alcohol, ketone --> secondary alcohol

  reagent: NaBH4 in aqueous ethanol

  conditions: room temperature and pressure
• carbonyls undergo nucleophilic addition as NaBH4 contain a source of nucleophilic hydride ions (:H-) which are attracted to the positive carbon in the c=o bond
• carbonyls can also be reduced using catalytic hydrogenation

  reagent: hydrogen and nickel catalysts

  condition: high pressure
• reduction of carbonyl with CN

  mechanism: nucleophilic addition

  reagent: NaCN and dilute sulfuric acid

  conditions: room temperature and pressure
• reagents in reduction of carbonyl with CN

  NaCN supplies nucleophilic :Cn-ions

  H2SO4 supplies H+ ions for second step of mechanism
• when naming hydroxy nitriles CN becomes part of the main chain and the carbon n1
• We use NaCN instead of HCN as it is less toxic and has higher concentration of CN- ions

  this is because HCN is a weak acid and will only partially ionise
• nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the trigonal planar carbon is attacked from both sides by HCN

  there is equal chance of either enantiomer forming so racemate is produced with no optical activity