Carbonyl compounds

• Product of oxidation of aldehydes

  Carboxylic acid
• Conditions/reagents for oxidation of aldehydes


  Reflux, acidified potassium dichromate
• Can ketones be oxidised?


  No
• How to get alcohols from aldehydes/ketones


  Warm with NaBH4
• What is happening to aldehydes/ketones when being warmed with NaBH4?

  Reduced
• Symbol for reducing agent

  [H]
• Kind of alcohol formed from nucleophilic addition of aldehydes with NaBH4


  Primary
• Kind of alcohol formed from nucleophilic addition of ketones with NaBH4

  Secondary
• Kind of reaction in the reduction of aldehydes/ketones with NaBH4

  Nucleophilic addition
• Product of nucleophilic addition of aldehydes/ketones with HCN

  Hydroxynitriles
• Alternative to HCN


  NaCN/H+
• Reason for using NaCN/H+ rather than HCN

  HCN boils slightly above room temperature and is extremely poisonous
• Why is the reaction between HCN and aldehydes/ketones useful?

  Extends carbon chain
• Kind of reaction between HCN and aldehydes/ketones

  Nucleophilic addition
• Mechanism in words for reaction between aldehydes/ketones and NaBH4


  Lone pair of electrons on hydride ion attracted to partially positive carbon, dative covalent bond forms between hydride ion and carbon atom, pi bond in C=O bond breaks by heterolytic fission, oxygen atom of intermediate donates pair of electrons to a hydrogen atom in a water molecule
• Mechanism in words for reaction between aldehydes/ketones and NaCN

  Lone pair of electrons from the cyanide ion attracted and donated partially positive carbon, dative covalent bond forms between the two carbons, pi bond in C=O breaks by heterolytic fission, donates lone pair of electrons to a hydrogen ion
• Uses of 2,4-DNP


  To detect the presence of a carbonyl group, to identify a carbonyl group from the melting point of the derivative
• Other name for 2,4-DNP

  Brady's Reagent
• Colour of precipitate formed from 2,4-DNP


  Yellow, orange
• How to do test for carbonyl groups

  5cm depth of 2-4-DNP in a clean test tube, add three drops of unknown compound, add sulfuric acid if no crystals form
• How to identify an aldehyde or ketone by melting point

  2,4-DNP test carried out, orange solid filtered off to separate the solid precipitate, solid recrystallised, melting point measured and recorded, compared to database
• How to get a sample of 2,4-dinitrophenylhydrazone

  Buchner filtration, recrystallisation
• How to do Buchner filtration

  Put sample in Buchner filter, part way through suction add about 1cm3 of CH3OH
• How to do recrystallisation

  Put crystals from filtration into boiling tube in a water bath, add hot ethanol from a test tube which is also in the water bath until the precipitate dissolves, cool in an ice-water mixture until crystals form, re-filter, wash with ethanol to cleanse of contaminated solvent, press crystals between 2 wads of filter paper, place on dry filter paper near a Bunsen burner
• How to choose a solvent for recrystallisation

  Solute must be soluble in it when it is hot but not when it is cold
• How to record the melting point for a sample of 2,4-dinitrophenylhydrazone

  Make a melting point tube, put crystals in, attach to a thermometer, place tube and thermometer and stirrer in a boiling tube, half-fill with dibutyl phthalate, put above a micro burner, stir constantly by moving the stirrer up and down, record the temperature at which melting is observed
• Use of Tollens' Reagent


  To identify presence of an aldehyde group, to distinguish between aldehydes and ketones
• Positive test for an aldehyde group with Tollens' Reagent

  Silver mirror
• Why does the silver mirror form with Tollens' Reagent?


  Silver ions are reduced to make silver
• Equations for reaction between aldehydes and Tollens' Reagent


  Ag+ + e- -> Ag, CH2O + [O] -> CHOOH
• Type of reaction between aldehydes and Tollens' Reagent


  Redox
• How to make Tollens' Reagent


  Use a clean test tube, add aqueous silver nitrate, add sodium hydroxide until brown precipitate of Ag2O forms, add dilute ammonia solution until brown precipitate dissolves
• How to test for an aldehyde group using Tollens' Reagent


  Make the reagent, put the unknown compound in a clean test tube, add the reagent, leave in a beaker of warm water
• Why the Tollens' Test works

  Aldehydes can be further oxidised to make carboxylic acids, ketones can't
• Why can carbonyl compounds undergo addition reactions?

  They have a carbon-oxygen double bond so are unsaturated
• Temperature of water bath in Tollens' Test

  50 degrees celsius
• Why doesn't the C=O react with electrophiles?

  The bond is polarised, electrophiles repelled by the partially positive carbon