Carbonyl chemistry

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					    Carbonyl chemistry

-Production of carbonyl compounds
-Oxidation and Reduction reactions
    -Further oxidation reactions.
Formation of carbonyl compounds
• Starting with an alcohol, react with an
  oxidising agent (such as acidified potassium
  dichromate)




• Propan-1-ol + [O]  Propanal +water
          The carbonyl functional group



     The bond is polar due to the difference in electronegativity.


                              ORBITAL
PLANAR                        OVERLAP
WITH                                                             NEW
BOND                                                             ORBITAL
ANGLES
OF 120°
        Aldehydes vs. Ketones
• Aldehydes have the carbonyl group at the end
  of a carbon chain (joined to only one carbon)
         H               CH3
               C=O              C=O
         H                 H

• Ketones have the carbonyl group bonded to
  two carbons.
         CH3            C2H5
                C=O             C=O
         CH3              CH3
          Naming aldehydes/ketones
Aldehydes    C2H5CHO          propanal



Ketones      CH3COCH3         propanone

             CH3CH2COCH3      butanone

             CH3COCH2CH2CH3   pentan-2-one

             CH3CH2COCH2CH3   pentan-3-one

             C6H5COCH3        phenylethanone
      Identification of carbonyls
• IR spectroscopy shows a strong peak around
  1400-1600 cm-1
         Oxidation of carbonyl
• Reacting an aldehyde with an oxidising agent
  (acidified dichromate ions) produces a
  carboxylic acid.




• Propanal + [O]  Propanoic acid +water
        Reduction of carbonyls
• Carbonyl groups can also be reduced.
• Reduction agent used is Sodium
  tetrahydroborate(III) (also called Sodium
  Borohydride)




• This is classed as a weak reducing agent.
        Reduction of carbonyls
• Propanal is reduced to a primary alcohol by
  sodium borohydride.




• Propanal + 2[H]  Propan-1-ol
         Reduction continued
• NaBH4 reacts with ketones also.




• Propanone + 2[H]  propan-2-ol
          Reduction explained
• The BH4 ion is acting as a source of hydride
  ions (:H-)
           » Hydride ions being a negatively charged hydrogen ion
             containing a lone pair of electrons
• The mechanism is an example of nucleophilic
  addition.
  – H- ion is attracted to the δ+C of the carbonyl,
    forming a new C-H bond
  – The resulting O- ion forms a dative covalent bond
    using the H+ from a water molecule
  Reduction reaction mechanism




• Propanal + 2[H]  propan-1-ol
          Carbonyl questions
• Write equations + mechanisms for the
  reduction of:
  – Butanal
  – 2-methylhexan-3-one
• Define the term nucleophile
• Show the mechanism for the reaction
  between butanone and NaBH4
   Carbonyl chemistry 2
chemical tests on carbonyls
 -How to detect a carbonyl group
     -Aldehyde or ketone?
         ILPAC experiment 8.4
• Using the instruction sheets, follow the
  method for experiment 8.4 A and 8.4 B
• DO NOT DO EXPERIMENT C
• Make full and detailed notes on what you
  observe during the reactions.
       Condensation reactions
• We can detect the presence of a carbonyl
  group using 2,4-dinitrophenylhydrazine
  (abbreviated to 2,4-DNP or 2,4-DNPH).
• 2,4-DNPH is also
  called Brady’s reagent when
  prepared with methanol and
  sulfuric acid.
            Using 2,4-DNPH
• When Brady’s reagent is added to an aldehyde
  or ketone, a yellow or orange precipitate is
  formed.
• The precipitate, called a 2,4-
  dinitrophenylhydrazone derivative confirms
  the presence of a carbonyl functional group.
           Aldehyde or ketone
• Aldehydes and ketones can be distinguished
  from each other by using Tollen’s reagent
                            Ammoniacal silver nitrate



• Tollen’s reagent is a weak oxidising agent, the
  carbonyl group is oxidised to form a carboxylic
  acid.
                Tollen’s reagent.
• Silver ions are reduced to silver metal
     • Ag+(aq) + e-  Ag(s)




• Aldehyde + [O]  Carboxylic acid
         Carbonyl questions 2
• The carbonyl compounds CH3COCH3 and
  CH3CH2CHO are structural isomers.
  – Name these compounds
  – State the reagents used and observations made to
    prove the presence of a carbonyl group in these
    compounds
  – State the reagents and observations made to
    distinguish between the two chemicals.
      Carbonyl chemistry 3
Identifying a carbonyl compound
    -Determining experimentally an
     unknown carbonyl compound
                  ILPAC 8.5
• Using the instruction sheets given last lesson,
  carry out experiment 8.5, parts A and B


• Complete a detailed method for identifying a
  carbonyl compound
  – Purifying by recrystallisation -> melting point
    determination -> compare melting point to known
    values.
         Carbonyl questions 3
• Explain how you would use the solid obtained
  from the reaction of an aldehyde with 2,4-
  DNPH to prove the original aldehyde was
  butanal.

• The reaction of 2,4-DNPH with ethanal gives
  water as a product. Suggest the type of
  reaction taking place.

				
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posted:10/14/2011
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