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CONTENTS Prior knowledge Bonding in carbonyl compounds Structural differences

ALDEHYDES & KETONES. CONTENTS Prior knowledge Bonding in carbonyl compounds Structural differences Nomenclature Preparation Identification Oxidation Nucleophilic addition Reduction 2,4-dinitrophenylhydrazine. ALDEHYDES & KETONES.

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CONTENTS Prior knowledge Bonding in carbonyl compounds Structural differences

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  1. ALDEHYDES & KETONES • CONTENTS • Prior knowledge • Bonding in carbonyl compounds • Structural differences • Nomenclature • Preparation • Identification • Oxidation • Nucleophilic addition • Reduction • 2,4-dinitrophenylhydrazine

  2. ALDEHYDES & KETONES • Before you start it would be helpful to… • know the functional groups found in organic chemistry • know the arrangement of bonds around carbon atoms • recall and explain the polarity of covalent bonds

  3. CARBONYL COMPOUNDS - BONDING Bondingthe carbon is sp2 hybridised and three sigma (s) bonds are planar PLANAR WITH BOND ANGLES OF 120°

  4. CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these P ORBITAL PLANAR WITH BOND ANGLES OF 120°

  5. CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond P ORBITAL PLANAR WITH BOND ANGLES OF 120°

  6. CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond P ORBITAL ORBITAL OVERLAP PLANAR WITH BOND ANGLES OF 120°

  7. CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond P ORBITAL ORBITAL OVERLAP PLANAR WITH BOND ANGLES OF 120° NEW ORBITAL

  8. CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond as oxygen is more electronegative than carbon the bond is polar P ORBITAL ORBITAL OVERLAP PLANAR WITH BOND ANGLES OF 120° NEW ORBITAL

  9. CH3 H C = O C = O H H CARBONYL COMPOUNDS - STRUCTURE Structurecarbonyl groups consists of a carbon-oxygen double bond the bond is polar due to the difference in electronegativity DifferenceALDEHYDES - at least one H attached to the carbonyl group

  10. CH3 CH3 C2H5 H C = O C = O C = O C = O CH3 H CH3 H CARBONYL COMPOUNDS - STRUCTURE Structurecarbonyl groups consists of a carbon-oxygen double bond the bond is polar due to the difference in electronegativity DifferenceALDEHYDES - at least one H attached to the carbonyl group KETONES - two carbons attached to the carbonyl group

  11. H O H H H H H C C C H H C C C O H H H H CH3 C2H5 C = O C = O CH3 H O O CARBONYL COMPOUNDS - FORMULAE MolecularC3H6O StructuralC2H5CHOCH3COCH3 Displayed Skeletal

  12. CARBONYL COMPOUNDS - NOMENCLATURE AldehydesC2H5CHO propanal KetonesCH3COCH3 propanone CH3CH2COCH3 butanone CH3COCH2CH2CH3 pentan-2-one CH3CH2COCH2CH3 pentan-3-one C6H5COCH3 phenylethanone

  13. CARBONYL COMPOUNDS - FORMATION ALDEHYDES Oxidation of primary (1°) alcoholsRCH2OH + [O] ——> RCHO + H2O beware of further oxidationRCHO + [O] ——> RCOOH Reduction of carboxylic acids RCOOH + [H] ——> RCHO + H2O KETONES Oxidation of secondary (2°) alcoholsRCHOHR + [O] ——> RCOR + H2O

  14. CARBONYL COMPOUNDS - IDENTIFICATION Method 1strong peak around 1400-1600 cm-1 in the infra red spectrum Method 2formation of an orange precipitate with 2,4-dinitrophenylhydrazine Although these methods identify a carbonyl group, they cannot tell the difference between an aldehyde or a ketone. To narrow it down you must do a second test.

  15. CARBONYL COMPOUNDS - IDENTIFICATION Differentiationto distinguish aldehydes from ketones, use a mildoxidising agent Tollen’s Reagentammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+ the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s) the test is known as THE SILVER MIRROR TEST

  16. CARBONYL COMPOUNDS - IDENTIFICATION Differentiationto distinguish aldehydes from ketones, use a mildoxidising agent Tollen’s Reagentammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+ the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s) the test is known as THE SILVER MIRROR TEST Fehling’s Solutioncontains a copper(II) complex ion giving a blue solution on warming, it will oxidise aliphatic (but not aromatic) aldehydes the copper(II) is reduced to copper(I) a red precipitate of copper(I) oxide, Cu2O, is formed The silver mirror test is the better alternative as it works with all aldehydes Ketones do not react with Tollen’s Reagent or Fehling’s Solution

  17. CARBONYL COMPOUNDS - CHEMICAL PROPERTIES OXIDATION • provides a way of differentiating between aldehydes and ketones • mild oxidising agents are best • aldehydes are easier to oxidise • powerful oxidising agents oxidise ketones to a mixture of carboxylic acids ALDEHYDESeasily oxidised to acids RCHO(l) + [O] ——> RCOOH(l) CH3CHO(l) + [O] ——> CH3COOH(l) KETONESoxidised under vigorous conditions to acids with fewer carbons C2H5COCH2CH3(l) + 3 [O] ——> C2H5COOH(l) + CH3COOH(l)

  18. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Mechanismoccurs with both aldehydes and ketones involves addition to the C=O double bond unlike alkenes, they are attacked by nucleophiles attack is at the positive carbon centre due to the difference in electronegativities alkenes are non-polar and are attacked by electrophiles undergoing electrophilic addition Group Bond Polarity Attacking species Result ALKENES C=C NON-POLAR ELECTROPHILES ADDITION CARBONYLS C=O POLAR NUCLEOPHILES ADDITION

  19. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Reagenthydrogen cyanide - HCN (in the presence of KCN) Conditionsreflux in alkaline solution Nucleophilecyanide ion CN¯ Product(s)hydroxynitrile (cyanohydrin) EquationCH3CHO + HCN ——> CH3CH(OH)CN 2-hydroxypropanenitrile NotesHCN is a weak acid and has difficulty dissociating into ions HCN H+ + CN¯ the reaction is catalysed by alkali which helps produce more of the nucleophilic CN¯

  20. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O STEP 1

  21. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2 A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2

  22. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2 A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2

  23. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2 A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2

  24. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

  25. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Watch out for the possibility of optical isomerism in hydroxynitriles CN¯ attacks from above CN¯ attacks from below

  26. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Watch out for the possibility of optical isomerism in hydroxynitriles CN¯ attacks from above CN¯ attacks from below

  27. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

  28. CARBONYL COMPOUNDS - REDUCTION WITH NaBH4 • Reagentsodium tetrahydridoborate(III) (sodium borohydride), NaBH4 • Conditionsaqueous or alcoholic solution • MechanismNucleophilic addition (also reduction as it is addition of H¯) • NucleophileH¯ (hydride ion) • Product(s)Alcohols Aldehydes are REDUCED to primary (1°) alcohols. • Ketones are REDUCED to secondary (2°) alcohols. • Equation(s)CH3CHO + 2[H] ——> CH3CH2OH • CH3COCH3 + 2[H] ——> CH3CHOHCH3 • NotesThe water provides a proton • QuestionNaBH4 doesn’t reduce C=C bonds. WHY? • CH2 = CHCHO + 2[H] ———> CH2 = CHCH2OH

  29. CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN Reagenthydrogen Conditionscatalyst - nickel or platinum ReactiontypeHydrogenation, reduction Product(s)Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols. Equation(s) CH3CHO + H2 ——> CH3CH2OH CH3COCH3 + H2 ——> CH3CHOHCH3 NoteHydrogen also reduces C=C bonds CH2 = CHCHO + 2H2 ——> CH3CH2CH2OH

  30. CARBONYL COMPOUNDS - REDUCTION IntroductionFunctional groups containing multiple bonds can be reduced C=C is reduced to CH-CH C=O is reduced to CH-OH CN is reduced to CH-NH2 Hydrogen H• H2 H+ (electrophile) H¯ (nucleophile) ReactionsHydrogen reduces C=C and C=O bonds CH2= CHCHO + 4[H] ——> CH3CH2CH2OH Hydride ion H¯ reduces C=O bonds CH2 = CHCHO + 2[H] ——> CH2=CHCH2OH Explanation C=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯

  31. CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? COMPOUND X H2 NaBH4

  32. CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? COMPOUND X H2 NaBH4 C=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯

  33. 2,4-DINITROPHENYLHYDRAZINE Structure Usereacts with carbonyl compounds (aldehydes and ketones) used as a simple test for aldehydes and ketones makes orange crystalline derivatives - 2,4-dinitrophenylhydrazones derivatives have sharp, well-defined melting points also used to characterise (identify) carbonyl compounds. Identification / characterisation A simple way of characterising a compound (finding out what it is) is to measure the melting point of a solid or the boiling point of a liquid. C6H3(NO2)2NHNH2

  34. CHO CHO CHO Cl Cl Cl 2,4-DINITROPHENYLHYDRAZINE C6H3(NO2)2NHNH2 The following structural isomers have similar boiling points because of similar van der Waals forces and dipole-dipole interactions. They would be impossible to identify with any precision using boiling point determination. Boiling point 213°C 214°C 214°C Melting point of 2,4-dnph derivative 209°C 248°C 265°C By forming the 2,4-dinitrophenylhydrazone derivative and taking its melting point, it will be easier to identify the unknown original carbonyl compound.

  35. REVISION CHECK What should you be able to do? Recall the structure of and bonding in the carbonyl group Explain the difference in structure between aldehydes and ketones Recall the different response to oxidation of aldehydes and ketones Recalland understand the mechanism of nucleophilic addition Recall the products from the reduction of carbonyl compounds CAN YOU DO ALL OF THESE? YES NO

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