THE CHEMISTRY OF ALDEHYDES AND KETONES A guide for A level students KNOCKHARDY PUBLISHING

INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available. Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at... Navigation is achieved by... either clicking on the grey arrows at the foot of each page orusing the left and right arrow keys on the keyboard ALDEHYDES & KETONES KNOCKHARDY PUBLISHING

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

CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp 2 hybridised and three sigma (s) bonds are planar PLANAR WITH BOND ANGLES OF 120°

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

CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp 2 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 (  ) bond PLANAR WITH BOND ANGLES OF 120° P ORBITAL

CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp 2 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 (  ) bond PLANAR WITH BOND ANGLES OF 120° P ORBITAL ORBITAL OVERLAP

CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp 2 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 (  ) bond PLANAR WITH BOND ANGLES OF 120° P ORBITAL ORBITAL OVERLAP NEW ORBITAL

CARBONYL COMPOUNDS - BONDING Bonding the carbon is sp 2 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 (  ) bond as oxygen is more electronegative than carbon the bond is polar PLANAR WITH BOND ANGLES OF 120° P ORBITAL ORBITAL OVERLAP NEW ORBITAL

CARBONYL COMPOUNDS - STRUCTURE Structurecarbonyl groups consists of a carbon-oxygen double bond the bond is polar due to the difference in electronegativity Difference ALDEHYDES - at least one H attached to the carbonyl group C = O H CH 3 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 Difference ALDEHYDES - at least one H attached to the carbonyl group KETONES - two carbons attached to the carbonyl group C = O H CH 3 C = O H H CH 3 C = O C2H5C2H5 CH 3

CARBONYL COMPOUNDS - FORMULAE Molecular C 3 H 6 O Structural C 2 H 5 CHOCH 3 COCH 3 Displayed Skeletal C = O H C2H5C2H5 CH 3 H C C C H H O H H H C C C O H H H H O O

CARBONYL COMPOUNDS - NOMENCLATURE Aldehydes C 2 H 5 CHOpropanal Ketones CH 3 COCH 3 propanone CH 3 CH 2 COCH 3 butanone CH 3 COCH 2 CH 2 CH 3 pentan-2-one CH 3 CH 2 COCH 2 CH 3 pentan-3-one C 6 H 5 COCH 3 phenylethanone

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

CARBONYL COMPOUNDS - IDENTIFICATION Method 1 strong peak around cm -1 in the infra red spectrum Method 2 formation 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.

CARBONYL COMPOUNDS - IDENTIFICATION Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent Tollen’s Reagentammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not ketones contains the diammine silver(I) ion - [Ag(NH 3 ) 2 ] + the silver(I) ion is reduced to silver Ag + (aq) + e¯ ——> Ag(s) the test is known as THE SILVER MIRROR TEST

CARBONYL COMPOUNDS - IDENTIFICATION Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent Tollen’s Reagentammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not ketones contains the diammine silver(I) ion - [Ag(NH 3 ) 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, Cu 2 O, 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

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) CH 3 CHO(l) + [O] ——> CH 3 COOH(l) KETONESoxidised under vigorous conditions to acids with fewer carbons C 2 H 5 COCH 2 CH 3 (l) + 3 [O] ——> C 2 H 5 COOH(l) + CH 3 COOH(l)

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 C=CELECTROPHILESALKENES CARBONYLS NON-POLAR C=OPOLARNUCLEOPHILES ADDITION BondAttacking speciesGroupPolarityResult

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Reagenthydrogen cyanide - HCN (in the presence of KCN) Conditionsreflux in alkaline solution Nucleophilecyanide ion CN¯ Product(s)hydroxynitrile (cyanohydrin) Equation CH 3 CHO + HCN ——> CH 3 CH(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¯

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

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 2A pair of electrons is used to form a bond with H + Overall, there has been addition of HCN STEP 2STEP 1

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 2A pair of electrons is used to form a bond with H + Overall, there has been addition of HCN STEP 2STEP 1

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 2A pair of electrons is used to form a bond with H + Overall, there has been addition of HCN STEP 2STEP 1

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

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

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

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH 4 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)CH 3 CHO + 2[H] ——> CH 3 CH 2 OH CH 3 COCH 3 + 2[H] ——> CH 3 CHOHCH 3 NotesThe water provides a proton QuestionNaBH 4 doesn’t reduce C=C bonds. WHY? CH 2 = CHCHO + 2[H] ———> CH 2 = CHCH 2 OH CARBONYL COMPOUNDS - REDUCTION WITH NaBH 4

CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN Reagent hydrogen Conditionscatalyst - nickel or platinum Reaction type Hydrogenation, reduction Product(s)AlcoholsAldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols. Equation(s)CH 3 CHO + H 2 ——> CH 3 CH 2 OH CH 3 COCH 3 + H 2 ——> CH 3 CHOHCH 3 NoteHydrogen also reduces C=C bonds CH 2 = CHCHO + 2H 2 ——> CH 3 CH 2 CH 2 OH

CARBONYL COMPOUNDS - REDUCTION IntroductionFunctional groups containing multiple bonds can be reduced C=Cis reduced to CH-CH C=Ois reduced toCH-OH C  Nis reduced toCH-NH 2 HydrogenHH 2 H + (electrophile)H¯ (nucleophile) Reactions Hydrogen reduces C=C and C=O bonds CH 2 = CHCHO + 4[H] ——> CH 3 CH 2 CH 2 OH Hydride ion H¯ reduces C=O bonds CH 2 = CHCHO + 2[H] ——> CH 2 =CHCH 2 OH ExplanationC=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯

CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? NaBH 4 H2H2 COMPOUND X

CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? C=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯ NaBH 4 H2H2 COMPOUND X

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. C 6 H 3 (NO 2 ) 2 NHNH 2

2,4-DINITROPHENYLHYDRAZINE C 6 H 3 (NO 2 ) 2 NHNH 2 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. CHO Cl

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 Recall and 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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© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING THE CHEMISTRY OF ALDEHYDES AND KETONES THE END