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Week 3 • Recognise and name aldehydes and ketones. © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 The carbonyl functional group © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 Formation of the C=O group π-bond © Pearson Education Ltd 2009 This document may have been altered from the original Naming aldehydes • See p. 20 A2 text book. • In an aldehyde the carbon atom of the carbonyl group is joined to at least 1 hydrogen atom. • Aldehydes end in ‘al’. • The parent chain is the longest unbranched carbon chain present in the molecule. • The carbonyl C atom is always the 1 carbon in the chain. Week 3 2-methylpentanal © Pearson Education Ltd 2009 This document may have been altered from the original Naming ketones • See p. 21 A2 text book. • In ketones the carbonyl group is in the middle of a chain with a C atom on either side. • Ketones end in ‘one’. • The parent chain is the longest chain containing the carbonyl group. • The carbonyl is numbered with the smallest possible number. Week 3 The structure of 2-methylpentan-3-one © Pearson Education Ltd 2009 This document may have been altered from the original Aromatic aldehydes and ketones • These contain the carbonyl functional group and a ring. • The simplest aromatic aldehyde is? • The simplest aromatic ketone is? • Aromatic carbonyls contribute to the flavours and smells of many stone fruits, plums, peaches, cherries etc. • Cinnamaldehyde is the characteristic smell and flavour of cinnamon. • Phenylethanone resembles strawberries etc. Week 3 Structures of benzaldehyde and phenylethanone © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 Structure of cinnamaldehyde © Pearson Education Ltd 2009 This document may have been altered from the original Physical Properties of Carbonyl Compounds • Aliphatic carbonyl compounds are generally less pleasant, particularly aldehydes which are described as lachrymatory. • E.g. Heptan-2-one is responsible for the flavour of blue cheese. • Lower members of both carbonyl series are soluble in water. • Solubility decreases with increasing RMM. • Volatility decreases with increasing RMM. Volatility Compound BPoC Ethanal 20 Propane -42 Ethanol 78 Week 3 • Describe the oxidation of primary alcohols to form aldehydes and carboxylic acids. • Describe the oxidation of secondary alcohols to form ketones. • Describe the oxidation of aldehydes to form carboxylic acids. © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 Ethanol oxidised to ethanal, and finally to ethanoic acid © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 Propan-2-ol can be oxidised to propanone © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 Oxidation of an aldehyde to a carboxylic acid © Pearson Education Ltd 2009 This document may have been altered from the original Oxidation • • • • • Oxidation is: Gain of oxygen Loss of hydrogen Loss of electrons In the oxidation of alcohols or aldehydes, ion electron equations can be written which are more informative than the [O] symbol. • See p. 23 in A2 text. Week 3 • Describe the reduction of carbonyl compounds to form alcohols. • Outline the mechanism for nucleophilic addition reactions of aldehydes and ketones with hydrides. © Pearson Education Ltd 2009 This document may have been altered from the original Reduction of carbonyl compounds • Aldehydes and ketones are made by the OXIDATION of alcohols. • It follows that with a suitable reducing agent carbonyl compounds should be reduced back to alcohols. • NaBH4 is used with warming and water or ethanol as a solvent. • The reaction can be considered as the addition of hydrogen across the double bond, though the actual reaction is nucleophilic addition using H- as the nucleophile. Week 3 Reduction of an aldehyde produces a primary alcohol © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 The primary alcohol propan-1-ol and the aldehyde propanal © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 Reduction of a ketone produces a secondary alcohol © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 NaBH4 readily generates hydride ions © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 Reduction of an aldehyde by nucleophilic addition © Pearson Education Ltd 2009 This document may have been altered from the original Week 3 • Describe the use of 2,4-dinitrophenylhydrazine to detect a carbonyl group and to identify a carbonyl compound. • Describe the use of Tollens’ reagent to detect the presence of an aldehyde group. © Pearson Education Ltd 2009 This document may have been altered from the original Test for the Presence of a Carbonyl Group • 2,4-dinitrophenylhydrazine will give an orange precipitate with any carbonyl compound (not acids, esters etc). • The reaction is an addition/elimination reaction or condensation reaction which involves addition across the double bond followed by loss of water. • 2,4-DNP or Brady’s reagent is used because the big molecule gives better precipitates of the hydrazone derivative with distinct and diagnostic melting points. Test for the Presence of a Carbonyl Group • If the hydrazone derivative is recrystallised from hot ethanol the melting point determination can be used to identify an unknown carbonyl compound. Week 3 Reaction of propanal with 2,4-dinitrophenylhydrazine © Pearson Education Ltd 2009 This document may have been altered from the original Aldehyde or Ketone? • Aldehydes are oxidised to carboxylic acids with mild oxidising agents but ketones are unaffected. • This can be used to distinguish between them. • Tollen’s Reagent – ammoniacal silver nitrate, will oxidise an aldehyde when left to stand in mildly warm,not hot water to a silver mirror. • The oxidation of the aldehyde is shown after this but the oxidising agent – Ag+ ions are reduced to metallic silver. • Ag+(aq) + e- → Ag(s) Week 3 Oxidation of an aldehyde using Tollens’ reagent © Pearson Education Ltd 2009 This document may have been altered from the original