2025 NYJC H2 Chem Carbonyl Lecture Notes

1 Content • Aldehydes (exemplified by ethanal) (i) oxidation to carboxylic acid (ii) nucleophilic addition with hydrogen cyanide (iii) characteristic tests for aldehydes • Ketones (exemplified by propanone and phenylethanone) (i) nucleophilic addition with hydrogen cyanide (ii) characteristic tests for ketones Learning Outcomes Candidates should be able to: (a) describe the formation of aldehydes and ketones from, and their reduction to, primary and secondary alcohols respectively (b) describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones (c) explain the differences in reactivity between carbonyl compounds and alkenes towards nucleophilic reagents, such as lithium aluminium hydride and hydrogen cyanide (d) describe the use of 2,4- dinitrophenylhydrazine (2,4-DNPH) to detect the presence of carbonyl compounds (e) deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of simple tests (i.e. Fehling’s and Tollens’ reagents; ease of oxidation) (f) describe the presence of a CH 3CO– group in a carbonyl compound from its reaction with alkaline aqueous iodine to form tri-iodomethane Reference texts 1. Understanding Chemistry for Advanced level, Ted Lester, Janet Renshaw 2. Chemistry for Advanced Level. Peter Cann, Peter Hughes 3. Organic Chemistry, John McMurry Nanyang Junior College Chemistry (9729) Lecture Notes 15 Carbonyl Compounds Lecturer: Ms Theresia Line Ishak JC2/2025

2 1 Introduction Carbonyl compounds contain the carbonyl group (C=O). They are further classified into Aldehydes and Ketones. General Formula: CnH2nO Functional group: Aldehydes • R = H or alkyl or aryl groups • Can be written as RCHO. • The aldehyde functional group –CHO is always at the end of a chain , i.e. a terminal group. Functional group: Ketones • R’ and R’’ = alkyl or aryl groups only. They cannot be H. • Can be written as R’COR’’, whe re CO represents the carbonyl group. • For the ketone functional group, the carbonyl group is never found at the end of a chain but between carbon atoms.

3 2 Nomenclature Aldehyde Ketone Name Structural Formula Name Structural Formula Methanal HCHO C H O H Propanone CH3COCH3 C H3 C CH3 O Ethanal CH3CHO CH3 C H O Pentan-2-one CH3COCH2CH2CH3 C H3 C CH2 O CH2 CH3 Cyclohexanecarbaldehyde C O H Cyclohexanone O Phenylethanal C6H5CH2CHO CH2 C O H Phenylethanone C6H5COCH3 C O CH3 Benzaldehyde C6H5CHO C O H

4 3 Structure of the carbonyl functional group • The carbonyl carbon atom is sp2 hybridised and joined to three other atoms by forming σ bonds. The carbonyl carbon atom and the three atoms bonded to it form a trigonal planar structure with bond angle of 120o. Orbitals of the carbonyl carbon The unhybridised 2p orbital of the carbonyl carbon overlaps side -on with a 2p orbital of the oxygen atom to form a π bond. • As the oxygen atom is more electronegative compared to the carbon atom, it causes the carbonyl carbon to be electron deficient. Hence it carries a slight positive charge (δ+) while the oxygen atom carries a slight negative charge (δ−). The electron deficient carbon is thus susceptible to attack by nucleophiles like CN−. 120o unhybridised 2p orbital 120o Side-on overlap of the 2p orbitals give rise to π bond Three sp2 orbitals are involved in head-on overlap to form three σ bonds. C O R' R'' δ+ δ-

5 Difference in reactivity between Carbonyl compounds and Alkenes • Although both alkenes (C=C) and carbonyl compounds (C=O) are unsaturated with double bonds, they behave differently towards nucleophilic reagents. • Carbonyl carbon is electron- deficient since it is bonded to electronegative oxygen. The electronegative oxygen pulls electrons from the carbonyl carbon, causing the carbonyl carbon to be electron deficient and attracts nucleophiles. • On the other hand, there is no electronegativity difference between the two C atoms in the C=C bond, hence the bond is non-polar and there is no electron deficient carbon for the nucleophile to attack. In addition, the π bond causes the C=C bond to be electron-rich, repelling the approach of a nucleophile. CH2 CH2 Br Br δ+ δ− C R H O δ+ δ− Alkene Carbonyl CN− 4 Physical properties 4.1 Boiling Point • C arbonyl compounds are polar in nature. They have stronger intermolecular permanent dipole – permanent dipole attraction which cause them to have higher boiling point than alkanes and other non-polar compounds of comparable relative molecular mass, Mr. compounds Mr boiling point / oC CH3CH2CH2CH3 58.0 – 0.5 CH3CH2CHO 58.0 48 CH3COCH3 58.0 56 • However they do not exhibit hydrogen bonding between molecules as there is no H attached to an electronegative atom like N, O, or F. Hence they have lower boiling points than alcohols and carboxylic acids of comparable relative molecular mass, Mr. compounds Mr boiling point / oC CH3CH2CHO 58.0 48 CH3CH2CH2OH 60.0 97 CH3COOH 60.0 118

6 • The order of boiling points for compounds with comparable Mr but different functional group is: carboxylic acids > alcohols > aldehydes/ketones > alkanes Carboxylic acids Alcohol Aldehydes/Ketones Alkanes Hydrogen bonding exists between molecules. Permanent dipole – permanent dipole attraction exist between molecules. Weak instantaneous dipole – induced dipole attraction exist between molecules. 4.2 Solubility • The lower aliphatic aldehydes and ketones (i.e. the carbonyl group is not attached to the benzene ring) are appreciably soluble in water due largely to their ability to form hydrogen bonds with water molecules. (Note that carbonyl compounds are however unable to form hydrogen bonding between themselves!) C O R' R lllllllllllll: H O Hδ+ δ− δ+ δ− hydrogen bond δ+ • Higher members containing more than 5 carbons and the aromatic substituents are virtually insoluble in water. As the chain length increases, the carbonyl molecule becomes largely non -polar. Instantaneous dipole – induced dipole attraction between the hydrocarbon chains in the carbonyl molecules become more significant. The energy evolved from the formation of instantaneous dipole – induced dipole attraction between water and carbonyl molecules is insufficient to overcome the instantaneous dipole – induced dipole attraction between carbonyl molecules and hydrogen bonding between water molecules.

7 Overview of carbonyl reactions K2Cr2O7, H2SO4(aq) heat with immediate distillation KMnO4, H2SO4(aq), heat under reflux LiAlH4, dry ether, rm temp OR NaBH4, rm temp OR H2(g), Ni catalyst, heat K2Cr2O7, H2SO4(aq), heat under reflux OR KMnO4, H2SO4(aq), heat under reflux LiAlH4, dry ether, rm temp OR NaBH4, rm temp OR H2(g), Ni catalyst, heat K2Cr2O7, H2SO4(aq), heat under reflux OR KMnO4, H2SO4(aq), heat under reflux HCN, trace NaOH(aq), 10-20 oC OR HCN, trace NaCN(aq), 10-20 oC K2Cr2O7, H2SO4(aq), heat under reflux OR KMnO4, H2SO4(aq), heat under reflux HCN, trace NaOH (aq), 10-20 oC OR HCN, trace NaCN (aq), 10-20 oC C CH2 R1C H3 disubstituted alkene C O R1C H3 ketone secondary alcohol C O H C H3 H H primary alcohol C O C H3 H aldehyde C O C H3 OH carboxylic acid C OH C H3 H CN hydroxynitrile C OH C H3 R1 CN hydroxynitrile NO2 O2N NH N H2 C CH3 H NO2 NO2 NH N2,4-dinitrophenylhydrazine, warm (2,4-DNPH) C O H-O C O CH3 O- C O CH3 O- I2(aq), NaOH(aq), warm Fehling's reagent, warm Tollens' reagent, warm OXIDATION REDUCTION OXIDATION OXIDATION NUCLEOPHILIC ADDITION CONDENSATION OXIDATION OXIDATION C O H C H3 H R1 OXIDATION OXIDATION REDUCTION NO2 O2N NH N H2 C CH3 R1 NO2 NO2 NH N CONDENSATION NUCLEOPHILIC ADDITION I2(aq), NaOH(aq), warm 2,4-dinitrophenylhydrazine, warm (2,4-DNPH) CHI3+ C O R1 -O CHI3+ OXIDATION OX