Raffles Institution H2 Chem Carbonyl Compounds (Student)

Page 1 of 22 Raffles Institution Year 6 H2 Chemistry 2025 Lecture Notes 18 – Carbonyl Compounds Lecturer: Dr Jackson Koh I Learning Objectives 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) deduce the presence of a CH 3CO− group in a carbonyl compound from its reaction with alkaline aqueous iodine to form tri-iodomethane II References 1. Understanding Advanced Organic and Analytical Chemistry by Kim Seng Chan & Jeanne Tan. 2. Chemistry in Context by G. C. Hill & J. S. Holman 3. ‘A’ Level Chemistry by E. N. Ramsden 4. Organic Chemistry by John McMurry III Contents 1. Introduction ........................................................................................................................................ 2 2. Nomenclature .................................................................................................................................... 3 3. Physical properties ............................................................................................................................ 4 3.1 Boiling points ............................................................................................................................... 4 3.2 Solubility ...................................................................................................................................... 4 4. Preparation methods for carbonyl compounds .................................................................................. 5 4.1 Oxidation of alcohols ................................................................................................................... 5 4.2 Oxidation of alkenes .................................................................................................................... 6 4.3 Electrophilic substitution of benzene ........................................................................................... 6 5. Chemical properties of carbonyl compounds .................................................................................... 7 5.1 Relative reactivities of carbonyl compounds towards nucleophilic attack .................................. 8 5.2 Formation of cyanohydrins – Nucleophilic addition .................................................................... 9 5.2.1 Nucleophilic addition mechanism .................................................................................. 10 5.2.2 Stereochemistry of addition products ............................................................................ 12 5.2.3 Difference in reactivity between carbonyl compounds and alkenes towards nucleophiles .................................................................................................................. 13 5.3 Hydrolysis and reduction of cyanohydrins ................................................................................ 13 5.4 Formation of alcohols – Reduction ........................................................................................... 14 5.5 Condensation reaction .............................................................................................................. 15 5.6 Formation of carboxylic acids – Oxidation ................................................................................ 16 5.7 Formation of carboxylate salts using Tollens’ Reagent ............................................................ 16 5.8 Formation of carboxylate salts using Fehling’s Solution ........................................................... 17 5.9 Formation of tri-iodomethane (iodoform) – Oxidation ............................................................... 18 6. Reactivity of aromatic carbonyl compounds .................................................................................... 19 7. Summary of reactions with oxidising and reducing agents in Organic Chemistry .......................... 20 8. Summary of distinguishing tests ...................................................................................................... 20 9. Summary of Nucleophilic Addition Mechanism ............................................................................... 21

Page 2 of 22 1 INTRODUCTION 1.1 Carbonyl Compounds – Aldehydes & Ketones 1.2 Examples of Naturally Occurring Carbonyl Compounds C O H benzaldehyde (bitter almond oil extract) C O H vanillin (Vanilla flavouring) OH OCH3 cinnamaldehyde or 3-phenylpropenal (in cinnamon) CHCHCHO OHCH3 H3C O testosterone (hormone)  Compounds with the general formula RCHO where −CHO is termed the aldehyde group. R may be H, alkyl or aryl group. Aliphatic aldehyde – R is an alkyl group; Aromatic aldehyde – R is an aryl group.  Have at least one H atom attached to the carbonyl carbon atom.  Compounds with the general formula RCOR’. R and R’ may be an alkyl or aryl group.  Have two alkyl or aryl groups attached to the carbonyl carbon atom. Carbonyl Functional Group Aldehyde Ketone O Oct-1-en-3-one (in cooked mushrooms)

Page 3 of 22 2 NOMENCLATURE For aliphatic aldehydes, ❑ the longest chain carrying the –CHO group is considered the parent structure and is named by replacing the –e of the corresponding alkane by –al. ❑ the position of a substituent is indicated by a number, the carbonyl carbon being considered as carbon-1. Structural Formula IUPAC Name Common Name HCHO methanal formaldehyde CH3CHO ethanal acetaldehyde CH3CH2CHO propanal CH3(CH2)2CHO butanal 3–phenylpropanal For aromatic aldehydes, ❑ the parent structure is named as a benzaldehyde. ❑ the carbon on the benzene ring bearing the carbonyl functional group is considered carbon-1, and the position of a substituent on the benzene ring takes reference from this carbon-1. Structural Formula IUPAC Name Common Name benzaldehyde benzenecarbaldehyde 4–bromobenzaldehyde For ketones, ❑ the name is obtained by replacing the final –e of the name of the corresponding alkane with –one. ❑ the chain is numbered such that the carbonyl carbon atom has the lower possible number, and this number is used to designate its position. Structural Formula IUPAC Name Common Name CH3COCH3 propanone acetone CH3CH2COCH3 butanone CH3CH2CH2COCH3 pentan–2–one CH3CH2COCH2CH3 pentan–3–one cyclohexanone Practice: Name the following compounds Aldehydes Ketones CH2CH2CHO O CHO CHOBr

Page 4 of 22 3 PHYSICAL PROPERTIES 3.1 Boiling points As polar compounds, aldehydes and ketones have higher boiling point s than alkanes of similar electron cloud size. Example: Table 1 pentane butanal butan-1-ol Structural formula Boiling point (°C) 36.0 76.0 117.2 Mr 72.0 72.0 74.0 Butan-1-ol has the highest boiling point as the hydrogen bonds are stronger and require