2025 NYJC H2 Chem Carbonyl Lecture Notes.pdf

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 ove