ASRJC 2025 H2 Chem Carbonyl Compounds Notes

2025 J2 H2 Carbonyl Compounds ©2025/ASRJC/CHEM 1 ANDERSON SERANGOON JUNIOR COLLEGE JC2 H2 CHEMISTRY CARBONYL COMPOUNDS Guiding Questions 1. How are carbonyl compounds synthesised? 2. Which class of reagents do carbonyl compounds react with and why? What types of reactions do carbonyl compounds undergo and why? 3. How do carbonyl compounds react with HCN in nucleophilic addition? 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) deduce the presence of a CH3CO– group in a carbonyl compound from its reaction with alkaline aqueous iodine to form tri–iodomethane References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. A–Level Chemistry by E.N. Ramsden 3. Understanding Chemistry for Advanced Level by Ted Lister and Janet Renshaw 4. Chemistry in Context by Graham Hill and John Holman Part I: Introduction A. The carbonyl group B. Aldehydes and ketones Part V: Reactions A. Nucleophilic addition reaction Mechanism for nucleophilic addition of HCN B. Reduction reaction C. Oxidation reaction D. Condensation (addition–elimination) reaction Part II: Nomenclature A. Naming aldehydes B. Naming ketones Part III: Physical Properties A. Boiling points B. Solubility Part IV: Preparation of aldehydes and ketones Oxidation of alcohols Part VI: Chemical tests A. Distinguishing carbonyl compounds B. Distinguishing aldehydes only C. Distinguishing aldehydes and ketones with CH3CO– group

2025 J2 H2 Carbonyl Compounds ©2025/ASRJC/CHEM 2 (A) The carbonyl group  The carbonyl group, C O, is present in aldehydes and ketones. Bonding in the carbonyl group:  and  bond in carbonyl group OC R' R  bond C O R' R  bond p orbital  The carbonyl carbon is sp2 hybridised and forms three  bonds, which are planar.  The unhybridised p orbital of the carbon atom overlaps sideway with the adjacent p orbital of the oxygen atom to form a  bond. Comparison between alkene and carbonyl group Alkene Carbonyl even electron density in alkene electron density pulled closer to electronegative carbonyl oxygen  Electron cloud is evenly distributed between the two sp2 hybridised carbon atoms.  Electron–rich C=C attracts electrophiles (electron–loving species) like H – Br.  Electrons in the  and  bonds are drawn towards the more electronegative oxygen atom. This makes the carbonyl group polar: C = O , creating an electron–deficient carbon atom.  The electrondeficient carbon attracts nucleophiles like CN and H (from LiAlH4). Part I: Introduction +  +  

2025 J2 H2 Carbonyl Compounds ©2025/ASRJC/CHEM 3 (B) Aldehydes and Ketones 1. General formula for both homologous series is CnH2nO. 2. The carbonyl groups in aldehydes and ketones differ in their positions in the carbon chain. Aldehydes Ketones HCR O R'CR O  Carbonyl group is at the end of carbon chain  RCHO represents the aldehyde functional group. (Do not write as “RCOH”, as it may indicate that the molecule possess es an –OH group)  Carbonyl group is between the carbon chain  RCOR’ represents the ketone functional group, where R and R’ are alkyl or aryl groups and not H atom. Note that the following compounds, though contain carbonyl group, have different reactive groups (functional groups) as that of aldehyde or ketone. (A) Naming Aldehydes 1. Identify the longest continuous ch ain that contains the –CHO group and the –CHO carbon is numbered as C–1. The alkane name provides the parent name. 2. Replace –e of the alkane name by –al. 3. Substituents are named as usual. Structural formula Name HCHO methanal CH3CHO ethanal CH3CHCHO CH3 CH3CH CHCHO Part II: Nomenclature of carbonyl compounds Amide Carboxylic Acid Acyl Chloride Ester C O N H C O OH C O Cl C O OCl R

2025 J2 H2 Carbonyl Compounds ©2025/ASRJC/CHEM 4 4. For aromatic aldehydes (in which –CHO is directly bonded to benzene ), the parent name is benzaldehyde. Substituents are named as usual. Structural formula Name CHO benzaldehyde CHO NO2 O2N (B) Naming Ketones 1. Identify the longest continuous chain that contains the >C=O group. The alkane name provides the parent name. 2. Replace the –e of the alkane name by –one. 3. Ensure lower numbering is used to locate the >C=O group (for 5 carbons and above). 4. Substituents are named as usual. Structural formula Name CH3COCH3 propanone CH3CH2COCH3 butanone CH3CH2CH2COCH2CH3 hexan–3–one CH3CH2CHCOCHCH3 CH3 CH3 O cyclohexanone 5. For aromatic ketones (in which >C=O is directly bonded to benzene ring ), the benzene ring is named as substituent – phenyl. Structural formula Name C O CH3 phenylethanone C O diphenylmethanone

2025 J2 H2 Carbonyl Compounds ©2025/ASRJC/CHEM 5 (A) Boiling points 1. Aldehydes and ketones have higher boiling points than alkanes of similar molecular mass but lower boiling points than alcohols. Explanation: >C=O group is polar. Permanent dipole –permanent dipole (pd–pd) attractions between polar aldehyde and ketone molecules are stronger than the instantaneous dipole–induced dipole (id–id) attractions between non–polar alkane molecules but weaker than the hydrogen bonds between polar alcohol molecules. Table 1: Comparison of boiling point of alkane, aldehyde and alcohol (B) Solubility in polar solvents 1. Shorter chain aldehydes and ketones are soluble in polar solvents. Explanation for higher water solubility compared to alkanes: The carbonyl oxygen can form hydrogen bonds with water molecules. The energy released during the formation of hydrogen bonding between carbonyl compound molecules and water molecules is able to compensate for the energy taken in to break the permanent dipole - permanent dipole attractions between carbonyl compound molecules and the hydrogen bonding between water molecules. Hydrogen bonds between carbonyl compound and water molecules C O R R' O H H    hydrogen bond Table 2: Physical properties of aldehyde and ketone Part III: Physical Properties of carbonyl compounds

2025 J2 H2 Carbonyl Compounds ©2025/ASRJC/CHEM 6 2. As the non–polar alkyl chain gets longer, the solubility in water decreases. Explanation: As the size of the non–polar alkyl chain increases, the instantaneous dipole–induced dipole (id–id) attractions become more significant. Hence, the energy evolved during the formation of hydrogen bonding between the water molecules and the carbonyl compound molecules is not enough to compensate for the energy required to break the stronger id -id attractions between carbonyl compound molecules and the hydrogen bonding between water molecules. (A) Aldehydes and ketones are obtained by oxidation of the corresponding alcohols.