VJC 2025 Carbonyl Compounds Notes

1 VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT JC2 H2 CHEMISTRY Carbonyl Compounds (Aldehydes & Ketones) _______________________________________________________________________________ LECTURE OUTLINE 1. Introduction 2. Physical Properties 2.1 Boiling point 2.2 Solubility in water 3. Laboratory Preparation 3.1 Oxidation of alcohols 4. Reactions 4.1 Reactions of the carbonyl group 4.1.1 Nucleophilic addition reaction of HCN 4.1.1.1 Reactions involving cyanohydrin 4.1.2 Condensation reactions 4.2 Reduction of aldehydes or ketones 4.3 Oxidation of aldehydes 4.3.1 With acidified K2Cr2O7 or KMnO4 4.3.2 With Tollens’ reagent 4.3.3 With Fehling’s solution 4.4 Tri-iodomethane formation 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 LiAlH4 and HCN (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 Lesson 1 2 Pages 1 – 9 10 – 14 Complete by 23 Feb 23 Feb scan this QR code to ask questions related to the lecture

2 1 Introduction (Nomenclature/Structure) • General formula of CnH2nO • Functional group is C=O • Include both (i) aldehydes, RCHO (where R = H, alkyl or aryl groups) and (ii) ketones, RCOR’ (where R, R’ = alkyl or aryl groups) aldehyde ketone • Carbonyl compounds are widely used in perfume industry (e.g. aliphatic aldehydes with 8 –13 carbon atoms have very pleasant floral odours and are used in feminine perfumery) • Nomenclature a) Aldehyde for linear chains containing only aldehyde functional group 1. count the number of carbon atoms in the chain 2. the ‘e’ of the –ane of the alkane with the same number of carbon atoms is replaced by ‘al’. Note that it is not required to number the position of the aldehyde as it is assigned position 1 within the chain. name structural formula skeletal formula methanal HCHO ethanal CH3CHO propanal CH3CH2CHO butanal CH3CH2CH2CHO pentanal CH3CH2CH2CH2CHO for branched chains or chains containing substituents (-OH, -NH2, -R (alkyl), -X (halide)) 1. find the longest alkyl chain, which is the parent chain, that contains the aldehyde group 2. assign the aldehyde carbon as carbon position 1 and name the parent chain 3. identify substituent(s) on the parent chain and include its position as prefixes to the name of the parent chain

3 3-methylpentanal 3-hydroxybutanal 2-bromo-4-chlorobutanal b) Ketone for linear chains containing only ketone functional group 1. count the number of carbon atoms in the chain 2. the ‘e’ of the –ane of the alkane with the same number of carbon atoms is replaced by ‘one’. Ketones containing more than 4 carbon atoms can exist as constitutional isomers, so the position of the carbonyl groups need to be specified in the name. name structural formula skeletal formula propanone CH3COCH3 butanone CH3COCH2CH3 pentan-2-one CH3COCH2CH2CH3 pentan-3-one CH3CH2COCH2CH3 Worked Example 1 Classify the following carbonyl compounds as aldehyde or ketone. Hence name the compounds. structure CH3CHClCH2CHO type of carbonyl name of compound

4 2 Physical Properties 2.1 Boiling point • Carbonyls are moderately polar because of permanent + charge on the carbonyl C atom and the − charge on the O atom of the C=O functional group. (recall that oxygen is more electronegative than carbon) • Hence they have higher boiling point than alkanes with similar number of electrons but lower boiling point than alcohols with similar number of electrons (comparable Mr). compound Mr b.p. / °C dominant intermolecular forces CH3CH2CH3 44 – 42 instantaneous dipole-induced dipole interactions CH3CHO 44 21 permanent dipole-permanent dipole interactions CH3CH2OH 46 78 hydrogen bonding Recall that for compounds with similar M r, more energy is required to overcome stronger intermolecular hydrogen bonds than permanent dipole-permanent dipole interactions! 2.2 Solubility in water • Both ketones and aldehydes are soluble in water as they are polar and are able to form hydrogen bonding with water molecules. • Both can dissolve polar and non-polar solutes. • Solubility in H 2O decreases as Mr increases because as the carbon chain increases, the extent of intermolecular hydrogen bonding between the carbonyl compounds and water (solvent-solute interaction) decreases. Thus, less energy is released which makes it more difficult to overcome the hydrogen bonding between water molecules (solvent -solvent interaction). 3 Laboratory Preparation Students should be able to a) describe the formation of aldehydes and ketones from, and their reduction to, primary and secondary alcohols respectively 3.1 Oxidation of alcohols • primary (1) alcohols aldehydes • secondary (2) alcohols ketones Oxidation of primary alcohols (1) to prepare aldehydes Reagents and conditions: K2Cr2O7(aq),, H2SO4(aq), heat with immediate distillation intermolecular hydrogen bonding between water and aldehyde molecule oxidation oxidation

5 Making thinking visible Why is immediate distillation of aldehyde necessary? If the aldehyde is not distilled immediately, further heating with acidified dichromate will oxidise the aldehyde to a carboxylic acid. (oxidation of alcohol to carboxylic acid will be covered in the topic of carboxylic acids and derivatives) Why is distillation of aldehyde possible in a mixture of the alcohol and aldehyde product? The aldehyde product will always have a lower boiling point than its corresponding alcohol and hence will be boiled off and collected first in the distillation setup. Why is KMnO4(aq) not used to oxidise primary alcohol to aldehyde? KMnO4(aq) is a very strong oxidising agent that will readily oxidise the aldehyde further to carboxylic acid. Oxidation of secondary alcohols (2) to prepare ketones Reagents and conditions: K2Cr2O7(aq), H2SO4(aq), heat or KMnO4(aq), H2SO4(aq), heat Making thinking visible Why is immediate distillation of ketone not necessary? Ketone cannot be further oxidised even with further heating under reflux. Therefore, ketone does not need to be distilled immediately. Worked Example 2 Write down the observations of the following reactions and draw the structures of the organic products.

6 4 Reactions 4.1 Reactions of the carbonyl group • Ketones are generally less reactive than aldehydes towards nucleophiles for two reasons: (i) electronic factor: carbonyl carbon of ketone is less electron-deficient as it is attached to two electron-donating alkyl groups (ii) steric hindrance: alkyl group is bulkier than hydrogen. This hinders the approach of the nucleophile to the electron-deficient carbon in carbonyl. 4.1.1 Nucleophilic addition of HCN Students should be able to b) describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones c) explain the differences in reactivity between carbonyls and alkenes towards nucleophilic reagent such as LiAlH4 and HCN Structure and reactivity • Although both alkenes and carbonyl compounds are unsaturated with double bonds, alkenes possess a C=C while carbonyl compoun