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Carbonyl Compounds (Chapter 18)
Note: This is a final version of the notes for chapter 18. There will not be any further version of notes for this, and this will be used for the final exam as preparation material Carbonyl Compounds: they are organic molecules that contain the carbonyl group C=O. The general formula is CnH2nO. There are two types: aldehydes and ketones. Aldehydes: the functional group is at the end of the carbon chain (1) Ketones: the functional group is in the middle of the chain (2)
Aldehydes
Here are some examples of aldehydes: Isomers of Butanal [C 4 H 8 O]:
Isomers of Pentanal [C 5 H 10 O]:
Oxidation/Reduction of Aldehydes
Type of Reaction: Oxidation Reagents: primary alcohol, weak oxidising agent like K 2 Cr 2 O 7 Conditions: heat and distill off [letting the vapors escape] Observation: color of K 2 Cr 2 O 7 changes from orange to green CH 3 CH 2 OH + [O] → CH 3 CHO + H 2 O [ethanol] [ethanal] Type of Reaction: Oxidation Reagents: aldehyde, oxidising agent Conditions: heat under reflux CH 3 CHO + [O] → CH 3 COOH [ethanal] [ethanoic acid] Note: aldehydes oxidised to form carboxylic acids ● KMnO 4 is a strong oxidising agent ● K 2 Cr 2 O 7 is a weak oxidising agent with LiAlH 4 [reducing agent] [strong] Reagents: aldehyde, LiAlH 4 Conditions: LiAlH 4 should be in dry ether CH 3 CHO + 2 [H] → CH 3 CH 2 OH [ethanal] [ethanol] With NaBH 4 [reducing agent] [weak] Reagents: aldehyde, NaBH 4 Conditions: NaBH 4 should be in aqueous, heat CH 3 CHO + 2 [H] → CH 3 CH 2 OH [ethanal] [ethanol] ● LiAlH 4 is a strong reducing agent ● NaBH 4 is a weak reducing agent
Oxidation/Reduction of Ketones
Type of Reaction: Oxidation Reagents: secondary alcohol, oxidising agent, for example, K 2 Cr 2 O 7 Conditions: heat under reflux Observation: color of K 2 Cr 2 O 7 changes from orange to green CH 3 CH(OH)CH 3 + [O] → CH 3 COCH 3 + H 2 O [propan-2-ol] [propanone] With LiAlH 4 Reagents: Ketone, LiAlH 4 Conditions: LiAlH 4 should be in dry ether CH 3 COCH 3 + 2[H] → CH 3 CH(OH)CH 3 [propanone] [propan-2-ol] With NaBH 4 Reagents: Ketone, NaBH 4 Conditions: NaBH 4 should be in aqueous, heat CH 3 COCH 3 + 2[H] → CH 3 C(OH)CH 3 [propanone] [propan-2-ol]
Testing For Carbonyl Compounds
Type of Reaction: Condensation Reagent: 2,4-DNPH Conditions: no conditions Observation: orange precipitate
Distinguishing between Aldehydes and Ketones
With Tollen’s Reagent Tollen’s Reagent: ammoniacal silver nitrate [alkaline silver nitrate dissolved in ammonia] Conditions: Heat Observations: silver mirror screen with aldehyde but no change with ketone 2 CH 3 CHO + 2 OH-^ + Ag+^ → 2 CH 3 COO-^ + Ag + H 2 O [ethanol] [ethanoate ion] With Fehling’s Solution Fehling’s Solution: alkaline solution with Cu+2^ ions Conditions: Heat Observations: change from pale-blue solution to brick-red precipitate with aldehyde but no change with ketone CH 3 CHO + 5 OH-^ + 2 Cu+2^ → CH 3 COO-^ + Cu 2 O + H 2 O [ethanol] [ethanoate ion]
The Nucleophilic Addition Mechanism
● Reagents: aldehyde/ketone and HCN in NaCN. Here, NaCN acts as a catalyst to provide the nucleophile ● Conditions: Room Temperature Note: curly arrows show movement of electron pairs
