Lecture Outline: Chemistry of Aldehydes and Ketones

Lecture outline Ch 18 — Additions to the carbonyl group; chemistry of aldehydes and ketones Before we begin studying reactions of aldehydes and ketones, it's worthwhile to revisit some chemistry that can be used for their preparation. We've seen several reactions recently that have been described as oxidations or reductions. Although these are familiar concepts, a brief discussion of these principles as they apply to organic molecules will help us recognize oxidation and reduction reactions of common organic compounds. This is covered in section 10.14 (p 380) of the text. Principles of oxidations and reductions as applied to organic cmpds a. The general method — Assign an oxidation level to each C involved in rxn by adding: +1 for every bond to a more electronegative atom (O, N, halogen) –1 for every bond to a less electronegative atom (H, metal, etc) 0 for every bond to another C increase => oxidation decrease => reduction e.g., OH –1 –1 +1 = –1 0 –1 CO2H +1 +1 +1 = +3 –1 –2 ∴ oxidation ∴ reduction Important: the numbers above are oxidation levels, not charges b. The easier method for alcohols and carbonyl cmpds — again focusing only on the C(s) involved in the rxn ... oxidation => gain of bonds to O and/or loss of bonds to H reduction => loss of bonds to O and/or gain of bonds to H How does this apply to the two examples above? Does this method lead to the same conclusions? More generally, how does the first method demonstrate that counting bonds to O and H is a valid shortcut for recognizing reductions and oxidations? oxidation series — oxidations that replace bonds to H with bonds to O are easy; once all the Hs are gone, the carbon cannot be oxidized further (without cleavage of CC bonds — this requires powerful oxidants and is normally of no use synthetically.) O R CH2 OH R CH O R C OH note that OH R CH OH is the "hydrate" of the aldehyde (i.e. RCH=O + H2 O) — hydrates are generally less stable than the carresponding carbonyl compound — this has the same ox. state as the aldehyde R2CH R3C OH R2C O OH Write the series of cmpds that would be produced by oxidation of methanol. Oxidation of alcohols to aldehydes, ketones, and carboxylic acids Two useful Cr (VI) oxidants — a. H2CrO4, H2O (aqueous chromic acid) R2CH OH H2CrO4 R2C O RCH O H2O RCH2 OH H2CrO4 — made from CrO3, H2SO4, H2O or from Na2CrO7, H2SO4, H2O H2O H2CrO4 H2O RCO2H chromic acid ox can't be stopped at the aldehyde partial mech: great l.g. O OH C O C H Cr OH OH H a protonated "chromate ester" E2 rxn O C OH2 (+ H2 CrO3 ) Cr (IV) Notice that the carbon undergoes a 2-e– oxidation and the chromium a 2-e– reduction. Cr (IV) is unstable. Three of these get together and disproportionate — they swap electrons to make 1 Cr (VI) and 2 Cr (III). This is a standard laboratory test for an oxidizable functional group — Cr (VI) is orange; Cr (III) is green — the color change that accompanies reduction of the Cr is a clear indication that something else (an alcohol, an aldehyde, etc) must have been oxidized. b. Milder oxidant — CrO3 and pyridine (with no water) — one popular variation of this is pyridinium chlorochromate (PCC) CrO3 oxidizes alcohols to carbonyl compounds just like aq H2CrO4; The only difference between CrO3 and aqueous chromic acid is that CrO3 does not oxidize aldehydes. e.g., R2CH RCH2 OH CrO3 pyr. OH CrO3 pyr. Draw the products of the following rxns — assume excess oxidant in each case. OH OH CrO3 pyr. OH OH H2CrO4 H2O Reactions of aldehydes and ketones — — carbonyl additions Nucleophiles (RO– , R3N:, H:–, "R:– ", etc) attack C C Electrophiles (H+, AlCl3, etc) attack O O I don't understand the reason for the order in which the text covers the reactions in this chapter, so I'm going to rearrange them. We'll start with the wimpy oxygen-based nucleophiles (sections 18.3 and 18.9), then move to nitrogen-based nucleophiles (18.8), then the powerful hydride and carbanionic nucleophiles. 1. H2O and ROH as nucleophiles — a. Hydration (reversible) OH C O + C H2O O H a geminal diol or "carbonyl hydrate" — base-catalyzed rxn O HO H2O HO OH mech: — acid-catalyzed rxn O H H2O mech: HO OH