Ketones resonance effects

In a, 3-unsaturated ketones, nitriles, and esters (e.g., 125), the y hydrogen assumes the acidity normally held by the position a to the carbonyl group, especially when R is not hydrogen and so cannot compete. This principle, called vinylology, operates because the resonance effect is transmitted through the double bond. However, because of the resonance, alkylation at the a position (with allylic rearrangement) competes with alkylation at the y position and usually predominates. 

Primary and secondary nitroalkanes, and substrates containing terminal em-dinitroaliphatic functionality, have one or more acidic a-protons, a consequence of inductive and resonance effects imposed by the nitro group. As a result, such compounds can behave like carbanions and participate in a number of addition and condensation reactions which are typical of substrates like ketones, aldehydes, and /S-ketoesters. Such reactions are extremely useful for the synthesis of functionalized polynitroaliphatic compounds which find potential use as explosives, energetic oligomers and plasticizers. 

Replacement of an alkyl group of a saturated aliphatic ketone by a hetero atom (G) shifts the carbonyl absorption. The direction of the shift depends on whether the inductive effect (a) or resonance effect (b) predominates. 

Figure 15.14 Effects on NMR signals of the resonance effect in mesomeric forms of a carbonyl. If the carbonyl of a ketone is compared with that of an ester, then it should be noted that for the ketone, the more electropositive of the two, the carbon has less protection than the ester. In NMR the carbonyl signal for a ketone is around 205 ppm while for an ester it is around 165 ppm.

Competitive resonance effects can be found. In general, C-H bonds alpha to ketones are more acidic than when alpha to ester carbonyls, which are more acidic relative to amide carbonyls, all three of which are more acidic than C-H bonds near carboxylates. The resonance stabilization gained by ionization of the C-H bond is increasingly lower in this series because the O, N, or 0 heteroatom on the ester, amide, or carboxylate, respectively, is increasingly involved in resonance with the carbonyl in the acid prior to ionization of the C-H bond. In such cases, we must consider the role of resonance in stabilizing the HA compound as well as A". 

Tanaka A, Hashimoto K, Kominami H (2012) Preparation of Au/Ce02 exhibiting strong surface plasmon resonance effective for selective or chemoselective oxidation of alcohols to aldehydes or ketones in aqueous suspensions under irradiation by green light. J Am Chem Soc 134(35) 14526-14533... 

An a hydrogen of an aide hyde or a ketone is more acidic than most other protons bound to carbon Aldehydes and ketones are weak acids with pK s in the 16 to 20 range Their enhanced acidity IS due to the electron withdrawing effect of the carbon yl group and the resonance stabi lization of the enolate anion... 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

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