NMR spectrum of acetaldehyde

Figure 19.18 1H NMR spectrum of acetaldehyde. The absorption of the aldehyde proton appears at 9.8 8 and is split into a quartet.

FIGURE 6.5 H NMR spectrum of acetaldehyde. Right CH3 resonance, with the two spin states of the aldehyde proton indicated. Left, CHO resonance, with the four spin orientations of the methyl protons indicated. 

Figure 24.18 High-resolution NMR spectrum of acetaldehyde. (Courtesy of Carl Esche, Dept, of Chemistry, University of Maryland.)...

Compared to the NMR spectrum of the protected polymer (top. Figure 1), the chemical shifts of both CHj and CH of the acetal group disappeared completely after the deprotection as the result of evolving gaseous acetaldehyde. The new spectrum. Figure 3, showed a mixture of two components broad chemical shifts for poly(vinylphenol) and two sets of sharp aromatic peaks for small molecular phenol. As expected, phenol remained with poly(vinylphenol) because it was not volatile under out experimental conditions. 

Photolysis experiments carried out with thin films gave essentially same results as those with solution NMR study. IH NMR spectrum of a reaction mixture after irradiation of a resist film provided evidence supporting our proposed imaging mechanism that only mass loss firom the deprotection was acetaldehyde. 

We have established that the formation of enols is catalysed by acids and bases. The reverse of this reaction—the formation of ketone, from enol—must therefore also be catalysed by the same acids and bases. If you prepare simple enols in the strict absence of acid or base they have a reasonable lifetime. A famous example is the preparation of the simplest enol, vinyl alcohol, by heating ethane-1,2-diol (glycol—antifreeze) to very high temperatures (900 °C) at low pressure. Water is lost and the enol of acetaldehyde is formed. It survives long enough for its proton NMR spectrum to be run, but gives acetaldehyde slowly. 

The ultimate fate of the enolate is obscured by the work-up conditions. Condensation products, rather than acetaldehyde, are observed there is always a peak in the proton NMR spectrum at 5.7 delta arising from vinylic protons. 

Denmark has spectroscopically examined the reaction of both allyl- and 2-bute-nylstannanes with aldehydes using the Lewis acids SnCU and BF3-OEt2 [73, 82]. First, the metathesis of both allyltributylstannane and tetraallyltin with SnCl4 was determined (by C NMR spectroscopy) to be instantaneous at -80 °C. The reaction of allyltributylstannane with a complexed aldehyde was detemiined to be significantly more complicated. When a molar equivalent of SnCU per aldehyde was employed, metathesis was determined to be the preferred pathway for aldehydes. When one half a molar equivalent of SnC per aldehyde is used, the reaction pathways and product distribution become very sensitive to both the aldehyde structure and addition order. A spectrum of mechanistic pathways was documented ranging from direct addition (acetaldehyde) to complete metathesis (pivalalde-hyde) to a competitive addition and metathesis (4-t-butylbenzaldehyde). The results obtained with a molar equivalent of SnCl4 are most relevant, as this reagent stoichiometry is most commonly used in the addition reactions. 

Clearly, the zwitterionic peroxide (213) is implicated (Scheme 27). Closure to the dioxetane (214) and scission to (210) is a minor event. Capture of (213) by aldehyde predominates. The epimeric cM-fused trioxanes (211) arise from the syn or anti arrangement (215). The major epimer, identified by x-ray, has the C3 methyl group disposed cis with respect to the indole ring. The trans epimer is unstable and cannot be isolated. The structure of the minor position isomer (212) follows from its NMR spectrum. It probably derives from the addition of the alternative benzylic zwitterionic peroxide (216) to acetaldehyde. 

Figure 9.11. The NMR spectrum at 500MHz in HCClj of a mixture of the (Z)- and (E)-oximes of ethanal (acetaldehyde, CH3CHO).

Further study of the add-catatyzed deprotection was carried out using NMR spectroscopy. In stead of using a solid-state film, a solution of poty[4-(l-pheno-tyretho ty)s ene] in deuterated DMF was chosen to conduct the study. The reason to use DMF as the solvent was because both protected and deprotected polymers were soluble in DMF. After a trace of triflic add was introduced into the polymer solution in a NMR tube and heated, gaseous molecules evolved from the solution, which were believed to be acetaldehyde. The solution was further heated at 110 °C for 2 minutes and by then gas evolution stopped. After cooling to room temperature, a proton NMR spectrum was taken, which is shown in Figure 3. 

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