With carbonyl group gases

Like alkene double bonds, carbonyl double bonds can be reduced by catalytic hydrogenation. Catalytic hydrogenation is slower with carbonyl groups than with olefinic double bonds, however. Before sodium borohydride was available, catalytic hydrogenation was often used to reduce aldehydes and ketones, but any olefinic double bonds were reduced as well. In the laboratory, we prefer sodium borohydride over catalytic reduction because it reduces ketones and aldehydes faster than olefins, and no gas-handling equipment is required. Catalytic hydrogenation is still widely used in industry, however, because H2 is much cheaper than NaBH4, and pressure equipment is more readily available there. 

Gas phase reactivity toward allyltrimethylsilane was used to compare the reactivity of several cyclic A-acyliminium ions and related iminium ions.203 Compounds with endocyclic acyl groups were found to be more reactive than compounds with exocyclic acyl substituents. Five-membered ring compounds are somewhat more reactive than six-membered ones. The higher reactivity of the endocyclic acyl derivatives is believed to be due to geometric constraints that maximize the polar effect of the carbonyl group. 

The nitroaldol reaction of methyl nitroacetate (199, Scheme 38) with 1,2 3,4-di-0-isopropylidene-a-D-ga/acfo-hexodialdo-l,5-pyranose (200) and 2,3-O-isopropyli-dene-D-glyceraldehyde (202) catalyzed by silica gel proceeded in almost quantitative yield, with high selectivity for attack on the aldehyde carbonyl group, giving derivatives 201 and 203, respectively. Two of the four possible diastereomers were detected as main products, and were obtained as a mixture. For the nitroaldol reaction with 200 gave similar results in either the presence or absence of silica gel, whereas the reaction with 202 did not proceed in its absence, showing that catalytic action of silica is mandatory in this case.176... 

To conclude this section on the effect of solvent on a-nucleophilicity, we refer to the current, rather controversial, situation pertaining to gas-phase smdies and the a-effect. As reported in our review on the a-effect and its modulation by solvent the gas-phase reaction of methyl formate with HOO and HO , which proceeds via three competitive pathways proton abstraction, nucleophilic addition to the carbonyl group and Sat2 displacement on the methyl group, showed no enhanced nucleophilic reactivity for HOO relative to This was consistent with gas-phase calculational work... 

Formaldehyde is a colorless, flammable gas with a distinctive pungent odor. It is the simplest aldehyde, which is a class of organic compounds with the carbonyl group bonded to at least one hydrogen atom. Formaldehyde was described by August Wilhelm von Hoffmann (1818—1892) in 1867 after the Russian Aleksandr Butlerov (1828—1886) had inadvertently synthesized it in 1857. Formaldehyde readily dissolves in water to produce a solution called formalin, which is commonly marketed as a 37% solution. 

By Reduction of Unsaturated Precursors The method of choice for labeling with tritium is the reduction of a suitable unsaturated precursor (containing a double bond, carbonyl group, etc.) with carrier-free tritium gas or tritiated metal hydrides. The major limitation of this method is the availability of a suitable unsaturated precursor of the desired compound. It is essential to carry out the synthesis in a non-hydroxylic solvent (dioxane, ethyl acetate, etc.). Reductions carried out in alcohol or water will lead to almost complete exchange of the tritium gas with the solvent. 

Pron et al.569) looked at polyacetylene treated from the gas phase with H2S04 which leads to HS04 counter-ions. They found that the conductivity drops in air with the appearance of C=O bands in the ir, although the rate of decay is much lower than would be expected for undoped samples. The polymer was more rapidly degraded by exposure to water but could be redoped with further acid treatment. Pron et al.570) have also reported hydrolytic instability in polyacetylene with A1C14 as the counterion. In both cases the proposed mechanism involves addition of OH" to the chain and keto-enol tautomerism to form carbonyl groups. 

The chain of atoms can include an oxygen atom. When the enophile is a carbonyl group, the reaction is a Prins reaction 6.16, which is usually acid-catalysed and not pericyclic, except perhaps in gas phase reactions like the intramolecular example 6.17 6.19 with a plausible transition structure... 

Several curious molecules with cumulated C=C bonds and a carbonyl group at the end have received attention. Problems were the separation of unstable compounds and the measuring technique, and also the direction of the dipole moments and sometimes even the geometry the exact linearity can be doubted in some cases. In the series of compounds 63 with an increasing number of cumulated double bonds, the dipole moments (in the gas phase) change in an irregular way156 the compound with n = 2 is not linear. 

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