Acetylene molecule, 100 Ammonia

Lewis first sketched his idea about the octet rule on the back of an envelope. The models show water, ammonia, methane, ethylene, and acetylene molecules. 

A novel cadmate complex has been formed by the reaction of Cd(NH2>2 with I C=CH in the presence of acetylene in liquid ammonia.250 The potassium salt, K2Gd(CCH)4-2NH3 191, has been structurally characterized. The cadmium center is tetrahedrally coordinated to four acetylide units with which it forms Cd-C bonds of 2.23-2.25 A (Figure 29). The acetylide ligands are 7r-coordinated to two crystallographically distinct potassium ions whose coordination sphere is completed by two ammonia molecules. 

Figure 5.2 Schematic representation of four atomic molecules (from top to bottom) linear (acetylene), planar (formaldehyde), a-planar (ammonia).

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Type 4A sieves. The pore size is about 4 Angstroms, so that, besides water, the ethane molecules (but not butane) can be adsorbed. Other molecules removed from mixtures include carbon dioxide, hydrogen sulphide, sulphur dioxide, ammonia, methanol, ethanol, ethylene, acetylene, propylene, n-propyl alcohol, ethylene oxide and (below -30°) nitrogen, oxygen and methane. The material is supplied as beads, pellets or powder. 

Over 25% of the elements, ineluding carbon, have been identified, as well as molecules of water, carbon monoxide, carbon dioxide, ammonia, ethane, methane, acetylene, formaldehyde, formic acid, methyl alcohol, hydrogen cyanide, and acetonitrile. When applied to the planets only, the science is called chemical planetology,... 

Direct ionization produces a staircaselike structure in the plot of ion current as a function of photon energy, where the height of each step is proportional to the probability of production of a certain vibronic state of the ion. Such favorable cases of staircaselike structure have been observed for ammonia87 and acetylene.88 The structure in ammonia is attributable to excitation of successive vibrational levels of the out-of-plane bending mode of the ion and in acetylene, to excitation of the C-C stretching mode. As a result, these molecules are favorable candidates for studying the effects of vibrational excitation on the cross sections for ion-molecule reactions. 

For example, if we wanted to deprotonate ethyne (acetylene, piC, 25), then hydroxide (the strongest base we could have in aqueous solution, pJTa 15.7) would establish an equilibrium where only 1 in 109 3 (1015 7/1025) ethyne molecules were deprotonated. This means about 1 in 2 billion of our ethyne molecules will be deprotonated at any one time. Since, no matter what base we dissolve in water, we will only at best get hydroxide ions, this is the best we could do in water. So, in order to deprotonate ethyne to any appreciable extent, we must use a different solvent that does not have a p. Ca less than 25. Conditions often used to do this reaction are sodium amide (NaNH2) in liquid ammonia. 

This synthetic route can be usefiil for organic molecules with exceptionally acidic protons, such as acetylenes or cyclopentadienes. These reactions often require the alkali metal to be present as very small particles. Sodium sand can be formed by refluxing in THF with minor agitation to break up the sodium into tiny particles. Reactions with potassium may require a mirror that can be formed by dissolving potassium metal in liquid ammonia followed by evaporation of the ammonia from the reaction vessel. Ultrasound has also been used to aid in the formation of organolithium compounds from lithium metal. 

The alkali metal-graphite compounds are extremely reactive. They ignite in air and may react explosively with water. In the controlled reaction with water or alcohol only alkali hydroxide and hydrogen result there is no acetylene or any other hydrocarbon. Fredenhagen concluded from this that the compounds could not be carbides. Mercury dissolves the alkali metal out of the lattice. When treated with liquid ammonia, CgMe gives up only a third of the alkali metal and takes in its place two molecules of ammonia (see Section IIIA4). 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

The simplest approach to avoid thc distillation and to separate the polyynes from PAHs and carbon is to precipitate them as copper salts, taking advantage of a particular property of the hydrogen-terminated acetylenes to react with Cu(I) salt solutions to produce acetylides. Thus, the polyyne acetylides precipitate as brown powders by stirring a concentrated solution of these molecules in decalin with a Cu(I) solution in ammonia. The filtration permits the recovery of these molecules as the copper salts. 

The partial reduction of C=C bonds by alkali metals in liquid ammonia can be applied also to ethynylcarbinols274 and acetylenic compounds that contain hydroxyl275 or acid groups276 in the molecule. 

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