Ketone molecules

By contrast, in the system propionic acid d) - methyl isobutyl ketone (2), (fi and are very much different when y 1, Propionic acid has a strong tendency to dimerize with itself and only a weak tendency to dimerize with ketone also,the ketone has only a weak tendency to dimerize with itself. At acid-rich compositions, therefore, many acid molecules have dimerized but most ketone molecules are monomers. Acid-acid dimerization lowers the fugacity of acid and thus is well below unity. Because of acid-acid dimerization, the true mole fraction of ketone is signi-... 

The aldol reaction is a carbonyl condensation that occurs between two aldehyde or ketone molecules. Aldol reactions are reversible, leading first to a /3-hydroxy aldehyde or ketone and then to an cr,/6-unsaturated product. Mixed aldol condensations between two different aldehydes or ketones generally give a mixture of all four possible products. A mixed reaction can be successful, however, if one of the two partners is an unusually good donor (ethyl aceto-acetate, for instance) or if it can act only as an acceptor (formaldehyde and benzaldehyde, for instance). Intramolecular aldol condensations of 1,4- and 1,5-diketones are also successful and provide a good way to make five-and six-inembered rings. 

In the aldol reaction the a carbon of one aldehyde or ketone molecule adds to the carbonyl carbon of another. Although acid catalyzed aldol reactions are known, the most common form of the reaction uses a base. The base most often used is OH, though stronger bases such as alkoxides (RO ) are sometimes employed. Hydroxide ion is not a strong enough base to convert substantially all of an aldehyde or ketone molecule to the corresponding enolate ion, that is, the equilibrium lies... 

The dimerization of ketones to 1,2-diols can also be accomplished photochemi-cally indeed, this is one of the most common photochemical reactions. The substrate, which is usually a diaryl or aryl alkyl ketone (though a few aromatic aldehydes and dialkyl ketones have been dimerized), is irradiated with UV light in the presence of a hydrogen donor such as isopropyl alcohol, toluene, or an amine. In the case of benzophenone, irradiated in the presence of 2-propanol, the ketone molecule initially undergoes n — k excitation, and the singlet species thus formed crosses to the T, state with a very high efficiency. 

The decay of the biradical produces ketone molecule in the triplet state, which is an emitter of light [222], The CL intensity was proved to be propotional to the rate of chain initiation, which is equal to the rate of chain termination. The observed luminescence spectra were found to be identical with the spectra of the subsequent ketone in the triplet state. The intensity of CL (/chi) produced by oxidized hydrocarbon is the following ... 

CL is weak in liquid-phase hydrocarbon oxidation and can be intensified. To increase the CL, activators are added to the oxidized hydrocarbon [17,220,223]. The activator takes an excess of energy from the excited ketone molecule and emits light in high yield ... 

Example More extensive substitution at the oxirane system brings additional dissociation pathways for the molecular ions. Nevertheless, one of the main reaction paths of molecular ions of glycidols gives rise to enol radical ions by loss of a aldehyde (R = H) or ketone molecule. [218] The reaction mechanism can be rationalized by the assumption of a distonic intermediate (Scheme 6.78) ... 

It turns out that a reaction still occurs because the carbonyl group itself is an electrophile. As the enolate forms, it can attack the carbonyl group of another aldehyde or ketone molecule. This is an aldol reaction or aldol condensation, also called an aldol addition. 

The attack of the initiator on the carbonyl group gives butyl lsopropenyl ketone with a release of lithium methoxide, and the ketone molecule formed soonattacks. most of the growing chains owing to its higher reactivity than that of MMA. 

In the aldol reaction500 the a carbon of one aldehyde or ketone molecule adds to the carbonyl carbon of another.501 The base most often used is OH, though stronger bases, e.g., alu-... 

The presence of chlorine atoms in the ketone molecule leads to a further complication, since the C-Cl bond dissociation energy may be of the order of 80 kcal. mol.-1, so that a photodecomposition of type C becomes energetically feasible. 

The dependence of the quantum yield on temperature, pressure, and the presence of foreign gas has been explained by a mechanism similar to that proposed for acetone by Noyes67 and quoted earlier. The efficiency of energy transfer from the excited ketone molecule to the foreign gas was shown to depend on the number of vibrational modes in the molecule. 

If you are in difficulty, remember that the A1 atom is electrophilic and therefore combines with nucleophiles, and think which end of the ketone molecule is nucleophilic. 

Hydrogen abstraction can occur from a position within the ketone molecule, and this generates a biradical that may cyclize by combination of the radical centres. The overall photocydization process is observed for a wide variety of compound types, and it has been used extensively to make cyclic or polycyclic systems, in an unconstrained system a ketone (n,n ) excited state shows a preference tor abstraction from the y-position (4.42), which can be understood on... 

The major features of the Norrish II reactions which are germane to this chapter are included in Scheme 41. Note that each structure in Scheme 41 represents a family of conformers which are related by similarities in both structure and reactivity. A ketone molecule in collision-free space can exist in a variety of conformations produced by rotation around single C—C bonds. In a linear alkanone, the conformation of lowest energy is all-trans although local minima can exist where there is one or more gauche arrangements. 

The solid phases of 81 are also well ordered macroscopically and their higher E/C ratios require that the hydroxy-1,4-biradical be in rather inflexible reaction cages with little excess free volume. Hydrogen bonding to neighboring ketone molecules may be partially responsible for the high photoproduct ratios found upon collapse of the biradicals in the solid phases, but the size, shape, and flexibility of the reaction cavity are clearly the more important factors. The highest E/C ratios observed in the second solid phase of 81a... 

Silica gel surfaces (Figure 9) offer an anisotropic environment to adsorbed ketone molecules in which motions are restricted (in a crude sense) to two dimensions. Under the best of circumstances, a distribution of site sizes and... 

Zeolites are somewhat like silica in their surface characteristics. Ketones and hydroxy-1,4-biradicals have very polar groups which can interact favorably with metal cations located along zeolite walls. The potential effect of the metal ions on the position of the reacting ketones is twofold. First, the cations may force a ketone molecule into a conformation or a site which it would normally not occupy based solely upon free-volume considerations. Second, the diffusion coefficient of a ketone or a hydroxy-1,4-biradical is probably much more than an order of magnitude smaller than that of benzene [289] so that the residence time of a ketone and its Norrish II intermediates in a zeolite site with at least one metal ion is expected to be closer to 100 ns than to 1 ns. 

With ketones which can eject radicals more stable than methyl, fragmentation competes more successfully with all physical processes than in acetone, and unsymmetrical ketones preferentially eject the more stable alkyl radical.309 Thus both methyl ethyl ketone310 and methyl isopropyl ketone311 yield chiefly acetyl and ethyl or isopropyl radicals. Half of the diethyl ketone molecules excited by 3130-A irradiation at 25° decompose from the excited singlet state before they can undergo intersystem crossing, and another 40% fragment from the triplet state.312 Both fluorescence and phosphorescence are extremely weak. The more rapid decomposition in both excited states relative to that observed in acetone almost eliminates competition from physical-decay processes. 

The comments made about the diradical hypothesis with respect to the photochemistry of cyclopentanone are equally applicable to cyclohexanone. Since the formation of none of the products listed in reactions 15-18, and (15,29) is quenched by even 10-20 mm. of oxygen the existence of diradical intermediates in this system is subject to question. The alternative mechanism would be one that causes a concerted split of the ketone molecule in the excited state into two (in the cases of reactions 15 and 16) or three (reaction 17) molecular fragments. Both 16 and 17 are analogous to reactions 3 and 2 in the photochemistry of cyclopentanone and do not involve a shift of hydrogen atom from one... 

The quantum yield for this process at 3130 A. has been determined to be of the same order of magnitude in the vapor phase, in the pure liquid and in methyl pentane solution (29). The significance of this surprising result is difficult to assess at present since the data are sparse. In photolysis in aqueous solution a reaction has been observed (17) which bears resemblance to both eq. 18 and 52. It leads to the formation of caproic acid, the net reaction being the addition of a molecule of water and cleavage of the ring in the ketone molecule. 

Recent studies of the photolysis of bicyclic ketones have led to the preparation of unusually strained systems by the eUmination of carbon monoxide from excited ketone molecules. Irradiation of Formula 286 gives bicydo [2.2.0[hexane (Formula 287) in low yield (118). Mercury-photosensitized decomposition of bicyclo [2.2.1 ]heptan-2-one (Formula 258) gives bicyclo [2.2.1 [hexane (Formula 289) in 20% yield (119). Camphor (Formula 290) undergoes a similar mercury photosensitized decomposition to 1,5,5-trimethylbicyclo [2.2.1 [hexane (Formula 291) (10%) (119). 

PrO[3H] is taking place in the presence of suspensions of inorganic semiconductors, CdS, ZnO and TiC>2. The amount of D and T incorporated into the ketone molecules was proportional to the irradiation time. 

There are some known unsuccessful attempts to carry out alkylation (Mel, Me2S04), halogenation (tert-butyl hypochloride) and nitration of aromatic dihydrobenzodiazepines [7, 105]. Such attempts only resulted in the destruction of the seven-membered heterocycle. As a rule, these destructive processes are typical of dihydrodiazepine systems and often manifest themselves during the synthesis and study of these compounds. Therefore, the results of the destruction of a seven-membered heterocycle are most widespread and include its decomposition into ortho-diamine and carbonyl compounds (Scheme 4.43, reactions A and B) [105, 106] and benzimidazole rearrangement accompanied by splitting out of a methyl aryl ketone molecule (Scheme 4.43, reaction C) [117]. 

Investigation of the reactions between ortho-diamines and dibenzoylethylene 182 causes many problems for researchers. Owing to the polyelectrophilicity of this ketone molecule as well as the possibility of a redox process in the reaction mixtures, there is a potential in such reactions for the formation of various heterocyclic systems. This leads to difficulties in establishing the structure of the reaction products and gives rise to a number of errors. In particular, during the years 1970-1975, five mutually inconsistent reports devoted to the reaction of dibezoylethylene with o-PDA (Scheme 4.53) were published [130, 131, 132, 133, 134]. In these papers five possible structures (183, 184, and 186-188) were... 

The enol attacks a protonated carbonyl group of a second ketone molecule. 

Thus, the partial or total removal of the interdigitation of the lipid alkyl chains by cholesterol seems to be an important factor in determining the ability of the ketone molecule to place the C = O group close to the polar interface, at low temperatures. As the interdigitation is removed, the contact between the C = O group of the 9HP molecule and the solvent increases. 

Di-n-heptyl ketone and the di-n-heptyl ketone prepared catalytically from l-octanol-2-d were examined in carbon tetrachloride solutions with a cell having a path length of 1.0 mm. The ketone prepared from 1-octa-nol-2-d showed an absorption band characteristic of C-D at 4.63 of di-heptyl ketone. The number of C-D bonds per ketone molecule was estimated to be in the range of 1.0 to 1.5. A more accurate estimate of the C-D content of the ketone would require a reference sample of deuterated ketone, which was not available. 

The tetrahydro-y-pyrone serves as a self-initiator through the light absorption of the ketone group in the molecule. The possibility that the reactive radical is formed through the collapse of the excited tetrahydro-y-pyrone molecule cannot be excluded, although excited ketone molecules do not tend to collapse in such a manner (69). 

---