Base strength aldehydes

Adolph Baeyer is credited with the first recognition of the general nature of the reaction between phenols and aldehydes in 1872 ([2,5-7] [18], Table 5.1). He reported formation of colorless resins when acidic solutions of pyrogallic acid or resorcinol were mixed with oil of bitter almonds, which consists primarily benzaldehyde. Baeyer also saw resin formation with acidic and basic solutions of phenol and acetaldehyde or chloral. Michael and Comey furthered Baeyer s work with additional studies on the behavior of benzaldehyde and phenols [2,19]. They studied a variety of acidic and basic catalysts and noted that reaction vigor followed the acid or base strength of the catalyst. Michael et al. also reported rapid oxidation and darkening of phenolic resins when catalyzed by alkaline materials. 

Early in 1963 Culbertson and Pettit6 reported the base strengths of ten simple polycyclic benzenoid aldehydes and ketones and attempted to relate the pKa values to molecular structure. They were able to show the existence of a linear correlation between -pKa and the protonation it energy, that is, the gain in n energy of the protonated over the neutral compound. In their treatment... 

Quaternary ammonium hydroxides immobilized on MCM-41 (MCM-410H1) [23] act a stronger base catalyst tban amine analogs. The condensation of salicyl-aldehyde with diethyl-2-pentenedicarboxylate in the presence of this catalyst led to a mixture of coumarin-3-acrylates and chromene derivatives [24], The ratio of the formation of coumarin-3-acrylates and chromene derivatives depended on the base strength of the catalyst (Scheme 5.8). 

A suitable solid base must have the appropriate base strength for the reaction under investigation. If the initial reaction step is the removal of a proton from a reactant of the form R1-CH2-R2 then the acidity of the proton to be removed depends on the identity of the R) and R2 groups (Table 1). The solid base selected should have sufficient base strength to carry out the reaction but should not have excessive base strength as this may lead to rapid catalyst deactivation or to side-product formation. For aldehyde and ketone condensation reactions therefore with a p.Ka of 19.7 - 20 a strong base is required but not a superbase material. Caustic can be used to carry out reactions with reactants with the removable proton having a pA a of up to around 20. 

The impetus for low temperature aldol processes in the absence or minimization of organic solvent has resulted in several new approaches. For example, Dewa et al. describe an aldehyde-ketone condensation catalyst that can be suspended in water, prepared by reacting lanthanum tris(isopropoxide) with anthracenebisresorcinol (ABR). A polymeric aquo complex with several 1,2-linked ABRs and two (LaOH) + groups is formed, coordinating 30 water molecules and relatively stable to decomposition. The base strength of this complex approached that of aqueous NaaCOs. 

As might be expected the base strengths of aldehydes seem to fall between those of the corresponding ketones and carboxylic acids. They are too weak to be titrated in acetic acid and the aliphatic members are too unstable to acid to have been studied successfully as Hammett bases. Most of our knowledge of pK s in this series comes from the careful studies of Schubert et al. (57,298,301-303) and the more recent report of Yates and Stewart (371) all of which are concerned with substituted benzaldehydes. 

Oxazines are bases of varying strength and stability. Generally speaking, those which were derived by cyclization using aliphatic aldehydes are more stable than those formed from aromatic aldehydes. 

Hull and Conant in 1927 showed that weak organic bases (ketones and aldehydes) will form salts with perchloric acid in nonaqueous solvents. This results from the ability of perchlonc aad in nonaqueous systems to protonate these weak bases. These early investigators called such a system a superacid. Some authorities believe that any protic acid that is stronger than sulfunc aad (100%) should be typed as a superaad. Based upon this criterion, fluorosulfuric arid and trifluoro-methanesulfonic acid, among others, are so classified. Acidic oxides (silica and silica-aluminai have been used as solid acid catalysts for many years. Within the last few years, solid acid systems of considerably greater strength have been developed and can he classified as solid superacids. 

Basic Protocols 8 and 9 look at specific groups of compounds, aldehydes and esters, which are of more interest to flavor houses and research scientists. For flavorists, the strength of citms flavor is determined from the aldehyde content, while the fruity aroma is attributed to the ester content. Over the years, these procedures have been modified. Basic Protocol 8 is a titration method to determine aldehyde content in citms oils. It was originally developed for lemon oil however, it is also applicable to other citms oils. Alternate Protocols 4 and 5 also produce similar results. Alternate Protocol 4 is based on... 

Aldehyde content is considered an important flavor note included in the standard of identities for citrus oils. The flavor strength of an oil is based on the aldehyde content, where higher is better. The two major aldehydes are acetaldehyde and octanal. The quantification of aldehydes is based on the reaction of citral in the sample with a hydroxylamine solution, followed by titration with KOH in the presence of ethyl orange indicator. It is modified from AOAC Method 955.32 (AOAC, 1990c Redd et al., 1986). This method was originally developed for lemon oils however, it is applicable to other citrus oils. 

Colorimetric Method (Rocques). This is based on the brown coloration given by alcohols (especially those with non-normal chains) when heated with concentrated sulphuric acid. It should be carried out on the alcohols brought to a definite strength and freed from aldehydes, which also give an intense brown coloration with sulphuric acid. The determination is made colorimetrically, by comparison with a suitable standard solution. 

In other aromatic carbonyl compounds, the relative binding strength is based mainly on steric considerations. For benzaldehydes (Entries 7 and 8),127,130 equilibrium constants are on the order of 104 for adduct formation and the borane binds syn to the sterically insignificant aldehyde proton. In the acetophenone adduct, the borane binds syn to the methyl group, but the equilibrium constant is an order of magnitude lower due to... 

Many product bases that the perfumer has to deal with may be either acidic or alkaline, and as we have already seen, under such conditions a number of materials such as esters, aldehydes, and ketones are likely to be unstable. The severity of the problem depends on the strength and type of acid or alkali concerned. For instance, hydrochloric acid and citric acid are examples of a strong and a weak acid, respectively, and sodium hydroxide and ammonia of a strong and a weak alkali. One way commonly used to express the degree of acidity or alkalinity is by means of the pH value. This is a numerical scale from 1 to 14 in which pH 1 is the most acidic and pH 14 the most alkaline. At pH 7 the product is said to be neutral. 

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