Aldehydes, Estimation

Anisidine Value measures the extent of oxidative deterioration, which spectrophoto-metrically estimates aldehydes, and secondary oxidation products. 

Noth. The above method can be successfully applied only to dilute solutions of formaldehyde which are free in particular from other alfphatic aldehydes, since the latter, if present, would undergo a similar oxidation. Formaldehyde, if mixed with other aldehydes, should be estimated by quantitative addition of potassium cyanide for details, see advanced text-books of quantitative organic analysis. 

Many compounds contain more than one functional group Prostaglandin Ei a hormone that regulates the relaxation of smooth muscles con tains two different kinds of carbonyl groups Classify each one (aldehyde ketone carboxylic acid ester amide acyl chloride or acid anhydride) Identify the most acidic proton in prostaglandin Ei and use Table 1 7 to estimate its pK ... 

Chain lengths of some oxidations can be quite long (>100), especially for substrates with easily abstractable hydrogens when they are oxidized under mild conditions at low conversions. Aldehydes are good examples of such substrates (26). Many other oxidations have chain lengths estimated from 3 to 10. At limiting rates, the chain length is near 1 (25). 

The thermal glass-transition temperatures of poly(vinyl acetal)s can be determined by dynamic mechanical analysis, differential scanning calorimetry, and nmr techniques (31). The thermal glass-transition temperature of poly(vinyl acetal) resins prepared from aliphatic aldehydes can be estimated from empirical relationships such as equation 1 where OH and OAc are the weight percent of vinyl alcohol and vinyl acetate units and C is the number of carbons in the chain derived from the aldehyde. The symbols with subscripts are the corresponding values for a standard (s) resin with known parameters (32). The formula accurately predicts that resin T increases as vinyl alcohol content increases, and decreases as vinyl acetate content and aldehyde carbon chain length increases. 

This reaction shows that the estimation of alcohols in essential oils containing aldehydes may lead to numbers which are too high. 

TSie processes depending on the use of sodium bisulphite or sulphite, and in which the aldehyde or ketone compounds dissolve in the solution of the reagent, are known as absorption processes, and are those most commonly employed for oils containing a high proportion of aldehydes and ketones, the use of sodium bisulphite being probably still the method most usually adopted for aldehydes, though the use of neutral sodium sulphite is the official process in the British Pharmacopoeia of 1914, and is also that most suitable for the estimation of ketones. 

In all the foregoing cases, the percentage of aldehyde or ketone is so high that the estimation by the above processes can be sufficiently accurately carried out on the original oil. With such oils as lemon, orange, hand-pressed lime, and citron or cedrat, however, the proportion of aldehydes is so small that it is not possible to satisfactorily determine it directly on the oil itself by absorption processes, and a preliminary concentration of the aldehydes in the oils by carefully fractionating out the hydrocarbons in vacuo has therefore been proposed by Burgess and Child who recommend the operation to be carried out as follows —... 

For the estimation of benzaldehyde, Eipper proposed a volumetric modification of the bisulphite process, the aldehyde being shaken with a measured volume of a standard solution of bisulphite, and the excess of bisulphite titrated back with iodine solution at a low tempe/atnrer Dodge found this give fairly accurate results, and recommends the iollowing method of carrying out the determination. About 0 15 gram... 

Several processes for the estimation of aldehydes and ketones have been based on these reactions, some depending on the separation and weighing of the insoluble hydrazone, others on treatment of the substance with an excess of pbenylhydrazine, and estimation of the unused reagent. 

Semioxamazide.—A gravimetric method for the estimation of cinnamic-aldehyde in cassia and cinnamon oils, but which appears to apply only to this aldehyde, has been devised by Hanus based on the formation of a crystalline semioxamazone when cinnamic aldehyde is treated -with semioxamazide, the reaction being—... 

Acetylation.—Gitronellal may be quantitatively estimated by the ordinary acetylation process i when the aldehyde is quantitatively converted into isopulegyl acetate, which is then determined by saponification with potash in the ordinary way. Dupont and Labaume have attempted to base a method for the separation of geraniol from citronellal in citron-ella oils on the fact that the citronellal oxime formed by shaking with hydroxylamine solution at the ordinary temperature is not converted into an ester by subsequent acetylation, but into the nitrile of citronellic acid which is stable towards" alkali during the saponification process. 

Dodge has based a process for the determination of benzaldehyde. A strong (2 5 N) alcoholic potash solution is required for the estimation, which is performed. by allowing a mixture of 10 c.c. of this solution with 1 to 2 grams benzaldehyde to stand at the ordinary temperature for twenty-four hours, after which the unabsorbed pota is titrated back with N/2 hydrochloric acid. A blank test is also made, and from the amount of potash entering into reaction, the percentage of aldehyde can be calculated. The process breaks down in the assay of natural oil of bitter almonds, probably due to the presence of benzaldehyde cyanhydrin. 

To a solution of 34.6 mmol of lithium 1 -(dimethylamino)naphthalenide34 in THF at — 60°C are added dropwise 3.20 g (15.7 mmol) of l-methylene-2-phcnylthiocyclohexane in 20 mL of THF. The color of the solution changes from dark green to brown when the reagent is consumed yield >90% (estimated from the yield of aldehyde adducts). 

The regioselectivity of this reaction is excellent (92 8), and the diastereomeric purity of 2 is estimated to be 93% de on the basis of the oxidation of 2 to (5)-2-cyclohexen-1-ol (93% ee). Similarly, the reaction of 2 with acetaldehyde provides (S.iS H- -cyclohexeny ethanol with an enantiomeric purity of 92% cc. Reactions of 2 with other aldehydes, however, have not yet been reported. 

Although the fate of Cr(IV) is uncertain, (cf. the alcohol oxidation), some characteristics of the intermediate chromium species have been obtained by Wiberg and Richardson from a study of competitions between benzaldehyde and each of several substituted benzaldehydes. The competition between the two aldehydes for Cr(VI) is measured simply by their separate reactivities that for the Cr(V) or Cr(IV) is obtained from estimation of residual aldehyde by a C-labelling technique. If Cr(V) is involved then p values for oxidation by Cr(VI) and Cr(V) are 0.77 and 0.45, respectively. An isotope effect of 4.1 for oxidation of benzaldehyde by Cr(V) was obtained likewise. 

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