Carbon skeleton determination hydrogenation

In all parts of this exercise we deduce the carbon skeleton on the basis of the alkane formed on hydrogenation of an alkene and then determine what carbon atoms may be connected by a double bond in that skeleton. Problems of this type are best done by using carbon skeleton formulas. 

Small isotope-effects can be detected by double-labeling techniques, in which the carbon skeleton is labeled with 14C, and the ratio of 14C to tritium is measured both in the substrate and the product. Care must be taken in the observation and interpretation of isotope effects determined from the hydrogen-isotope content of the product. Just as in non-enzymic reactions (see p. 154), discrimination against the substrate containing deuterium or tritium leads to an increase in the isotopic content of the substrate, and this decreases the apparent isotope-effect towards the end of the reaction. 

The mechanistic background for such a comparison is illustrated in Figure 10 which represents in more detail the pathway of hydroisomerization and hydrocracking of two n-alkanes. Branched carbenium ions are formed via n-alkenes and linear carbenium ions. Then, either desorption or -scission may occur in parallel reactions. Desorption (followed by hydrogenation) of a given carbenium ion yields an iso-alkane with the same carbon skeleton. /S - scission, on the other hand, yields fragments of definite carbon numbers ( /3 -scissions which would yield or are excluded). Thus a comparison between relative concentrations of the iso-alkanes and relative probabilities of the cracking reactions may be informative since both sets of data are determinable independently from each other. 

Carbon-skeleton chromatography was reported and subsequently developed by Beroza and co-workers (B6-B8, B10-B12). This technique involves catalytic hydrogenation of substances at elevated temperatures to produce hydrocarbons, which are then separated by GLC and identified by retention times and, sometimes, by other means (e.g., mass spectrometry). Under certain conditions, the hydrocarbon produced is the carbon skeleton of the original substance. Such a process is obviously of great value in providing an additional parameter in the problem of determination of chemical structure. Beroza (B9) has reviewed the developments in this field up to 1966. 

Oxidation with lead dioxide in 5% phosphoric acid yielded acetaldehyde and formaldehyde. Catalytic reduction in the presence of platinum resulted in the absorption of 1.5 mole equivalents of hydrogen and the isolation, by its steam volatility, of nearly 0.6 mole of a-methylbutyric acid. The carbon skeleton of sarracinic acid was thus determined, and the accompanying hydrogenolysis (some nonvolatile acid was also obtained) established that the double bond and hydroxyl constituted an allylic alcohol moiety. Since further data (spectroscopic data would be especially valuable) were not available, Danilova and Kuzovkov (127) were limited to the conclusion that sarracinic acid could be represented by one of three possible structures (CLXIa-c) ... 

In the determination of the carbon skeleton, thoroughly elaborated reaction methods associated with the participation of hydrogen are most often used. In the literature (e.g., ref. 1) the methods for determining the carbon skeleton are considered only on the basis of this reaction. However, for determining the carbon skeleton, other reactions can also be employed successfully, although their field of application is narrower. Thus, methylation is valuable for the identification of hydrocarbons (see Chapter 1). Diazomethane reacts under strong ultraviolet irradiation in the cold in accordance with the following equation ... 

The method for determining the carbon skeleton can be realized in several versions. First, the hydrogenolysis, hydrogenation or dehydrogenation of substances can be carried out independently of their chromatographic determination, by using known chemical methods (e.g., refs. 9-11). 

Of course, the carbon skeleton of cyclopropane is flat— three points determine a plane—and planarity causes still other problems for cyclopropane. Figure 5.4 shows a Newman projection looking down one of the three equivalent carbon-carbon bonds of cyclopropane. In cyclopropane, all carbon-hydrogen bonds are eclipsed. Remember. The solid wedges are coming toward you, and the dashed wedges are retreating from you. 

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