Hydrogenation cumulative double bonds

The cumulated double bonds of an allenic system are of relatively high energy The heat of hydrogenation of allene is more than twice that of propene... 

Table 7.20 Absorption Frequencies of Single Bonds to Hydrogen Table 7.21 Absorption Frequencies of Triple Bonds Table 7.22 Absorption Frequencies of Cumulated Double Bonds Table 7.23 Absorption Frequencies of Carbonyl Bands...

The two most commonly applied systems for naming polycyclic parents are in some ways complementary. Fusion nomenclature provides names for structures containing the maximum number of non-cumulative double bonds von Baeyer nomenclature (Section 1.02.3.4) names fully saturated structures. Thus names for partially hydrogenated structures can be arrived at either by adding hydro prefixes to fusion names or ene , diene , etc. suffixes to von Baeyer names (see examples 29 and 30). If needed, rules are available for... 

Most parent structures consist essentially of an assembly of rings and/or chains, the degree of hydrogenation of which is defined (usually completely saturated or containing the maximum number of non-cumulative double bonds in cyclic portions), and having no attached functional substituents (the carbohydrates are a notable exception to this). The stereochemistry at all (or most) chiral centres is defined thus such parent structures are sometimes referred to as stereoparents . Some examples are shown (77)-(83). 

The heat of hydrogenation of one carbon-carbon double bond of allene is 41 kcal mol-1, whereas that of an ordinary alkene is around 29 kcal mol-1. Thus the cumulated double bond of allene liberates 12 kcal mol-1 more than that of a simple alkene on hydrogenation. Accumulation of two carbon-carbon double bonds imparts an extra reactivity to allene, making it a remarkably active component participating in a variety of cycloaddition reactions as a two-carbon unit. 

The demethanizer, deethanizer, and debutanizer are fractionating columns that separate the lighter and heavier compounds from each other. Traces of triple bonds are removed by catalytic hydrogenation with a palladium catalyst in both the C2 and C3 stream. Cumulated double bonds are also hydrogenated in the C3 fraction. These are more reactive in hydrogenation than ethylene or propylene. The C2 and C3 splitters (Fig. 8.4) are distillation columns that can be as high as 200 ft. The mechanism of cracking was previously discussed in Chapter 7, Section 6. 

When a name applies equally to two or more isomeric condensed parent ring systems with the maximum number of non-cumulative double bonds and when the name can be made specific by indicating the position of one or more hydrogen atoms in the structure, this is accomplished by modifying the name with a locant, followed by italic capital H for each of these hydrogen atoms. Such symbols ordinarily precede the name. The said atom or atoms are called indicated hydrogen . The same principle is applied to radicals and compounds derived from these systems. 

The names of ortho-fused or ortho- and peri-fused polycyclic hydrocarbons with less than maximum number of non-cumulative double bonds are formed from a prefix dihydro- , tetrahydro- , etc., followed by the name of the corresponding unreduced hydrocarbon. The prefix perhydro- signifies full hydrogenation. When there is a choice... 

When the two conditions of paragraph (a) are not fulfilled, positions in the skeleton of the corresponding hydrocarbon that are occupied by hetero atoms are denoted by a prefixes, and the parent heterocyclic compound is considered to be that which contains the maximum number of conjugated or isolated double bonds, but the corresponding hydrocarbon is named in the form in which it contains the maximum number of non-cumulative double bonds. Hydrogen additional to that present in the parent heterocyclic compound is named by hydro prefixes and/or as H in front of the a terms. 

Because penta-1,2-diene has a larger heat of hydrogenation than penta-1,4-diene, we conclude that the cumulated double bonds of allenes are less stable than isolated double bonds and much less stable than conjugated double bonds. Figure 15-1 summarizes the relative stability of isolated, conjugated, and cumulated dienes and compares them with alkynes. 

The hydrogenation of cumulative double bonds (allenes) takes place at a rate comparable to that of the corresponding acetylene but no isomerization to the acetylene is observed during the reaction. "5 Selective hydrogenation to the alkene occurs since the allene readily displaces the product olefin from the catalyst before the hydrogenation can take place. Selectivities for monoalkene formation of greater than 95% were commonly obtained in the hydrogenation of various substituted allenes." This reaction selectivity is probably facilitated by the fact that the n electron clouds of the two double bonds are perpendicular to... 

In most cases, 1,2-addition of hydrogen to the C=C bond forms the end result. However, 1,4-addition has been observed to some conjugated enyne systems,260,263,264 whereby, it is interesting to note, compounds with cumulated double bonds result e.g. ... 

If an a-halogen is available hydrogen chloride can be eliminated, thereby generating a cumulative double bond system or a 1,3-dipole. 

What happens if two double bonds are even closer together than in the conjugated case Successive double bonds with no intervening single bonds are called cumulated double bonds. Consider 1,2-pentadiene, which contains cumulated double bonds. Such 1,2-diene systems are also called allenes, after the simplest member of the class, 1,2-propadiene or allene, H2C = C=CH2- The heat of hydrogenation of 1,2-pentadiene is —292 kJ/mol (—69.8 kcal/mol). 

Cumulative double bonds apparently undergo electrophilic addition as expected to form the most stable carbocation. Although relatively little work has been done to examine the details of the reaction with electrophiles (because compounds with cumulative double bonds are rare), it is nevertheless interesting that addition of hydrogen chloride (HCl) to 1,2-propadiene (aUene [CH2=C=CH2]) produces 2-chloropropene [CH2=C(C1)-CH3] rather than the isomeric 3-chloro-l-propene [CH2=CH-CH2C1] (Scheme 6.36). Thus, it appears that a secondary vinylic carbocation (CH2=C CH3) is preferred to a primary allylic carbocation (CFl2 CH=CH2) where the empty orbital is out of phase with the filled rr-system. 

Ketenes are oxo compounds with cumulated carbonyl and carbon—carbon double bonds of the general stmcture R R2C—C—O, where and R2 may be any combination of hydrogen, alkyl, aryl, acyl, halogen, and many other functional groups. Ketenes with R = sometimes called aldoketenes,... 

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