Hydrogenation alkene mixtures

In 1875 Alexander M Zaitsev of the University of Kazan (Russia) set forth a gen erahzation describing the regioselectivity of p eliminations Zaitsev s rule summarizes the results of numerous experiments m which alkene mixtures were produced by p elim matron In its original form Zaitsev s rule stated that the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the f3 carbon hav mg the fewest hydrogens... 

When optically pure 2 3 dimethyl 2 pentanol was subjected to dehydration a mixture of two alkenes was obtained Hydrogenation of this alkene mixture gave 2 3 dimethylpentane which was 50% optically pure What were the two alkenes formed in the elimination reaction and what were the relative amounts of each" ... 

The proton can be lost in two different ways, and a mixture of alkene isomers is obtained. The alkene mixture is known as diisobutylene and has a number of commercial uses. Hydrogenation yields 2,2,4-trimethylpentane... 

TABLE 3. Hydrogenation of alkene mixtures in the presence of Pt catalysts (548 K, flow reactor)... 

TABLE 4. Shape-selective hydrogenation of alkene mixtures in the presence of 1% Pt-ZSM-5 catalyst reduced in a mixture of alkenes and H2... 

The problem of selective hydrogenation of one double bond of a nonconjugated diene is essentially the same as that of competitive hydrogenation of alkene mixtures (see Section IV.A. 1). The less substituted double bond of the diene, consequently, can preferentially be hydrogenated under appropriate reaction conditions. The use of nonisomerizing catalysts, in general, is recommended to achieve such regioselective reaction. 

The neutral molecular sieve, aluminum phosphate (ALPO, Chapter 10) can also serve as a shape selective support for metal catalysts. These catalysts are normally prepared by the incipient wetness process followed by appropriate drying and reduction procedures. Fig. 13.19 illustrates the selectivity observed using a Ni/ALPO catalyst to hydrogenate a mixture of styrene and a methyl styrene (Eqn. 13.13). The rates of hydrogenation of cyclic alkenes over a Rh/ALPO catalyst decreased in the order cyclopentene cyclohexene > cycloheptene > cyclooctene but no selectivity was observed in a competitive hydrogenation of a mixture of cyclohexene and cyclooctene. 75... 

As discussed in Chapter 13, the hydrogenation of mixtures of simple alkenes and olefinic alcohols in non-polar solvents results in the selective saturation of the hydroxy olefin because of the enhanced adsorption of this material caused by the presence of the hydroxy group. In alcoholic solvents, however, this haptophilic assistance is negated by the adsorption of the solvent molecules on the catalyst, and hydrogenation selectivity is controlled by the steric environments of the competing olefins.7i.72... 

Since this reaction is a unimolecular example of the selective hydrogenation of an alkene mixture, the successful saturation of the less-substituted double bond should take place most readily over those catalysts that are most effective for the preferential saturation of one olefin in a mixture. Ruthenium has not been used extensively for such hydrogenations, but P-2 Ni(B) has been effective in promoting the selective hydrogenation of one of the double bonds in 46, methylene norbornene (49) (Eqn. 15.30), dicyclopentadiene (50) (Eqn. 15.31), and 2-methyl-1,5-hexadiene (51) (Eqn. 15.32).6 9,80,81... 

Recently, the hydrogenation a mixture of toluene, styrene and 1-octene, representing a model feed for hydrotreating in the refining industry, was performed in monolith reactors [37]. One is a y-alumina monolith of diameter 1 cm and 15 or 30 cm long and the other is a more conventional cordierite monolith with a wall-coated layer of y-alumina. In both monoliths, the channels size is 1-2 mm and the catalyst is based on Ni. Substantial alkene conversions of more than 50% were observed in the small-channel reactors, which was attributed to the intensified mass-transfer rate generally measured in monolith reactors [16]. 

Scheme 6.33. A representation of the path of the dimerization of 2-methylpropene [isobutylene (CH3)2C=CH2] to form a carbocation that loses a proton (H ) to give the two alkenes, (path a) 2,4,4-trimethyl-2-pentene [(CH3)3CH=C(CHj)2] and (path b) 2,4,4-trunethyl-l-pentene [(CH3)3CCH2C(CH3)=CH2].The catalytic (Pt.Ni.etc.) reduction (hydrogenation) of the alkene mixture yields 2,2,4-trimethylpentane [isooctane, (CH3)3CCH2CH(CH3)2].

What are we trying to accomplish We need to find a way for an isomerized alkene to emerge from a catalytic hydrogenation reaction mixture. 

Alkynes can be hydrogenated under the same conditions used to hydrogenate alkenes. Typically, platinum or palladium on charcoal is suspended in a solution containing the alkyne and the mixture is exposed to a hydrogen atmosphere. Under these conditions, the triple bond is saturated completely. 

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