2- Methylbutane isomerization

Treatment of 3 methyl 2 butanol with hydrogen chloride yielded only a trace of 2 chloro 3 methylbutane An isomeric chloride was isolated in 97% yield Suggest a reasonable structure for this product... 

There are three isomeric pentanes, ie, saturated aHphatic hydrocarbons of molecular formula C H 2- They are commonly called / -pentane [109-66-0] isopentane [78-78-4] (2-methylbutane), and neopentane [463-82-1] (2,2-dimethylpropane). 

As already discussed for aldol and Robinson annulation reactions, proline is also a catalyst for enantioselective Mannich reactions. Proline effectively catalyzes the reactions of aldehydes such as 3-methylbutanal and hexanal with /V-arylimines of ethyl glyoxalate.196 These reactions show 2,3-syn selectivity, although the products with small alkyl groups tend to isomerize to the anti isomer. 

When 2-bromo-3-methylbutane is reacted with hot ethanolic potassium hydroxide, a mixture of two isomeric alkenes is formed. Give the structural formulae of the two alkenes. (2)... 

With rhodium, aldehyde 27 is also the main isomeric aldehyde, but no aldehyde 26 is formed. Furthermore, slightly more erythro than threo isomer is formed. Thus, hydroformylation on / carbon is different for allylbenzene and for 3-phenyl-l-butene. The lower yield of 3-phenyl-2-methylbutanal is very likely the result of steric hindrance from the methyl group on the a carbon. 

In a multistep synthesis, the overall percent yield is the product of the fractional yields in each step times 100 and decreases rapidly with the number of steps. For this reason, a low-yield step along the way can mean practical failure for the overall sequence. Usually, the best sequence will be the one with the fewest steps. Exceptions arise when the desired product is obtained as a component of a mixture that is difficult to separate. For example, one could prepare 2-chloro-2-methylbutane in one step by direct chlorination of 2-methyl-butane (Section 4-5A). But because the desired product is very difficult to separate from the other, isomeric monochlorinated products, it is desirable to use a longer sequence that may give a lower yield but avoids the separation problem. Similar separation problems would be encountered in a synthesis that gives a mixture of stereoisomers when only one isomer is desired. Again, the optimal synthesis may involve a longer sequence that would be stereospecific for the desired isomer. 

Interestingly, the hydrogen/deuterium exchange of both (M — H) ions with D20 is not observed to go to completion, but stops after a certain reaction time. Evidence has been presented that this phenomenon is due to an irreversible isomerization of the (M — H) ions to the enolate anion [13] of 3-methylbutanal, which also gives the same product ions in reactions with hexafluorobenzene (Peerboom et al., 1985). The results obtained clearly point to a complex mixture of interconverting ion structures as shown in (67a-e). 

It is clear from the proposed mechanism that the initial step will be facilitated by the presence of a second hydroxyl group in the phenol, oriented meta to the first. In view of the importance of polyphenolic molecules in natural product chemistry, this synthesis takes on a special significance. Thus, the reaction of dihydric phenols with citral (65) has been used to synthesize a diverse range of natural products. However, such compounds can, and in some cases do, give rise to a mixture of isomeric products. Olivetol (64), for instance, yields a mixture of mono- and bis-chromenes of which cannabichromene (66) is a significant component (71JCS(C)796). On the other hand, the phenol (67) affords a h igh yield of isoevodionol (68) on reaction with 4,4-dimethoxy-2-methylbutan-2-ol (71JCS(C)8U). 

The kinetic parameters (apparent activation energies, orders versus hydrogen and hydrocarbon) have been determined for all the isomerization and hydrocracking reactions of n-pentane and 2-methylbutane on a Pt/AljOj catalyst of low dispersion (10% Pt d = 90 A) 40). n-Pentane-2- C and 2-methylbutane-2- C were used to estimate the contributions of cyclic type and bond shift isomerization, respectively. As shown in Table II, the reactions... 

Fig. 15. Particle size effect in isomerization of 2-methylbulane-2- C (O) n-pentane/ neopentane ratio ( ) n-pentane/2-methylbutane-3- C ratio.

One can notice that the easiest route on all catalysts is the isomerization of 2-methylbutane-2- C to 2-methylbutane-3- C, involving certainly a symmetrical intermediate. This result, and the earlier mentioned one, fast isomerization of 3-methylhexane-3- C to 3-methylhexanes-2- C and -4- C (see Section III, p. 27), support the view that, irrespective of the exact nature of the mechanism, bond shift reactions involving symmetrical intermediates are favored over the other reaction pathways. 

Isoprene epoxidized on the vinyl group (l,2-epoxy-3-methyl-3-butene, 14) would also be an attractive synthon for monoterpenoids if it were more readily accessible. Among the possible preparative methods (Scheme 2), acid-catalyzed isomerization of 2,3-epoxy-l-halo-3-methylbutanes (epoxidized prenyl halides) and dehydrohalogenation of the resulting isomers seemed the best route until chlorination of prenyl acetate (15, R = OAc) with hypochlorous acid was published. Dehydration of the mixture of products gave a maximum of the chloroacetate 16, which yielded the epoxide 14 with sodium hydroxide in methanol. ... 

Verma AL, Murphy WF, Bernstein HJ. Rotational isomerism XI. Raman spectra of n-butane, 2-methylbutane, and 2,3-dimethylbutane. J Chem Phys 1974 60 1540-1544. 

C (8) and n-butane-l-C (9) were contacted in the presence of aluminum bromide promoted with water. In the study however, of the liquid-phase isomerization of 2-methylbutane-l-C over an aluminum bromide catalyst... 

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