Propane methylene group

Note however that Table 4 3 mcludes two entries for propane The second entry corresponds to the cleavage of a bond to one of the hydrogens of the methylene (CH2) group It requires slightly less energy to break a C—H bond m the methylene group than m the methyl group... 

Calculations and Experiments on the Stereomutation of Cyclopropane. In 1965, Hoffmann published a seminal paper on trimethylene, another name for propane-1,3-diyl (8). He used extended hiickel (EH) calculations and an orbital interaction diagram to show that hyperconjugative electron donation from the central methylene group destabilizes the symmetric combination of 2p-n AOs on the terminal carbons in the (0,0) conformation of this diradical. Hoffmann s calculations predicted that the resulting occupancy of the antisymmetric combination of 2p-n AOs in 8 should favor conrotatory opening of cyclopropane (7), as depicted in Figure 22.8. 

In contrast, an extremely low activity was observed for the gallium-modified silicalite-1. scrambling started first at 723 K, which clearly indicates that Bronsted acid sites are necessary to activate propane adsorbed on zeolites Ga/ HZSM-5 179,181. A low activity was also observed for C-2-propane adsorbed on zeolite HZSM-5 in the absence of gallium. On this catalyst, C scrambling was observed after heating at 573 K for 20 min, and the theoretical 2 1 ratio of the signal intensities of methyl and methylene groups was reached after 80 min at 573 K. 

Since oxygen is much smaller than a methylene group, the same kind of situation occurs in XVII as was discussed in the previous section. The barrier to methyl rotation in dimethyl ether is 2.7 kcal/mole >, only slightly lower than in propane, where the beirrier is 3.4 kcal/mole. Oxocane should therefore have the BC-1 conformation, as in methylenecyclooctaue rather than the BC-3 and BC-7 conformations. The presence of only a single process in the proton spectrum of XVII is immediately consistent with the BC-1 conformation, but requires rapid pseudorotation between the BC-3 and BC-7 forms at —170 °C if the latter two forms are the correct conformations. The pseudorotation barrier in XVII should be higher than in cyclooctane, and probably comparable to that in cyclooctanone (6.3 kcal/mole). Thus, pseudorotation of the BC-3 form should not be rapid at —170 °C, and further support for this h5q)othesis is provided by 1,3-dioxocane (see below). It is therefore probable that oxocane has the BC-1 conformation. 

Hydrogenolysis of the C-S bond can be achieved both by dissolving metal systems (sodium in liquid ammonia) or by catalytic methods, particularly with a finely divided reactive form of nickel known as Raney nickel. When the latter is combined with dithioacetal formation, using either ethanedithiol or propane-1,3-dithiol, the result is a mild method for reducing a carbonyl group to a methylene group. 

The reluctance of phenyl-substituted methylenecyclopropanes to rearrange to products in which the phenyl group is located at the exocyclic methylene group was also demonstrated with l-methylene-2-phenylcyclopropane and l-methyl-2-methylene-l-phenylcyclopropane. When used as single enantiomers, both compounds underwent facile racemization at 100 C in chloroform without formation of benzylidenecyclopropane or (l-phenylethylidene)cyclo-propane. Similar results were obtained when a mixture of as- and tra i-l-methyl-2-methyl-ene-3-phenylcyclopropane (Table 1, R = R = R = R = H R = Me R = Ph) was isomerized under thermal or photochemical conditions. 

All three levels correctly calculate the decreasing dipole moment trend in the NH3, CH3 NH2, (CH3 )2 NH and H2 O, CH3 OH, (CH3 )2 O series. Similarly, the increase in going from HCHO to CH3CHO is handled adequately by all three methods. The dipole moments of propane, propene, and propyne are of interest since these molecules are the simplest stable polar hydrocarbons. The extended basis sets handle the dipole moments in these molecules somewhat more successfully than does STO-3G. The dipole moment in cyclopropene is calculated to have its negative end on the methylene group. From studies of the g values of cyclopropene and its 1,2-dideuterio derivative, Benson and Flygare31 have found the opposite result. However, it should be noted that the experimental error... 

The Knoevenagel reaction (Scheme 6.20) involves the reaction of aromatic aldehydes with a variety of molecules CH2XY. The groups X and Y may be the same or different, but are invariably electron withdrawing, so creating an activated methylene group from which the carbanion CHXY is produced. The reaction is usually carried out in pyridine solution, with piperidine as the basic catalyst. The reactions of benzaldehyde with propane-1,3-dinitrile [malononitrile, CH2(CN)2] and diethyl propane-1,3-dioate [diethyl malonate, CH.,(CO,Et)2] are illustrative. In both cases, manipulation of the CH=CX2 group in the product allows the synthesis of other compounds. 

---