2- cyclopentane- alkanal

Elemental sulfur reacts with alkanes such as cyclopentane in the presence of superacidic trifluoromethanesulfonic acid to give symmetrical dialkyl sulfides in moderate yields. 

Hydrocarbons, compounds of carbon and hydrogen, are stmcturally classified as aromatic and aliphatic the latter includes alkanes (paraffins), alkenes (olefins), alkynes (acetylenes), and cycloparaffins. An example of a low molecular weight paraffin is methane [74-82-8], of an olefin, ethylene [74-85-1], of a cycloparaffin, cyclopentane [287-92-3], and of an aromatic, benzene [71-43-2]. Cmde petroleum oils [8002-05-9], which span a range of molecular weights of these compounds, excluding the very reactive olefins, have been classified according to their content as paraffinic, cycloparaffinic (naphthenic), or aromatic. The hydrocarbon class of terpenes is not discussed here. Terpenes, such as turpentine [8006-64-2] are found widely distributed in plants, and consist of repeating isoprene [78-79-5] units (see Isoprene Terpenoids). 

NBS can also be used to brominate alkanes. For example, cyclopropane, cyclopentane, and cyclohexane give the corresponding bromides when irradiated in a solution of NBS in dichloromethane. Under these conditions, the succinimidyl radical appears to be involved as the hydrogen-abstracting intermediate ... 

Petroleum contains hydrocarbons other than the open-chain alkanes considered to this point. These include cycloalkanes in which 3 to 30 CH2 groups are bonded into closed rings. The structures of the two most common hydrocarbons of this type are shown in Figure 22.5 (p. 585). Cyclopentane and cyclohexane, where the bond angles are close to the ideal tetrahedral angle of 109.5°, are stable liquids with boiling points of 49°C and 81°C, respectively. 

The chain and branched chain saturated hydrocarbons make up a family called the alkanes. Some saturated hydrocarbons with five carbon atoms are shown in Figure 18-11. The first example, containing no branches, is called normal-pentane or, briefly, n-pentane. The second example has a single branch at the end of the chain. Such a structural type is commonly identified by the prefix iso- . Hence this isomer is called /50-pentane. The third example in Figure 18-11 also contains five carbon atoms but it contains the distinctive feature of a cyclic carbon structure. Such a compound is identified by the prefix cyclo in its name—in the case shown, cyclopentane. 

Thermal rearrangement of trans-l,2-dibromo compounds is known in the literature (refs. 6-10). In all case studies only one pair of bromine in each organic molecular was studied. Bellucci (ref. 10), for example, studied the kinetics of such trans-l,2-cyclo alkanes as cyclopentane, hexane, octane, etc. The intermediates suggested as an explanation for the experimental results are bromonium bromide I in polar solvents and four center transition state II in non-polar solvents. 

Fig. 12.4. Vapor-to-water transfer data for saturated hydrocarbons as a function of accessible surface area, from [131]. Standard states are 1M ideal gas and solution phases. Linear alkanes (small dots) are labeled by the number of carbons. Cyclic compounds (large dots) are a = cyclooctane, b = cycloheptane, c = cyclopentane, d = cyclohexane, e = methylcyclopentane, f = methylcyclohexane, g = cA-l,2-dimethylcyclohexane. Branched compounds (circles) are h = isobutane, i = neopentane, j = isopentane, k = neohexane, 1 = isohexane, m = 3-methylpentane, n = 2,4-dimethylpentane, o = isooctane, p = 2,2,5-tri-metbylhexane. Adapted with permission from [74], Copyright 1994, American Chemical Society...

Droege, A.T., Tully, F.P. (1987) Hydrogen-atom abstraction from alkanes by OH. 6. Cyclopentane and cyclohexane. J. Phys. Chem. 91, 1222-1225. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

It is not exactly known how large a Pt ensemble must be which can catalyze the multiple H/D exchange with D2 of alkanes such as cyclopentane, but it stands to reason that at least two adjacent Pt atoms are required (probably more). It follows that a catalyst which has its Pt atoms predominantly isolated from each other should NOT show this product pattern, but give a product distribution typical of stepwise exchange. Such a product should follow the binomial law i.e. no predominant peak at C5H5D5 the concentrations of the CsHio- iD products at low exchange should show a monotonous decrease with x. 

C5 Cyclization of various alkanes 38, 38a) over platinum on carbon was first observed in 1954. Barron et al. 15a) postulated the formation of a surface C5 cyclic intermediate, which may desorb as a cyclopentanic hydrocarbon or may produce skeletal isomers without desorption. 

With platinum and palladium supported on acidic alumina, cyclopentanes are important intermediates of aromatization (44, 123-124). For example, n-heptane gave about 2-3 times more aromatic product than 2,4-dimethyl-pentane, whereas the formation of C5 cyclic products was about the same from both alkanes. Alkylcyclopentanes aromatized at a reasonable rate (123a). 

My initial plan involved dehydrogenation of cyclic alkanes to give stable arenes, particularly of cyclopentane to give cyclopentadienyl derivatives. My own experimental efforts with Pt were unavailing, but Jeimifer Quirk, a graduate student of mine working on Ir, took an interest in the project and found a small yield of Cp... 

In the case of iridium, complex [IrH2(PPh3)2(acetone)2] BF4 (11) was the first to carry out catalytically the dehydrogenation of cycloalkanes [13, 14]. However, it was later realized that the halocarbons used as solvents reacted with 11 to produce the stable species [HL2lr(p-Cl)2(. i-X)IrL2H]BF4 (X = Cl (14) or H (15)) [16] (Scheme 13.8), and that elimination of the solvent by running the reactions in neat alkane not only improved yields but also permitted the activation of other previously unreactive cycloalkanes, such as methyl- and ethyl-cyclopentane. However, it was also noted that the system in some cases was not catalytic, due mainly to decomposition of the catalyst at the temperatures employed [16]. 

SiO)2Ta Cp[ activates, at room temperature, the C-H bonds of cyclic alkanes (from cyclopentane to cyclooctane) to form the corresponding surface tantalum-cycloalkyl stoichiometrically with evolution of hydrogen (Scheme 3.2) [20] ... 

Tantalum hydride(s) also catalyzes the hydrogenolysis of cyclic alkanes (substituted or not) but the reachvity order decreases with the cycle size as cycloheptane > methylcyclohexane > cyclohexane > methylcyclopentane > cyclopentane for the latter no reaction is actually observed (Figure 3.8). Activity decreases with hme and becomes low after 20 h. 

A study of the stoichiometric cyclopentane reaction over Ta-H has revealed that tantalum hydride very easily achvates cyclopentane, forming the corresponding cyclopentyl derivative. However, the latter is very quickly transformed into a cyclo-pentadienyl compound, as shown by NMR and EXAFS studies. This cyclopenta-dienyl derivative presents no achvity in alkane hydrogenolysis ... 

Indeed, cyclopentane formation during all cyclic alkane hydrogenolysis and cyclopentane transformation into a cyclopentadienyl derivative, inert in hydrogenolysis, explains the rapid deactivation process of this catalyst in the presence of cyclic alkanes. 

The photodecomposition of -alkanes at excitation energies slightly above the absorption onset involves both C-H and C-C bond decompositions [18]. The dominant process is the C-H scission, (H2) 0.8-0.9, and the contribution of C-C decomposition is small. In the photolysis of cyclohexane, cycloheptane, cyclooctane, and cyclodecane, however, only hydrogen evolution was observed [[Pg.375]

The yield determined in a certain type of experiment usually strongly depends on the assumptions made about the formation mechanism. In the older literature, the excited molecules were often assumed to be produced solely in neutral excitations [127,139-143] and energy-transfer experiments with Stern-Volmer-type extrapolation (linear concentration dependence) were used to derive G(5 i). For instance, by sensitization of benzene fiuorescence, Baxendale and Mayer established G(5 i) = 0.3 for cyclohexane [141]. Later Busi [140] corrected this value to G(5 i) = 0.51 on the basis that in the transfer, in addition to the fiuorescing benzene state S, the S2 and S3 states also form and the 82- 81 and 83 81 conversion efficiencies are smaller than 1. Johnson and Lipsky [144] reported an efficiency factor of 0.26 0.02 per encounter for sensitization of benzene fluorescence via energy transfer from cyclohexane. Using this efficiency factor the corrected yield is G(5 i) = 1.15. Based on energy-transfer measurements Beck and Thomas estimated G(5 i) = 1 for cyclohexane [145]. Relatively small G(5 i) values were determined in energy-transfer experiments for some other alkanes as well -hexane 1.4, -heptane 1.1 [140], cyclopentane 0.07 [142] and 0.12 [140], cyclooctane 0.07 [142] and 1.46 [140], methylcyclohexane 0.95, cifi-decalin 0.26 [140], and cis/trans-decalin mixture 0.15 [142]. 

Figure 7 G(Si) value as a function of the carbon atom numbers in the molecules. When more than one measured value was published, we tried to select the most probable value. Alkanes (1) propane, (2) w-butane, (3) w-pentane, (4) cyclopentane, (5) w-hexane, (6) cyclohexane, (7) w-heptane, (8) cycloheptane, (9) methylcyclohexane, (10) w-octane, (11) cyclooctane, (12) isooctane, (13) w-decane, (14) cyclodecane, (15) cw-decalin, (16) trawx-decalin, (17) w-dodecane, (18) dicyclohexyl, (19) n-hexadecane. (From Refs. 18, 29, 65, 92, 148, and 155.)...

Cycloalkanes. Cycloalkanes are conformationally restricted alkanes. Three rings are employed in drug design cyclopropane, cyclopentane, cyclohexane (the latter two are... 

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