Isomers, branched

In practice, the reference base is usually taken not as a-methylnaphthalene but as heptamethyinonane (HMN), a branched isomer of n-cetane. The HMN has a cetane number of 15. In a binary system containing Y% of n-cetane, the cetane number CN vyOl be, by definition (./ - V ... 

FIGURE 2 13 Tube (top) and space filling bottom) models of (a) pentane (b) 2 methylbutane and (c) 2 2 dimethylpropane The most branched isomer 2 2 dimethylpropane has the most compact most spherical three dimensional shape... 

Table 2 3 lists the heats of combustion of several alkanes Unbranched alkanes have slightly higher heats of combustion than their 2 methyl branched isomers but the most important factor is the number of carbons The unbranched alkanes and the 2 methyl branched alkanes constitute two separate homologous senes (see Section 2 9) m which there is a regular increase of about 653 kJ/mol (156 kcal/mol) m the heat of combustion for each additional CH2 group... 

We have seen in this chapter that among isomenc alkanes the unbranched isomer is the least stable and has the highest boiling point the most branched isomer is the most stable and has the lowest boiling point Does this mean that one alkane boils lower than another because it is more stable" Explain... 

Paraffin Isomerization. Another weU-estabhshed commercial process which employs zeoflte catalysts is the isomerization of normal paraffins into higher octane, branched isomers. The catalyst for the Hysomet process of the Shell Oil Co. is dual-functional, and consists of a highly acidic, latge-pote zeoflte loaded with a small amount of a noble-metal hydrogenation component. This catalyst possesses the same... 

Bisphosphites such as (7) combine excellent reactivity, straight-chain selectivity, and high resistance to the typical phosphite degradation reactions (29). Further, the corresponding 0x0 catalysts are excellent olefin isomerization catalysts so that high normal-to-branched isomer ratios are obtained even from internal olefins, enabling, in certain instances, the use of inexpensive mixed isomer olefin feedstocks. 

There are three commercial routes to ADN in use. The first method, direct hydrocyanation of 1,3-butadiene [106-99-0] has replaced an older process, cyanation via reaction of sodium cyanide with 1,4-dichlorobutane [110-56-5] owing to the lower cost and fewer waste products of the new process. During the initial steps of the direct hydrocyanation process, a mixture of two isomers is generated, but the branched isomer is readily converted to the linear 3-pentenenitrile [4635-87-4]. 

The linear isomer is more valuable than the branched isomer (see Butyraldehyde). The product aldehydes ate hydrogenated to give so-called 0x0 alcohols long-chain products are converted iato sulfonates and used as detergents. 

Isomerization. Isomerization of low octane and / -paraffins in gasoline streams into higher octane branched isomers is an important... 

Carbonylation, or the Koch reaction, can be represented by the same equation as for hydrocarboxylation. The catalyst is H2SO4. A mixture of C-19 dicarboxyhc acids results due to extensive isomerization of the double bond. Methyl-branched isomers are formed by rearrangement of the intermediate carbonium ions. Reaction of oleic acid with carbon monoxide at 4.6 MPa (45 atm) using 97% sulfuric acid gives an 83% yield of the C-19 dicarboxyhc acid (82). Further optimization of the reaction has been reported along with physical data of the various C-19 dibasic acids produced. The mixture of C-19 acids was found to contain approximately 25% secondary carboxyl and 75% tertiary carboxyl groups. As expected, the tertiary carboxyl was found to be very difficult to esterify (80,83). 

This effect is not simply due to the better packing possible with the branched isomers. The lumpy brcuiched structures impede rotation about the carbon-carbon bond on the main chain, thus giving a stiffer molecule with consequently higher transition temperature. 

Part A of Table 1.5 shows all the acyclic C4-C6 and some of the Cg hydrocarbons. A general trend is discernible in the data. Branched-chain hydrocarbons are more stable than straight-chain hydrocarbons. For example, A/fj for -octane is —49.82 kcal/mol, whereas the most highly branched isomer possible, 2,2,3,3-tetramethylbutane, is the most stable of the octanes, with of —53.99 kcal/mol. Similar trends are observed in the other series. 

FIGURE 2.12 Bolling points of unbranched alkanes and their 2-methyl-branched Isomers. (Temperatures In this text are expressed In degrees Celsius, °C. The SI unit of temperature Is the kelvin, K. To convert degrees Celsius to kelvins add 273.15.)... 

Abstraction of a hydride ion from a tertiary carbon is easier than from a secondary, which is easier than from a primary position. The formed car-bocation can rearrange through a methide-hydride shift similar to what has been explained in catalytic reforming. This isomerization reaction is responsible for a high ratio of branched isomers in the products. 

Alkanes with long, unbranched chains tend to have higher melting points, boiling points, and enthalpies of vaporization than those of their branched isomers. The difference arises because, compared with unbranched molecules, the atoms of neighboring branched molecules cannot get as close together (Fig. 18.5). As a result, molecules with branched chains have weaker intermolecular forces than their unbranched isomers. 

The strength of the London forces between alkane molecules increases as the molar mass of the molecules increases hydrocarbons with unbranched chains pack together more closely than their branched isomers. Alkanes are not very reactive. but they do undergo oxidation (combustion) and substitution reactions. 

The difference can be traced to the weaker London forces that exist in branched molecules. Atoms in neighboring branched molecules cannot lie as close together as they can in un branched isomers. 

In polysulfide solutions there exist chainlike dianions with practically arbitrary chain lengths n. Berghof, 8ommerfeld, and Cederbaum have investigated the onset of stabihty in the 8 (n=2-8) series of dianions based on 8CF calculations with the DZPD basis set [89]. The isolated chainlike 8 isomers were found to exhibit twisted structures, and the onset of electronic stabihty (with respect to electron auto detachment) was predicted to occur at 71=7. Branched isomers were found to be electronically more stable than the... 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

Fig. 3 Structures of two branched isomers of hexasulfane H2S6 calculated at the B3LYP/ 6-31G(2df) level of theory (internuclear distances in pm) [52]...

Butanes are chosen as the simplest models for the normal and branched isomers. Both branched and normal isomers contain a C-C bond (2 ) interacting with the terminal C-H bonds (2 and 2 ) (Scheme 26a). The cyclic -aj-a2 -a3 a2- interaction (Scheme 26b) occurs in the polarization of the middle C-C a-bond by the interactions with the antiperiplanar C-H a-bonds. The orbital phase is continuous in the branched isomer and discontinuous in the normal isomer (cf Scheme 4). The branched isomer is more stable. The basic rule of the branching effects on the stability of alkanes is ... 

Terminal branching gives one more cross conjugation number (nj) than inner branching (Scheme 25b). For example, the number is 2 for 2-methylpentane and 1 for 3-methylpentane, in agreement with the relative stability. Inner branched isomers have almost the same heat of formation. For example, the difference is very small (0.12 kcal moF ) between 3- and 4-methylheptanes (n = 1). 

For the ir-conjugated systems as well as the tr-type diradicals, the triplet branched isomers are more stable than the linear ones (e.g., 1 vs 3 5 vs 4 21 vs 22). 

The first few experiments in the continuous flow reactor yielded inconsistent octene conversions (Figure 28.3). The experiment ran for 218 hours. Initially the conversion was consistent at 3-4% for several hours, then improved significantly to 16% and then rapidly dropped off to less than 2% (Figure 28.3). The selectivity was also very good for this ran, with an average normal to branch isomer ratio of 7 1. 

The major problem remains control of regioselectivity in favor of the branched regioisomer. While aryl alkenes as well as heteroatom-substituted alkenes favor the chiral branched isomer, for aliphatic alkenes such an intrinsic element of regiocontrol is not available. As a matter of fact branched-selective and asymmetric hydroformylation of aliphatic alkenes stands as an unsolved problem. In this respect regio- and enantioselective hydroformy-... 

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