Unfavorable thermodynamics

As shown by the data in Fig. 31, the chain transfer constant of this initiator, Q = 1.0. In this context it is of interest to remember that the effect of initiator concentration on the molecular weight of HSi-PaMeSt was negligible, probably because of unfavorable thermodynamics (Sect. III.B.3.b.iv.). In contrast, with isobutylene chain transfer from the propagating carbenium ion to initiator is thermodynamically favorable (see Sect. IH.B.4.b.i.). Thus it is not surprising to find a large Q. The chain transfer mechanism has been illustrated in Scheme 5. 

A technique used to overcome the unfavorable thermodynamics of one reaction is to couple that reaction with another process that is thermodynamically favored. For instance, the dehydrogenation of cyclohexane to form benzene and hydrogen gas is not spontaneous. Show that, if another molecule such as ethene is present to act as a hydrogen acceptor (that is, the ethene reacts with the hydrogen produced to form ethane), then the process can be made spontaneous. 

In these reactions, a er-bond is formed at the expense of two re-bonds and, thus, the process leads to a net loss of one chemical bond that is intrinsically unfavorable thermodynamically. Formation of the new er-bond leads to ring closure, whereas the net loss of a bond leads to the formation of two radical centers, which can be either inside (the endo pattern in Scheme 1) or outside of the newly formed cycle (the exo pattern). Note that er-radicals are formed through the endo path, while exo-closures may produce either a er-radical when a triple bond is involved or a conjugated re-radical when the new bond is formed at the central carbon of an allene. The parent version of this process is the transformation of enediyne 1 into p-benzyne diradical2 (the Bergman cyclization), shown in Scheme 2. 

The peak at m/z 100 belongs to H loss and is also due to a-cleavage as can be easily recognized (Scheme 6.13). There are three different positions to cleave off the radical, and even seven almost equivalent hydrogens are available in total (for clarity only one them has been shown at any position in the scheme). Despite this multiple chance, the peak at m/z 100 is very weak, the reason for this being the unfavorable thermodynamics of H loss as compared to methyl loss (Table 2.2). 

An important exception to this regularity is the cyclization of aromatic alkoxides containing aromatic radical moieties. In these cases, C-0 bond formation is not observed, but C-C bond formation is achieved instead. As Galli and Gentili (1998) pointed out, this is primarily due to the unfavorable thermodynamic driving force for C-0 bond formation compared to C-C bond formation. Thus, the photostimulated reaction depicted in Scheme 7.39 results in the formation of a six-membered carbocycle rather than an octa-membered oxa-heterocycle. The carbocycle is formed in 75% yield (Barolo et al. 2006). This product is a precursor to the thalicmidine biomolecule of the alkaloid group. 

Besides the activation of the olefinic partner by a metal, the unfavorable thermodynamics associated with the addition of an enolate to a carbon—carbon multiple bond could be overwhelmed by using a strained alkene such as a cyclopropene derivative286. Indeed, Nakamura and workers demonstrated that the butylzinc enolate derived from A-methyl-5-valerolactam (447) smoothly reacted with the cyclopropenone ketal 78 and subsequent deuterolysis led to the -substituted cyclopropanone ketal 448, indicating that the carbometallation involved a syn addition process. Moreover, a high level of diastereoselectivity at the newly formed carbon—carbon bond was observed (de = 97%) (equation 191). The butylzinc enolates derived from other amides, lactams, esters and hydrazones also add successfully to the strained cyclopropenone ketal 78. Moreover, the cyclopropylzincs generated are stable and no rearrangements to the more stable zinc enolates occur after the addition. 

Any condensation of methane to ethane and subsequently to higher hydrocarbons must overcome the unfavorable thermodynamics. This can be achieved in condensation processes of oxidative nature, where hydrogen is removed by the oxidant. SbF5- or FS03H-eontaining superacid systems also act as oxidants. The oxidative condensation of methane was subsequently found to take place with more economical cooxidants such as halogens, oxygen, sulfur, or selenium 91... 

Alkylation with Alkanes. Alkylation of aromatic hydrocarbons with alkanes, although possible, is more difficult than with other alkylating agents (alkyl halides, alkenes, alcohols, etc.).178 This is due to the unfavorable thermodynamics of the reaction in which hydrogen must be oxidatively removed. 

Addition of water (36) or alcohols (37—39) direcdy to butadiene at 40—100°C produces the corresponding unsaturated alcohols or ethers. Acidic ion exchangers have been used to catalyze these reactions. The yields for these latter reactions are generally very low because of unfavorable thermodynamics. At 50°C addition of acetic acid to butadiene produces the expected butenyl acetate with 60—100% selectivity at butadiene conversions of 50%. The catalysts are ion-exchange resins modified with quaternary ammonium, quaternary phosphonium, and ammonium substituted ferrocenyl ions (40). Addition of amines yields unsaturated alkyl amines. The reaction can be catalyzed by homogeneous catalysts such as Rh[P(C(5H5)3]3Q (41) or heterogeneous catalysts such as MgO and other solid bases (42). 

This reaction, called the Claisen condensation, is interesting because, from consideration of bond and stabilization energies, it is expected to be unfavorable thermodynamically with AH° (vapor) equal to 6 kcal mole-1. This expectation is realized in practice, and much effort has been expended to determine conditions by which practical yields of the condensation product can be obtained. 

It is significant that the only stable formyl complexes isolated to date (58-60) are coordinatively saturated, which eliminates the possible conversion to a carbonyl hydride without the prior loss of a ligand. The unfavorable thermodynamics of (5) for formyl formation are a necessary consideration in developing schemes for CO reduction by this method. 

Olah et al.532,533 studied trihalomethyl cations (CX3+, X = Cl, Br, I) under stable ion conditions. 13C NMR chemical shift values correlate well with the decreasing order of back-donation (Cl > Br > I). Similar correlation was also found for dimethylhalo-carbenium ions 258. The CF3+ fluoro analog, however, could not be observed under any conditions. This can be attributed to a combination of unfavorable thermodynamics (generation of CF3+ from CF4 is endothermic by about 20lstarting materials and a suitably strong Lewis acid.534... 

In order to overcome unfavorable thermodynamics, hydrogen must be oxidatively removed (either by superacid or added oxidant). Considering the abundance of methane in nature, the conversion of natural gas into branched liquid hydrocarbons in the gasoline range is of immense interest. 

Although the direct C02-based route to dialkyl carbonates offers selectivity improvement (almost 100% for DMC synthesis), limited yields may result due to unfavorable thermodynamics and catalyst deactivation by water. Until now, mechanistic studies have been fruitful in identifying some of the key steps of the... 

The statistical distribution of isomers 7a, b and c would be 1 2 1. The near absence of isomer 7a indicates unfavorable thermodynamic stability for the phosphorus fused in two five membered rings compared to 5,6- and 6,6-fusion. Greater stability for the larger ring fusion is also exhibited for compounds 8, and which show low field to high field peaks in the approximate ratios 1 8 and 1 2 respectively (statistical abundances are 1 1). 

This overall reaction is already carried out industrially via the intermediate forma-mide (HCONH2) which, together with the use of disparate operating conditions (high pressures and mild temperatures for formamide synthesis, vacuum and high temperatures for its decomposition), overcomes the unfavorable thermodynamics of... 

Sodium borohydride contains 10.8 wt.% of hydrogen, but it shows unfavorable thermodynamics for use as reversible hydrogen storage material. Over 100 synthesis methods for the preparation of NaBH4 have been described, but only two have reached practical significance. In the Schlesinger process (Eq. (5.7)), trimethyl borate is boiled together with NaH in hydrocarbon oil at 250 °C [19] ... 

The respective DSC measurements of the decomposition reactions revealed that reaction (7.5) is exothermic with an enthalpy of about 15 kJ moO and reaction (7.6) is endothermic with an enthalpy of 13 kJ moO. Thus, this system again has rather unfavorable thermodynamic properties and is not reversible by gas-phase H2 loading. 

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