Alkenes compared

Substituents affect the heats of hydrogenation of alkynes m the same way they affect alkenes Compare the heats of hydrogenation of 1 butyne and 2 butyne both of which give butane on taking up two moles of H2... 

We will now explore each aspect of this two-step process. In the first step, a peroxyacid (RCO3H), sometimes called a per-acid, reacts with the alkene. Compare the structure of a carboxylic acid with the structure of a peroxy acid ... 

Shi s method gives good results for disubstituted /f-alkenes compared to the Jacobsen epoxidation previously described, which is more specific for disubstituted Z-alkenes. Concerning the epoxidation of trisubstituted alkenes, the epoxidation of 1-phenyl-1-cyclohexene could not be validated because of... 

The basic solution in water containing NaCo(CO)4 is treated with sulphuric acid in the presence of syn-gas and HCo(CO)4 is regenerated. This can be extracted as is shown in the drawing from water into the substrate, alkene. The catalyst is returned to the reactor dissolved in the alkene. Compared to other schemes (BASF, Ruhrchemie) the elegant detail of the Kuhlmann process is... 

Problem 6.8 How do boiling points and solubilities of alkenes compare with those of corresponding alkanes ... 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies.659 663 The use of C6F5IO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. 59 The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fev-oxametallacyclobutane intermediate.660 Picket-fence porphyrin (TPiVPP)FeCl-imidazole, 02 and [H2+colloidal Pt supported on polyvinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes.663... 

A 4 kJ/mol difference between cis and trans isomers is typical for disubstituted alkenes. Figure 7-8 summarizes the relative stabilities of alkenes, comparing them with... 

In contrast to alkyne ligands, alkenes are observed to bond side-on to one metal center in a cluster complex. This may reflect the poorer n-acceptor properties of an alkene compared to an alkyne. In all cases reported for single alkene units the ligand has been coordinated to a metal atom in a trinuclear cluster unit, and in the majority of cases the... 

The chemical character of liquid products usually coincides with that of gaseous ones. The gaseous products over clay catalysts are predominantly alkenes compared with alkanes over US-Y [16, 17]. Similarly over ZSM-5 gaseous products are predominantly alkenes, although more aromatics are formed too [8]. 

Samarium iodide has also been used for reducing epoxides to alkenes. Compared with other low-valent metal systems this reagent requires prolonged reaction time at room temperature, but the stereoselectivity, with c -epoxides giving m-alkenes, is high. " ... 

Why do cross-conjugated alkenes absorb at shorter wavelengths than linearly conjugated alkenes Compare hexa-1,3,5-triene and 1,1-divinylethene. [PMO]... 

Figure 5.5, where it is seen that methyl groups approach each other very closely in cis-2-butene, but the trans isomer is free of strain. An effeet that results when two or more atoms are close enough in spaee that a repulsion occurs between them is one type of steric effect. The greater stability of trans alkenes compared with their cis counterparts is an example of a sterie effeet. 

C2-symmetric diamine derivative 7 gave superior results. Excellent enantiose-lectivities over a range of substrates were, however, first obtained by Tomioka and Koga [13,14,15,16,17] with the D -syimnetric hgand 8. All the reactions were performed with a 1 1 complexbetween8andosmiumtetroxideat-100 °C, and gave superior results for alkyl-substituted alkenes compared with the DHQD/DHQ acetates. 

Figure 7. Effect of the extent of cracking (condensed-oil basis) on a naphthalene/ (Cu + C12), where Clt and C12 are the sums of the respective n-alkanes and 1-alkenes. Comparing these results with those in Figure 6 shows that cracking to 30% conversion produces primarily alkenes and that further cracking produces primarily aromatic compounds.

The hydroboration of alkenes is known to be activated either by pressure or catalysis. Consequently, the combination of these techniques might open the way for the hydroboration of particularly unreactive substrates. Maddaluno el al. recently investigated the hydroboration of some functionalized alkenes, comparing different reagents (catecholborane (CBH) versus pinacolborane (PBH)), and activation by Wilkinson catalyst (RhCl[PPh3]3) and pressure [24]. While bromoalkenes and al-lylamines were found to give the best results with CBH at ambient pressure, 2,3-dihydrofuran (52a, Scheme 7.17) was hydroborated most effectively by PBH in the... 

Addition Reactions Section B of Table 11.3 gives some rates of addition reactions involving carbon-carbon double bonds and aromatic rings. Comparison of Entries 23 and 24 shows that the phenyl radical is much more reactive toward addition to alkenes than the benzyl radical. Comparison of Entries 26 and 27 shows the same effect on additions to an aromatic ring. Delocalized benzyl and cumyl radicals have somewhat reduced reactivity. Additions to aromatic rings are much slower than additions to alkenes (compare Entries 23 and 27). This kinetic relationship shows that it is more difficult to disrupt an aromatic ring than an alkene tt bond. 

Significant reversal occurs only for precursors of (Z)-alkenes (compare Table 7, entry 18 and Table 6, entry 29 see also Table 6, entries 7, 8, 9, 10, 24, 25). Exceptions are known at high temperatures see Table 7, entries 14, 15. 

The dehydration of certain secondary and tertiary alcohols is complicated by the formation of more than one alkene because there is more than one adjacent carbon with a hydrogen for elimination. An example is 2-butanol. 1-Butene is formed by the loss of H from CH3 while 2-butene is formed by loss of H from CH2. 1-Butene and 2-butene are not formed in equal amounts. 2-Butene is the major product because it is the more stable alkene compared to 1-butene. 

The orbital interaction leading to the fission of the M-R bond and the formation of the new bonds is the typical synergetic interaction of transition metals with n-acceptor ligands. The a(M-R) and 7t(alkene) orbitals, which are occupied, interact in a four-centered transition state with the a (M-R) and 7t (alkene) orbitals, which are vacant. This interaction produces a mixing of the rr(alkene) and 7t (alkene) orbitals that polarizes the alkene Compared to the simple alkene coordination (Fig. 6.1, left), where the donor and acceptor alkene orbitals interact only with the metal orbitals (this is the classical Chatt-Dewar model), the interaction with o(M-R) and o (M-R) leading to insertion polarizes the donor alkene orbital towards C ... 

Hydroboration of 15 is thought to occur wathe conformation with the allylic hydrogen atom eclipsing the alkene (compare with reactions of allylic derivatives in Section 1.1.8) ... 

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