Thermodynamic stability alkenes

Double bonds tend to migrate to more highly substituted positions within a substrate that is, terminal alkenes isomerize to disubstituted or trisubstituted alkenes, disubstituted alkenes tend to migrate to trisubstituted, and trisubstituted to tetrasubstituted alkenes. Of course, migration can go both ways, and adsorbed surface species may not exhibit the same thermodynamic stability as their desorbed relatives. (The rate of migration is strongly catalyst dependent for example, it frequently occurs rapidly on Pd and slowly on Pt.)... 

Hyperconjugation has also been invoked to account for the greater thermodynamic stability of alkenes in which the double bond is not terminal, e.g. (30), compared with isomeric compounds in which it is, e.g. (31) in (30) there are nine hyperconjugable a-hydrogen atoms, compared with only five in (31) ... 

Finally, the major structural features in the substrate promoting E2 elimination are those that serve to stabilise the resultant alkene or, more particularly, the T.S. that precedes it. Such features include increasing alkyl substitution at both a- and //-carbon atoms (leading to alkenes of increasing thermodynamic stability), or introduction of a phenyl group that can become conjugated with the developing double bond. 

Support for the actual elimination step, in each case, being E2 is provided by the fact that changing the alkyl substituents on C and Cfi results in reaction rates that, in general, increase with the relative thermodynamic stability of the product alkene. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

At the higher temperature, the thermodynamic stability of the product is the important consideration, with the 1,4-adduct, a disubstituted alkene, being more stable than the 1,2-adduct, which is a monosub-stituted alkene. An essential part of the reasoning is... 

Now for the relative proportions of products. Only 3% of the unrearranged product shows just how unfavourable the secondary carbocation is compared with the rearranged tertiary carbocation. The relative proportions of the other two alkenes are explained by the increased thermodynamic stability of the more-substituted alkene, though this is not sufficient to produce just the single product. 

The thermodynamic stability of isomeric hydrocarbons is determined by burning them to CO2 and H O and comparing the heat evolved per mole -AH combustion). The more stable isomer has the smaller -AH) value. Trans alkenes have the smaller values and hence are more stable than the cis isomers. This is supported by the exothermic AH negative) conversion of cis to irons isomers by ultraviolet light and some chemical reagents. 

Such cycloadditions involve the addition of a 2tt- electron system (alkene) to a 4ir- electron system (ylide) and have been predicted to occur in a concerted manner according to the Woodward-Hoffmann rules. The two most important factors involved in the cycloaddition reactions are (i) the energy and symmetry of the reacting orbitals and (ii) the thermodynamic stability of the cycloadduct. The reactivity of 1,3-dipolar systems has been successfully accounted for in terms of HOMO-LUMO interactions using frontier MO theory (71TL2717). This approach has been extended to explain the 1,3 reactivities of the nonclassical A,B-diheteropentalenes <77HC(30)317). 

The diagram above refers to thermodynamic stability. When we discuss addition reactions you will see that the most stable alkene when mixed with an electrophile is the most reactive according to this diagram. This paradox is due to the intermediate, usually a carbocation. Since a tertiary carbocation is more stable, the energy of activation is lowered and a reaction with a tertiary intermediate proceeds more quickly in general, to predict the alkene product, use the above diagram as a reference, but to predict the most reactive alkene to an electrophile, the order is based on cation formation and is nearly reversed. 

The term alkene (olefin) metathesis refers to the equilibrium reaction shown in equation (1) in which the alkylidene groups of a pair of alkenes are exchanged with one another in the presence of a transition metal-containing catalyst. The reaction involves the net cleavage of the bonds of the substrate(s) and formation of the new carbon-carbon double bonds of the prodncts. Once equilibrium has been established, the resultant prodnct mixture has a distribution of alkenes (including isomers) that is determined solely by the relative thermodynamic stabilities of the prodncts. 

A general picture for the mechanism is shown in Scheme 4, which is based upon a theoretical analysis by Thom and Hoffmann. Here distinction between (2) and (2a) reflects the general assumption, supported by calculations, that the insertion step requires the M—H and C==C groups to be cis and coplanar, which need not be the case for the first-formed and/or thermodynamically most stable alkene complex (2). Thom and Hoffmann conclude that most or all metal hydrides will have some pathway that leads to hydrometallation without a large kinetic barrier, so long as none of the key intermediates along the way is too stable. The same inference was drawn for the bent metallocene systems discussed earlier (Figure 1) a kinetic barrier to insertion, found only for the cP-cases, is a consequence of the thermodynamic stabilization of alkene complex (2). ... 

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