The Relative Stabilities of Alkenes

Solution to 33b The product has both a new C—O bond (signifying an oxidation) and a new C—H bond (signifying a reduction). Thus, the two cancel each other the reaction is neither an oxidation nor a reduction. 

What alkene would you start with if you wanted to synthesize methylcyclohexane  

You need to choose an alkene with the same number of carbons, attached in the same way, as those in the desired product. Several alkenes could be used for this S5mthesis, because the double bond can be located anywhere in the molecule. 

Now use the strategy you have just learned to solve Problem 34. 

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The differences permit the measurement of the relative stabilities of alkene... 

In addition to having synthetic applications, catalytic hydrogenation is useful for analytical and thermochemical purposes. The analysis of a substance for the number of carbon-carbon double bonds it contains is carried out by measuring the uptake of hydrogen for a known amount of sample. Measurement of the heat evolved in the hydrogenation of alkenes gives information as to the relative stabilities of alkenes, provided that the differences in AS0 values are small (see Exercise 11-7). 

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... 

Q Predict the relative stabilities of alkenes and cycloalkenes, based on their structure and stereochemistry. 

In Chapter 7, we used heats of hydrogenation to compare the relative stabilities of alkenes. For example, the heats of hydrogenation of pent-l-ene and fran.s-pent-2-ene show that the disubstituted double bond in trans-pent-2-ene is 10 kJ/mol (2.5 kcal/mol) more stable than the monosubstituted double bond in pent-l-ene. 

Predict the relative stability of alkene double bonds. 

AH° values obtained for a series of similar reactions are one set of experimental data used to determine the relative stability of alkenes. Explain how the following data suggest that c/s-2-butene is more stable than 1 -butene (Section 12.3A). 

Another, more general, way to determine the relative stabilities of alkenes is to take advantage of the fact that alkenes undergo a hydrogenation reaction on treatment with H2 gas in the presence of a catalyst such as palladium or platinum ... 

Section 4.11 Addition of Hydrogen The Relative Stabilities of Alkenes... 

Alkene hydrogenation is significantly exothermic, and it is not always easy to keep the catalyst isothermal except at low rates. Heats of hydrogenation for a number of alkenes were measured many years ago (Table 7.1), the use of a catalyst ensuring that calorimetry could be conducted at ambient temperature. The values are similar to but perhaps more accurate than those derived from heats of combustion, where subtraction of two large numbers is involved they reflect the extents to which the jt electrons interact with the electrons in the C—H bonds by hyperconjugation. This electron delocalisation is also reflected in the relative stabilities of alkene complexes with Ag+ cations. ... 

Under these conditions, the more stable enolate anion predominates. The factors that determine the relative stabilities of enolate anions are the same as those that determine the relative stabilities of alkenes the more substituted the double bond of the enolate anion, the greater its stability. Thus, the composition of the enolate anion mbcture formed under conditions of thermodynamic control reflects the relative stabilities of the individual enolate anions. 

In addition to the utility of a process that allows alkenes and alkynes to be converted to alkanes, measurement of the heat of hydrogenation (rAH°) allows the relative stabilities of alkenes to be gauged and relationships to be explored. 

We saw in Section 6.13 that the relative stabilities of alkenes can be determined by their heats of hydrogenation. Recall that the most stable aUcene has the smallest heat of hydrogenation it gives off the least heat when it is hydrogenated because it has less energy to begin with. 

When more than one elimination product is possible, can we reliably predict which will be the major product Usually, the major product of an elimination reaction is the one that is most stable. But what can we say about the relative stabilities of alkenes In the diagram below, alkenes are ranked in order of increasing stability ... 

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