The CatalystAlkene Stabilization by Alkyl SubstituentsBond Stability(kcal/mol,1atm,25o)” id=”MathJax-Element-7-Frame” role=”presentation” style=”position:relative;” tabindex=”0″>/mol,1atm,2
After completing this section, you should be able to
explain why cis alkenes are generally less stable than their trans isomers. explain that catalytic reduction of a cis alkene produces the same alkane as the catalytic reduction of the trans isomer. explain how heats of hydrogenation (ΔH°hydrog) can be used to show that cis alkenes are less stable than their trans isomers, and discuss, briefly, the limitations of this approach. arrange a series of alkenes in order of increasing or decreasing stability. describe, briefly, two of the hypotheses proposed to explain why alkene stability increases with increased substitution. <Note: This problem is a typical example of those instances in science where there is probably no single “correct” explanation for an observed phenomenon.>
Make certain that you can define, and use in context, the key terms below.
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catalytic hydrogenation heat of hydrogenation, (ΔH°hydrog) hyperconjugation
The two alkenes, cis-CH3CH=CHCH3 and (CH3)2C=CH2 have similar heats of hydrogenation (−120 kJ/mol and −119 kJ/mol, respectively), and are therefore of similar stability. However, they are both less stable than trans-CH3CH=CHCH3 (−116 kJ/mol).
You may wonder why an sp2 –sp3 bond is stronger than an sp3-sp3 bond. Bond strength depends on the efficiency with which orbitals can overlap. In general, s orbitals overlap more efficiently than do p orbitals; therefore, the s–s bond in the hydrogen molecule is stronger than the p–p bond in fluorine. In hybrid orbitals, the greater the s character of the orbital, the more efficiently it can overlap: an sp2 orbital, which has a 33% s character, can overlap more effectively than an sp3 orbital, with only 25% s character.
Alkene hydrogenation is the addition of hydrogen gas (H2) to an alkene which saturates the bond and forms an alkane. Alkene hydrogenation reactions require a transition metal catalyst, such as Pt or Pd, to speed up the reaction. The hydrogenation reaction is used in this section to investigate the stability of alkenes, however, it will be discussed in greater detail in Section 8.7. Hydrogenation reactions are exothermic and the enthalpy change in this reaction is called the heat of hydrogenation (ΔH°hydrog). Since the double bond is breaking in this reaction, the energy released during hydrogenation is proportional to the energy in the double bond of the molecule. By comparing the heat of hydrogenations from a series of alkenes that produce the same alkane, a quantitative measure of relative alkene stabilities can be produced. These experiments will lead to an general understanding of structural features which tend to stabilize or destabilize alkenes.
A catalyst increases the reaction rate by lowering the activation energy of the reaction. Although the catalyst is not consumed in the reaction, it is required to accelerate the reaction sufficiently to be observed in a reasonable amount of time. Catalysts commonly used in alkene hydrogenation are: platinum, palladium, and nickel. The metal catalyst acts as a surface on which the reaction takes place. This increases the rate by putting the reactants in close proximity to each other, facilitating interactions between them. With this catalyst present, the sigma bond of H2 breaks, and the two hydrogen atoms instead bind to the metal (see #2 in the figure below). The (pi) bond of the alkene weakens as it also interacts with the metal (see #3 below).
Since both the reactants are bound to the metal catalyst, the hydrogen atoms can easily add, one at a time, to the previously double-bonded carbons (see #4 and #5 below). The position of both of the reactants bound to the catalyst makes it so the hydrogen atoms are only exposed to one side of the alkene. This explains why the hydrogen atoms add to same side of the molecule, called syn-addition.