Alkenes, hydrogenation exothermicity

The uncatalyzed addition of hydrogen to an alkene although exothermic is very slow The rate of hydrogenation increases dramatically however m the presence of cer tain finely divided metal catalysts Platinum is the hydrogenation catalyst most often used although palladium nickel and rhodium are also effective Metal catalyzed addi tion of hydrogen is normally rapid at room temperature and the alkane is produced m high yield usually as the only product... 

Section 6 2 Hydrogenation of alkenes is exothermic Heats of hydrogenation can be... 

Formation of new o-bonds at the cost of the loss of the 7t-bond of the alkene during alkene hydrogenation, polymerisation etc. makes the overall processes of alkenes thermodynamically feasible and the process is highly exothermic. 

Hydrogenation of alkenes is exothermic. Heats of hydrogenation can be measured and used to assess the stability of various types of double bonds. The information parallels that obtained from heats of combustion. 

Thermodynamically the insertion of an alkene into a metal-hydride bond is much more favourable than the insertion of carbon monoxide into a metal-methyl bond. The latter reaction is more or less thermoneutral and the equilibrium constant is near unity under standard conditions. The metal-hydride bond is stronger than a metal-carbon bond and the insertion of carbon monoxide into a metal hydride is thermodynamically most often uphill. Insertion of alkenes is also a reversible process, but slightly more favourable than CO insertion. Formation of new CT bonds at the cost of the loss of the ji bond of the alkene during alkene hydrogenation etc., makes the overall processes of alkenes thermodynamically exothermic, especially for early transition metals. 

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. ... 

Alkene hydrogenation is considerably exothermic (Table 7.1), and maintaining isothermal conditions in a catalyst bed is, except at lowest conversions, not a... 

Although the process is exothermic, there is usually a high free energy of activation for uncatalyzed alkene hydrogenation, and therefore, the uncatalyzed reaction does not take place at room temperature. However, hydrogenation will take place readily at room temperature in the presence of a catalyst because the catalyst provides a new pathway for the reaction that involves lower free energy of activation (Fig. 7.9). 

Although the addition of hydrogen to an alkene is exothermic, reduction is immeasurably slow in the absence of a catalyst. Commonly used transition metal catalysts include platinum, palladium, ruthenium, and nickel. Because the conversion of an alkene to an alkane involves reduction by hydrogen in the presence of a catalyst, the process is called catalytic reduction or, alternatively, catalytic hydrogenation. 

FIGURE 10.6 In a typical hydrogenation, the Jt bond of the alkene and the O bond of hydrogen are broken. Two new carbon-hydrogen bonds are made. The hydrogenation of an alkene is exothermic by approximately 30 kcal/mol. 

The reaction is exothermic and is characterized by a negative sign for AH°. Indeed, hydrogenation of all alkenes is exothermic. The heat given off is called the heat of hydrogenation and cited without a sign. In other words, heat of hydrogenation = -A//°. 

The uncatalyzed addition of hydrogen to an alkene, although exothermic, is very slow, but its rate is increased in the presence of certain finely divided metal catalysts, such as platinum, palladium, nickel, and rhodium. The reaction is normally rapid at room temperature, and the alkane is the only product. 

The addition of hydrogen to an alkene (hydrogenation, Sections 4.16A and 7.13) is an exothermic reaction the enthalpy change involved is called the heat of reaction or, in this specific case, the heat of hydrogenation. 

The heat of hydrogenation of an alkyne is greater than twice the heat of hydro genation of an alkene When two moles of hydrogen add to an alkyne addition of the first mole (triple bond double bond) is more exothermic than the second (double bond single bond)... 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

The reaction of an alkene with hydrogen is an exothermic reaction the enthalpy change involved is called the heat of hydrogenation. 

Hydrosilylation of alkenes is highly exothermic the heat of reaction is comparable to that of the hydrogenation of alkenes (-120 kJ.mol ). As a result, a very vigorous reaction may follow in the laboratory when a catalyst is added to an alkene and HSi(CH3)Cl2 (the exotherm will be between 100 and 200 °C for a product with MW of 240 without solvent ). Thus it is necessary to control the reaction by using solvent or by reducing the amount of catalyst. 

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