Hydrogenation heat of

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

The heats of formation of most organic com pounds are derived from heats of reaction by arith metic manipulations similar to that shown Chemists find a table of AH values to be convenient because it replaces many separate tables of AH° values for indi vidual reaction types and permits AH° to be calcu lated for any reaction real or imaginary for which the heats of formation of reactants and products are available It is more appropriate for our purposes however to connect thermochemical data to chemi cal processes as directly as possible and therefore we will cite heats of particular reactions such as heats of combustion and heats of hydrogenation rather than heats of formation... 

Heats of hydrogenation can be used to estimate the stability of double bonds as structural units even m alkenes that are not isomers Table 6 1 lists the heats of hydro genation for a representative collection of alkenes... 

Remember that a catalyst af fects the rate of a reaction but not the energy relation ships between reactants and products Thus the heat of hydrogenation of a particu lar alkene is the same irre spective of what catalyst is used... 

The pattern of alkene stability determined from heats of hydrogenation parallels exactly the pattern deduced from heats of combustion... 

Decreasing heat of hydrogenation and increasing stability of the double bond... 

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

Match the following alkenes with the appropriate heats 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)... 

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... 

The cumulated double bonds of an allenic system are of relatively high energy The heat of hydrogenation of allene is more than twice that of propene... 

We reach a similar conclusion when comparing benzene with the open chain con jugated tnene (Z) 13 5 hexatriene Here we compare two real molecules both conju gated tnenes but one is cyclic and the other is not The heat of hydrogenation of... 

FIGURE 11 2 Heats of hy drogenation of cydohexene 1 3 cydohexadiene a hypo thetical 1 3 5 cydohexa triene and benzene All heats of hydrogenation are in kilojoules per mole... 

One of these isomers has a heat of hydrogenation of 101 kJ/mol (24 1 kcal/mol) and the heat of hydrogenation of the other is 113 kJ/mol (27 1 kcal/mol) Match the heat of hydrogenation with the appropriate dihydronaphthalene... 

Cyclooctatetraene is relatively stable but lacks the special stability of benzene Unlike benzene which we saw has a heat of hydrogenation that is 152 kJ/mol (36 kcal/mol) less than three times the heat of hydrogenation of cyclohexene cycloocta tetraene s heat of hydrogenation is only 26 kJ/mol (6 kJ/mol) less than four times that of CIS cyclooctene... 

The extent to which benzene is more stable than either of the Kekule structures is its resonance energy, which is estimated to be 125-150 kJ/mol (30-36 kcal/mol) from heats of hydrogenation data... 

We saw in Chapter 12 that aromaticity reveals itself in various ways Qualitatively aro matic compounds are more stable and less reactive than alkenes Quantitatively their heats of hydrogenation are smaller than expected Theory especially Huckels rule furnishes a structural basis for aromaticity Now lets examine some novel fea tures of their NMR spectra... 

Heat of hydrogenation (Section 6 1) Heat evolved on hydro genation of a substance It is the value of — A//° for the ad dition of H2 to a multiple bond... 

Ma.nufa.cture. Butenediol is manufactured by partial hydrogenation of butynediol. Although suitable conditions can lead to either cis or trans isomers (111), the commercial product contains almost exclusively iVj -2-butene-l,4-diol Trans isomer, available at one time by hydrolysis of l,4-dichloro-2-butene, is unsuitable for the major uses of butenediol involving Diels-Alder reactions. The Hquid-phase heat of hydrogenation of butynediol to butenediol is 156 kj/mol (37.28 kcal/mol) (112). 

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