Approximate heats of hydrogenations

Scheme 2.1 Approximate heats of hydrogenation of representative unsaturated organic compounds.

From Table 7-1, approximate heats of hydrogenation can be determined for similarly substituted alkenes. The energy difference is approximately 6 kJ/mole (1.4 kcal/mole), the more highly substituted alkene being more stable. 

TABLE 3.1 Approximate Heats of Hydrogenations of Representative Unsaturated organic Compounds"... 

Naphthalene and other polycyclic aromatic hydrocarbons show many of the chemical properties associated with aromaticity. Thus, measurement of its heat of hydrogenation shows an aromatic stabilization energy of approximately 250 kj/mol (60 kcal/mol). Furthermore, naphthalene reacts slowly with electrophiles such as Br2 to give substitution products rather than double-bond addition products. 

In view of the electrophilic character of ground state oxygen atoms in their reactions with olefins, there should be an approximate correlation of the rates with the ionization potentials of the olefins. Such a correlation does exist (27) but it is only approximate (28). Approximate correlations exist also with some other related physical properties (27), such as heats of hydrogenation and spectroscopic excitation energies, and with theoretically derived properties (28) (obtained by molecular or-... 

The delocalisation of electrons means that benzene is unusually stable. It has a heat of hydrogenation which is approximately I50k.lmol 1 less than that which would be expected for a cyclic conjugated triene. This is known as the resonance energy. 

It is to be expected that the total heat of hydrogenation of a compound containing more than one double bond would be approximately the sum of the heats of hydrogenation of the individual bonds, as found in closely related compounds. Thus, the heat of hydrogenation of 1,5-hexadiene, — 60 53 kcal/mole, is very close to twice the value of 30 34 kcal/mole found for butene-This is taken for comparison, since it contains the CH2CH=CH2 grouping, as does 1,5-hexadiene. With limonene,... 

Heats of hydrogenation are used to assess the stabilities of isolated versus conjugated double bonds. Comparing the measured heats of hydrogenation (solid lines) of the four compounds shown gives the values shown by the dashed lines for the heats of hydrogenation of the terminal double bond of 1,4-pentadiene and ( )-l,3-pentadiene. A conjugated double bond is approximately 15 kJ/mol more stable than an isolated double bond. 

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