Functional group enthalpy

A chemical reaction involves making and breaking bonds, with the gain, loss, or transformation of a functional group. Enthalpy is the term used to measure bond dissociation energy, and AH° for a reaction is the difference in bond dissociation energy for bonds made (products) minus that for bonds broken (starting materials). This is products minus reactants. 

Numerous aromatic nitro compounds have explosive properties, and thus it is important to understand the role that enthalpy of formation has on the sensitivity and long-term stability of these compounds. We will examine three nitro-substituted aromatic families for which thermochemical data can be found in the literature2,84 derivatives of nitrobenzene, aniline and toluene. The choice of these three families allows us to compare the various electronic effects exerted by the parent functional group. The parent compounds differ electronically with respect to the aromatic ring in that ... 

Oxime ethers have a >C=N—O—C—substrucmre. Table 5 presents the enthalpy of formation data for such species where there is little structural commonality save the functional group of interest. 

There is reported enthalpy of formation data for methyl hydroperoxide. The enthalpy of formation of the methyl derivative for any functional group, MeZ, is expected to deviate somewhat from the linear relationship established by the n-alkyl members of the homologous series with the same Z. It is unclear where the best straight line lies for the n-alkyl hydroperoxides and so the extent of deviation for methyl hydroperoxide is likewise unclear. However, methyl hydroperoxide, like other methyl derivatives with electron-withdrawing functional groups, should exhibit an enthalpy of formation that is more positive than other members of the series. 

The liquid enthalpy of formation difference between 1-hexyl and 1-heptyl hydroperoxides is almost twice that of a normal enthalpy of formation methylene increment of about 25 kJmol . But which of these two, if either, is correct For hydrocarbon snb-stituents bonded to electronegative functional groups, the secondary isomers are more stable than the n-isomer. Accordingly, either the 1- or 4-heptyl hydroperoxide, or both, have an inaccurate enthalpy of formation because the primary isomer is reported to have the more negative enthalpy of formation. All of the enthalpies of formation for the Cg and C7 hydroperoxides cited in Reference 2 come from a single source. There is a reported value for the gas phase enthalpy of formation of fert-butyl hydroperoxide that is 11 kJ mol less negative than the value in Reference 2. 

The enthalpy of formation data for peroxides containing hydroxyl and alkoxyl functional groups are in Table 2. 

TABLE 2. Enthalpies of formation of hydroperoxides and peroxides containing single-bonded oxygen functional groups (kJ mol )... 

A simple method for estimating enthalpies of vaporization is the CHLP protocol . The quantities and req in equation 1 refer respectively to the number of quaternary carbon atoms and non-quaternary carbon atoms in the compound, and is a value that is characteristic of the functional group bonded to the hydrocarbon parent. 

---