Bond Dissociation Energies of Alkanes

In Section 3.7, we discussed bond dissociation energies for small molecules such as methane and ethane. Now we will extend that discussion to more complex alkanes. Much of the chemical reactivity of hydrocarbons is associated with the carbon-hydrogen bond. The bond dissociation energy, A//°, of the C—H bond of methane is 438 kj mole. In Section 3.7, we saw that this bond dissociation energy is given by the A//° for the following reaction. When we refer to bond dissociation energies, we use the term DH° 

A species with an unpaired electron, such as CH3 , is called a radical. The strength of a C—H bond in an alkane or cycloalkane depends upon the stability of the radical produced in the dissociation reaction. The strength of the C—H bond depends upon the structure of the hydrocarbon. Table 4.8 lists the variation of C—H bond strengths with structure. The energy required to cleave the R—H bond homolytically to give a free radical, R, decreases in the order 

This order reflects the stability of the radical products. Table 4.8 also hsts the strengths of C — 3 bonds. These radicals follow the same order of stability. This is not surprising since in each case a primary, secondary, or tertiary radical is produced, and the methyl radical is the same in all these cases. 

The methyl and ethyl radicals are electron-deficient species that have a carbon atom with only seven electrons in its valence shell. The DH° for the C—H bond of ethane is smaller than that for the C—H bond of methane because it is stabilized by the inductive effect of the CH3 group bonded to the radical center. 

If alkyl groups stabilize a free radical by an electron-donating inductive effect, then we should see a difference in the DH° values for the two different C—bonds in propane since one is to a primary carbon and the other to a secondary carbon. And that is exactly what we see. 

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D. Bond Dissociation Energies of Hydrocarbons Table 1. Bond dissociation energies of alkanes... 

From C-H bond-dissociation energies of alkanes (see Table 4-6), the ease of formation and stabilities of the carbon radicals is seen to follow the sequence tertiary > secondary > primary. By analogy, the secondary l-bromo-2-propyl radical, 5, is expected to be more stable and more easily formed than the primary 2-bromo-1-propyl radical, 6. The product of radical addition should be, and indeed is, 1-bromopropane ... 

Appendix C Bond Dissociation Energies of Hydrocarbons TABLE 1 Bond Dissociation Energies of Alkanes"... 

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

The benzyhc position m alkylbenzenes is analogous to the allyhc position m alkenes Thus a benzyhc C—H bond like an allyhc one is weaker than a C—H bond of an alkane as the bond dissociation energies of toluene propene and 2 methylpropane attest... 

The chemistry of the alkanes is dominated by the abstraction of a hydrogen atom by various free radicals. The bond dissociation energies of C-H bonds decrease in the order primary > secondary > tertiary > benzylic and allylic, and thus free radical reactions tend to occur at tertiary, benzylic or allylic centres. 

Figure 5.2 Energy diagram relating the proton affinity of neutral alkyl radicals, PA(R ), to the ionization energy of the corresponding alkane, lE(RH), with the bond dissociation energy of the appropriate C-H bond, BDE(C-H), and the ionization energy of atomic hydrogen, IE(H ), as additional parameters.

Wodrich, M. D. Schleyer, P. v. R. Org. Lett. 2006, 8, 2135 and supporting information. Moreover, Stanger was able to correlate C -H dissociation energies of alkanes on the basis of the calculated hybridization of the orbital used for C-H bonding, which was in turn a function of molecular geometry. Stanger, A. Eur. j. Org. Chem. 2007, 5717. 

One of the arguments for the stabilization of radicals by hyperconjugation is that the C-H homolytic bond dissociation energies for alkanes vary as shown in Table 5.6. However, the C-F homolytic dissociation energies do not follow the same trend. Are these results consistent with the hyperconjugation model for radical stability ... 

An early report on computational estimation of the thermodynamic properties of (difluoramino)alkanes, outside of the context of the American and Russian research programs, was by inel (Bogazi i University, Turkey), who estimated heats of formation, entropies, and bond dissociation energies of several simple difluoramines [141,142], derived from pubhshed experimental data on perfluorinated amines and thermochemical relationships. 

We know that free rotation around carbon—carbon single bonds is fast at room temperature (Section 4.11). Therefore, alkanes can exist in many conformations. Free rotation does not occur around the carbon—carbon double bond of an alkene at room temperature because of its 7t bond, which forms by side-by-side overlap of two 2p orbitals. About 240 kj mole is required to break a it bond (Figure 5.2). This quantity is the difference between the bond dissociation energies of a carbon—carbon double bond and a carbon—carbon single bond. 

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... 

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