Bond dissociation energies give the energy necessary for breaking a bond into two radicals (homolytic process).
The dissociation energy of a bond, can be calculated from the heats of formation of the molecule and of the two radicals formed by breaking that bond.
Heats of formation of the neutral molecule and the two radicals formed by breaking a bond are calculated by the method presented in the previous chapter.
Values calculated
BDE(A-B): bond dissociation energy of a bond between the atoms A and B
if the considered bond is part of a 
-system the BDE gives the energy for abolishing
the -interactions. The total amount of breaking that bond (
- and -interactions) is given by (see Figure 3)
TBDE(A-B): total bond dissociation energy
Figure 3. Difference between BDE and TBDE
Example
The bond dissociation of an ether bond in methylethylether may serve as an example.
|
|
-216.4 |
-17.2 |
+143.9 |
|
|
-215.4 |
-21.0 |
+142.2 |
BDE (exp.) = +343.1 kJ/mol
BDE (calc.) = (142.2 - 21.0 + 215.4) kJ/mol = +336.6 kJ/mol
Scheme 1. Calculation of the BDE of the Me-O-bond in methylethylether
Results
The example in the following table illustrates that even small changes in the BDE of C-H and C-C bonds can be reproduced.
|
Bond |
BDE(calc.) |
BDE(exp.) |
Bond |
BDE(calc.) |
BDE(exp.) | |
|
C1-H |
406.1 |
410.0 |
C3-C4 |
339.4 |
342.3 | |
|
C3-H |
386.6 |
397.5 |
C4-C5 |
337.5 |
336.4 | |
|
C5-H |
379.6 |
384.9 |
C3-C2 |
332.3 |
325.1 | |
|
C6-C7 |
354.7 |
354.8 | ||||
|
C5-C10 |
346.8 |
352.3 | ||||
|
C1-C2 |
341.1 |
342.3 | ||||
|
in kJ/mol |
Table 6. Bond dissociation energies of 2,2,5-trimethylheptane
Applications
The bond dissociation energy is a fundamental factor influencing chemical reactivity. It is of paramount importance in radical processes. It has also been shown that the BDE can influence reactivity in polar processes.
Reference
