Describing a Reaction: Bond Dissociation Energies

We’ve just seen that heat is released (negative ΔH) when a bond is formed because the products are more stable and have stronger bonds than the reactants. Conversely, heat is absorbed (positive ΔH) when a bond is broken because the products are less stable and have weaker bonds than the reactants. The amount of energy needed to break a given bond to produce two radical fragments when the molecule is in the gas phase at 25 °C is a quantity called the bond strength, or bond dissociation energy (D).

Each specific bond has its own characteristic strength, and extensive tables of such data are available. For example, a C−H bond in methane has a bond dissociation energy D = 439.3 kJ/mol (105.0 kcal/mol), meaning that 439.3 kJ/mol must be added to break a C−H bond of methane to give the two radical fragments ·CH₃ and ·H. Conversely, 439.3 kJ/mol of energy is released when a methyl radical and a hydrogen atom combine to form methane. Table 6.3 lists some other bond strengths.

Table 6.3 Some Bond Dissociation Energies, D

Bond	D (kJ/mol)	Bond	D (kJ/mol)	Bond	D (kJ/mol)
H―H	436	(CH3)2CH―H	410	C2H5―CH3	370
H―F	570	(CH3)2CH―CI	354	(CH3)2CH―CH3	369
H―CI	431	(CH3)2CH―Br	299	(CH3)3C―CH3	363
H―Br	366	(CH3)3C―H	400	H2C═CH―CH3	426
H―I	298	(CH3)3C―CI	352	H2C═CHCH2―CH3	318
CI―CI	242	(CH3)3C―Br	293	H2C═CH2	728
Br―Br	194	(CH3)3C―I	227		427
I―I	152	H2C═CH―H	464		325
CH3―H	439	H2C═CH―CI	396		374
CH3―CI	350	H2C═CHCH2―H	369	HO―H	497
CH3―Br	294	H2C═CHCH2―CI	298	HO―OH	211
CH3―I	239		472	CH3O―H	440
CH3―OH	385		400	CH3S―H	366
CH3―NH2	386		375	C2H5O―H	441
C2H5―H	421		300		352
C2H5―CI	352		336	CH3CH2O―CH3	355
C2H5―Br	293		464	NH2―H	450
C2H5―I	233	HC≡CHC≡C―HH	558	H―CN	528
C2H5―OH	391	CH3―CH3	377

Think again about the connection between bond strengths and chemical reactivity. In an exothermic reaction, more heat is released than is absorbed. But because making bonds in the products releases heat and breaking bonds in the reactants absorbs heat, the bonds in the products must be stronger than the bonds in the reactants. In other words, exothermic reactions are favored by products with strong bonds and by reactants with weak, easily broken bonds.

Sometimes, particularly in biochemistry, reactive substances that undergo highly exothermic reactions, such as ATP (adenosine triphosphate), are referred to as “energy-rich” or “high-energy” compounds. Such a label doesn’t mean that ATP is special or different from other compounds, it only means that ATP has relatively weak bonds that require a relatively small amount of heat to break, thus leading to a larger release of heat when a strong new bond forms in a reaction. When a typical organic phosphate such as glycerol 3-phosphate reacts with water, for instance, only 9 kJ/mol of heat is released (ΔH°′ = −9 kJ/mol), but when ATP reacts with water, 30 kJ/mol of heat is released (ΔH°′ = −30 kJ/mol). The difference between the two reactions is due to the fact that the bond broken in ATP is substantially weaker than the bond broken in glycerol 3-phosphate. We’ll see the metabolic importance of this reaction in later chapters.