Addition vs. Elimination A Thermodynamic Perspective

In many cases, an addition reaction is simply the reverse of an elimination reaction  

These two reactions represent an equilibrium that is temperature dependent. Addition is favored at low temperature, while elimination is favored at high temperature. To understand the reason for this temperature dependence, recall that the sign of AG determines whether the equilibrium favors reactants or products (Section 6.3). AG must be negative for the equilibrium to favor products. The sign of AG depends on two terms  

Let s consider these terms individually, beginning with the enthalpy term (AH). Many factors contribute to the sign and magnitude of AH, but the dominant factor is generally bond strength. Compare the bonds broken and formed in an addition reaction  

Notice that one Tt bond and one ct bond are broken, while two ct bonds are formed. Recall from Section 1.9 that ct bonds are stronger than Tt bonds, and therefore, the bonds being formed are stronger than the bonds being broken. Consider the following example  

The actual AH for this reaction, in the gas phase, has been measured to be -17 kcal/mol, confirming that bond strengths are, in fact, the dominant factor in determining the sign and magnitude of AH. 

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