Describing a Reaction Energy Diagrams and Transition States

For a reaction to take place, reactant molecules must collide and reorganization of atoms and bonds must occur. Let s again look at the three-step addition reaction of H2O and ethylene  

As the reaction proceeds, ethylene and HaO must approach each other, the ethylene tt bond and an H-0 bond must break, a new C-H bond must form in the first step, and a new C-0 bond must form in the second step. 

FIGURE 6.4 An energy diagram for the first step in the reaction of ethylene with HjO. The energy difference between reactants and transition state, ACt, defines the reaction rate. The energy difference between reactants and carbocation product, AC°, defines the position ofthe equilibrium. 

FIGURE 6.5 A hypothetical transition-state structure for the first step of the reaction of ethylene with HjO. The C=C 7T bond and O-H bond are just beginning to break, and the C-H bond is just beginning to form. 

As an analogy, you might think of reactants that need enough energy to climb the activation barrier from reactant to transition state as similar to hikers 

Think for a moment about the connection between bond strengths and chemical reactivity. In an exothermic reaction, more heat is released than is absorbed. But since making product bonds releases heat and breaking reactant bonds absorbs heat, the bonds in the products must be stronger than the bonds in the reactants. In other words, exothermic reactions are favored by stable 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 

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ATP is special or different from other compounds they mean only that ATP has relatively weak bonds that require a smaller amount of heat to break, thus leading to a larger release of heat on reaction. When a typical organic phosphate such as glycerol 3-phosphate reacts with water, for instance, only 9 kj/mol of heat is released (AT/° = -9 kj/mol), but when ATP reacts with water, 30 kj/mol of heat is released (AH3 = -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. 

9 Describing a Reaction Energy Diagrams and Transition States 

A small activation energ results in a rapid reaction because almost all collisions occur with enough energy- for the reactants to reach the transition state. 

A hypothetical transition-state structure for the first step of the reaction of  

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DESCRIBING A REACTION ENERGY DIAGRAMS AND TRANSITION STATES... 

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