Activation energy requirement for reaction

Reactions proceed more quickly at higher temperature A higher fraction of collisions have total energy of colliding particles greater than the activation energy required for reaction. 

A gas-phase reactivity model that assumes molecules react as a result of the collision of reactant molecules. The basic idea is that the kinetic energy of the impacting molecules exceeds the activation energy required for reaction. Classical mechanics is used to estimate the fraction of the collisions with enough energy to allow re-... 

A transition state is an unstable, high-energy configuration assumed by reactants in a chemical reaction on the way to making products. Enzymes can lower the activation energy required for a reaction by binding and stabilizing the transition state of the substrate. 

The activated complex lies at the top of the PE profile. The vertical distance AB corresponds to the critical energy required for reaction. The vertical distance CD corresponds to the critical energy for the back reaction. AC gives the PE change between reactants and products, approximately equal to A if of reaction. 

It is believed that an enzyme considerably reduces the activation energy required for each of the stages in an enzyme controlled reaction (Figure A7.1). 

Figure 4.1.3 For the uncatalyzed reaction (blue), the activation energy required for the reaction is high. If catalysis takes place (red), a transition state in which the reactant has adsorbed on the catalyst surface exists. In such a case, the required activation energy for the conversion of A to B is considerably lower.

Figure 2.12 A free energy diagram for the reductive elimination of acetone from a model complex. Note the high activation energies required for the forward and the backward reactions.

At other times, however, the situation can be helped (finally some good news) sometimes a catalyst can be found that will speed up the desired reaction. A catalyst is a substance that lowers the activation energy required for the reaction. A catalyst does not change the amount of product produced it just makes the reaction faster. A catalyst would be like a moving sidewalk at an airport it doesn t bring the terminals any closer together, but it does reduce the amount of energy required to move... 

The selectivity problem has hardly been treated by kinetic analysis. Recently Bourne published a detailed kinetic selectivity study on the aldehyde formation [106b]. The activation energy required for the formation of n- and isobutanal was determined to be 54 and 82 kJ/mol, respectively. The reaction rate models r and /"iso (eqs. (9) and (10)) explained the observed n/i ratios at different reaction conditions within 8 % standard deviation from the experimental results. 

MeifPhJCBr will react more rapidly in SNl reactions. The intermediate tertiary carhocation is furl her stabilized by delocalization of the positive charge onto the ortho and para positions on the benzene ring, which will lower the activation energy required for its formation. 

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