Orgo 5 - 5. Reactions of Alkenes and Alkynes Characteristic...

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5. Reactions of Alkenes and Alkynes Characteristic Reactions of Alkenes - The most characteristics of alkenes is addition to the carbon-carbon double bond in such a way that the pi bond is broken, and in its place sigma bonds are formed to two new atoms or groups of atoms. Some examples of characteristic reactions of alkenes include hydrochlorination (hydrohalogenation), hydration, bromination (halogenation), hydroboration, and hydrogenation (reduction). In regards to the chemical industry, the single most important reaction of ethylene and other low-molecular-weight alkenes is the production of chain-growth polymers. In the presence of certain catalysts called initiators, many alkenes form polymers by the addition of monomers to a growing polymer chain. n CH 2 =CH 2 -- (initiator )-> -(- CH 2 CH 2 -)- n Reaction Mechanism - A reaction mechanism describes in detail how a chemical reaction occurs, step by step. During a chemical reaction, bond breaking corresponds to an increase in energy, and bond forming corresponds to a decrease in energy. We use an energy diagram to show the changes in energy that occur during a chemical reaction, the graph consists of energy plotted on the y-axis ad the progress of the reaction on the x-axis. The reaction coordinate is a measure of the progress of a reaction, plotted on the x-axis in an energy diagram. - The difference in energy between the reactants and products is called the heat of reaction, H. If the energy of the products is lower than that of the reactants heat is released and the reaction is called exothermic. If the energy of the products is higher than that of the reactants, heat is absorbed and the reaction is called endothermic. A transition state is the point on the reaction coordinate at which the energy is at a maximum. At the transition state, sufficient energy has become concentrated in the proper bonds so that bonds in the reactants break. As they break, energy is redistributed and new bonds form, giving products; once the transition state is reached, the reaction proceeds to give products, with the release of energy. A transition state has a definite geometry, a definite arrangement of bonding and nonbonding electrons, and a definite distribution of electron density and charge. - The difference in energy between the reactants and the transition state is called the activation energy. The activation energy is the minimum energy required for a reaction to occur, it can be considered an energy barrier for the reaction. The activation energy determines the rate of a reaction, if the activation energy is large very few molecular collisions occur with sufficient energy to reach the transition state and the reaction is slow. Each step in a reaction has its own transition state and activation energy. A reaction intermediate corresponds to an energy minimum between two transition states. The energies of the reaction intermediates are higher than the energies of either the reactants or the products, these intermediates are highly reaction, and rarely one can be
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This note was uploaded on 02/09/2012 for the course SCIENCE 2213 taught by Professor Lee during the Spring '11 term at UWO.

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Orgo 5 - 5. Reactions of Alkenes and Alkynes Characteristic...

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