13.5 Reactions of Alkenes & Alkynes

A. Addition Reaction:  The addition of atoms/groups of atoms across the C=C.

-The pi bond is broken and replaced by 2 sigma bonds. This requires changes the hybridization states of the central carbons.

1. Reaction Mechanism: A description of which bonds are broken and which bonds are formed, along with the relative order of each step.

-can also describe the roles of solvents & catalysts

a. Potential Energy Diagrams & Transition states

  Consider:  A  +  B --> C  +  D

the potential energy is the energy stored in the chemical bonds due to the position of electrons in atomic and molecular orbitals.  Remember, electrons closer to a nucleus/bonded nuclei represent lower energy.  So electrons in pi bonds which are further from the bonded nuclei, on average, represent a higher energy state.

Activation Energy: The energy associated with transition state formation. Attributed to reactant bond changes. Heat of reaction (QR or DH): The difference in potential energies between the reactants and products. If DH is negative (P.E.products < P.E.reactants) this is called an exothermic reaction. If DH is positive (P.E.reactants< P.E.products ) this is called an endothermic reaction. Transition State. The species within a reaction that represents a hybrid between the reactant and product.  This high-energy particles is not stable and therefore either decomposes into product or back to reactant. The "bonds" within a transition state complex represents strained bonds or atoms that exceed the octet thus producing an unstable entity.