McMurry (Kap. 6,9) - describing a reaction: energy diagrams and transition states Flashcards
(10 cards)
What happens during a reaction?
1) Reactant molecules must collide
2) reorganization of atoms and bonds must occur.
What is an energy diagram?
A graph to depict graphically the energy changes that occur during a reaction.
The vertical axes: the total energy of the reactants.
The horizontal axes: (called the reaction coordinate/progress) - represents the progress of the reaction from beginning to end.
Before the two reactants collide, where are we in the energy diagram?
In the beginning.
What happens when the two reactants collide?
Their electron clouds repel each other, so the energy level rise, however the reactants continue to approach each other until the new bonds start to form - at some point a structure of maximum energy is reached (called TRANSITION STATE).
What is a transition state?
Represents the highest energy structure involved in the steps of a reaction.
It is unstable and cant be isolated, but it is the part of the reaction where some bonds are partially formed, while others are partially broken.
What is the activation energy?
The energy difference between reactants and transition state - determines HOW RAPIDLY THE REACTION OCCURS at a given temperature.
A large activation energy = slow reaction (because few collision occur with enough energy for the reaction to reach transition state).
A small activation energy = rapid reaction (almost all collisions occur with enough energy for the reactants to transition to transition state).
Does reactions have a larger or smaller activation energies at higher temperatures generally?
Reactions with higher activation energies require higher temperatures - to give reactants enough energy to climb to the transition state.
What happens when the transition state is reached?
The reaction can either continue on to give (the carbocation or other molecule product) or revert back to reactants.
IF it reverts back, the transition state structure comes apart and an amount of free energy corresponding to the (activation energy) is released.
IF the reaction continues on to give the (carbocation OR other product) the new bond is fully formed, and an amount of the energy corresponding to the difference between the transition state and the product is released (net energy change - difference between reactants and products).
If the product is higher in energy then the reactant, is the reaction endergonic or exergonic?
Endergonic, and the reaction absorbs energy (positive value of net energy change).
Does all the reactions have the same energy diagram?
NO, each reaction have its own energy profile.
FAST (small activation energy)
SLOW (large activation energy)
ENDEROGONIC/absorb energy (positive net energy change)
EXEROGONIC/release energy (negative net energy change).
