CHAPTER 6

enthalpy is the heat exchanged between a chemical reaction and its
surroundings. Breaking a bond requires the system to absorb energy.
Bonds can break homolytically or heterolytically. In this course, we will see heterolytic
bond cleavage most often.
We can use Bond Dissociation Energies (BDE) to calculate enthalpy (you did this math
in general chemistry). Most important thing to remember is lower BDE correlates to a
weaker bond that is first to break.

Exothermic reactions:
Products are lower in energy (more stable)•
delta H is negative•
Temperature of surroundings increases•

Endothermic reactions:
Products higher in energy (less stable)
delta H is positive•
Temperature of surroundings decreases•
Entropy of Chemical Reactions:
entropy is described as the randomness and molecular disorder associated with
a chemical reaction.
When entropy is positive, a reaction is spontaneous

Types of Reactions Where Entropy is Positive:
When there are more moles of product than reactant1.
2. When a Ring breaks open
Gibbs Free Energy:
Negative delta G indicates a spontaneous reaction

Reaction Coordinate Diagrams for Gibbs Free Energy:
Endergonic Reaction: Favors reactants
Exergonic Reaction: Favors products

Kinetics:
While delta G describes if a reaction is spontaneous, it tells nothing about how fast the
chemical reaction occurs. That is where we study kinetics or the rate of a chemical
reaction. The order of the reaction describes the degree of change in the concentration of the
reactants.

rate= k[A] is first order (if [A] is doubled, rate is doubled)

rate= k [A][B] is second order (if [A] and [B] are each doubled, the rate is quadrupled)

rate= k[A]²[B] is third order (if [A] is doubled, the rate is quadrupled)

Factors that Affect Reaction Rate:
1. Activation Energy- Energy required to transition from reactants to products. 2.Temperature- raising a reaction’s temperature increases the rate of a reaction
3. Steric Hindrance- since molecules have to collide in the correct orientation,
sometimes if a molecule is too crowded with large substituents, it blocks the collision
4. Catalyst- lowers the activation energy barrier for the reaction thus speeding up the
reaction rate.

Transition states are the highest point on a reaction coordinate diagram. A
transition state is where bond breaking and bond making occur simultaneously. The
timescale where this occurs is too fast to be observed by any instrumentation (ie NMR,
Mass Spec, UV Vis, IR etc)

Intermediates are formed during the course of a reaction. While they can be unstable,
it is possible to isolate them and analyze them through instrumentation. On a reaction
coordinate diagram, they are located as an energy minima between transition states.
Think “valley of stability”. The intermediate that we will discuss the most this semester
is the one shown above the carbonation.

Nucleophiles & Electrophiles:
Many organic chemistry reactions hinge on the principle that negative charges are
attracted to positive charges (and it’s the way we draw electron pushing arrows!) or
electron rich species nucleophile are attracted to electron poor species electrophile.
Note: charges (negative or positive) don’t have to be formal for a species to be
classified as a nucleophile or electrophile. A dipole moment that induces partial
positive or negative charges is enough to make something nucleophilic or electrophilic
nucleophile: lewis base, neutral or negative charge

electrophile: lewis acid, neutral, positive charge

Mechanisms & Arrow Pushing:
Curved Arrows are the way we describe how electrons move in an organic reaction
and they describe which bonds are being broken and being formed.
There are four main ways that electrons move in reactions:
1. Nucleophilic Attack
2. Loss of Leaving Group
3. Proton Transfer (Bronstead Acid/Base)
4. Rearrangements
Note: It is very common to use multiple of the four electron pushing types above to
describe a single organic reaction.

rearrangements: occur only at carbons adjacent to the carbonation; these shifts will only occur if it makes the carbonation more stable