TOPIC 3 REACTION SUPPLEMENT
As we begin to study how organic reactions occur in more detail, we must
first have a general understanding of four important concepts:
(1) types of organic reactions
(3) reaction energy diagrams
(2) carbocations
(4) the Hammond Postulate
These two concepts are important in studying many organic reactions so
we will examine them here before we proceed to study electrophilic
addition reactions of alkenes.
( 1 ) TYPES OF ORGANIC REACTIONS
 ADDITION REACTIONS:
- 2 reactants add together to form a single product
- A + B  C
H
These two
reactants
H
H
C
+
C
H
H
Br
H
H
Br
C
C
H
H
H
Add to give
this one
product
 ELIMINATION REACTIONS:
- A single reactant splits into two products
- A  B + C
This one
reactant
H
H
Br
C
C
H
H
H
H
C
H
H
+
C
H
H
Br
Gives these
two products
( 1 ) TYPES OF ORGANIC REACTIONS
 SUBSTITUTION REACTIONS:
- 2 reactants exchange parts to give two new products
- A-B + C-D  A-C + B-D
H
These two
reactants
H
C
H
+
H
Cl
Cl
H
C
H
Cl
H
+
H
Cl
Add to give
these two
products
 REARRANGEMENT REACTIONS:
- A single reactant undergoes rearrangement of atoms and
bonds to yield an isomeric product
This single
reactant
CH3CH2
H
H+ catalyst
C
C
C
H
H
1-butene
H
H3C
C
H
CH3
2-butene
Gives this
isomeric
product
( 2 ) CARBOCATIONS
 CARBOCATION: (carbo-cat-ion)
- A trivalent carbon atom w/ 6 e- Has a positive charge
C
 CARBOCATION STABILITY:
- More substituted carbocations are more stable than less
highly substituted ones
- Major Reason Why: inductive effects
- Electrons from alkyl groups can shift towards the +
charge thereby stabilizing it
- Order of stability: (R = alkyl group)
>
3° (tertiary)
>
2° (secondary)
>
1° (primary)
methyl
( 3 ) REACTION ENERGY DIAGRAMS
 REACTION ENERGY DIAGRAM:
- A graphical representation of the energy changes that occur
during a reaction
- y-axis: energy
- x-axis: reaction progress (reactants  products)
Here are some key concepts represented on reaction energy diagrams:
 TRANSITION STATE:
- Highest energy species formed in a reaction (very unstable)
 ACTIVATION ENERGY (∆G‡):
- Difference in energy between reactants and transition state
- Determines how quickly a reaction occurs @ a given temp.
- Larger ∆G‡ = slower reaction
- Smaller ∆G‡ = faster reaction
( 3 ) REACTION ENERGY DIAGRAMS
Parts of a reaction energy diagram:
activation energy
( 3 ) REACTION ENERGY DIAGRAMS
SOME POSSIBLE REACTION ENERGY DIAGRAMS:
A.) fast exergonic reaction
B.) slow exergonic reaction
- small ∆G‡ , negative ∆G°
- large ∆G‡ , negative ∆G°
∆G‡
Energy
Energy
∆G‡
Reaction progress 
Reaction progress 
( 3 ) REACTION ENERGY DIAGRAMS
SOME POSSIBLE REACTION ENERGY DIAGRAMS:
C.) fast endergonic reaction
D.) slow endergonic reaction
- large ∆G‡ , positive ∆G°
Energy
Energy
- small ∆G‡ , positive ∆G°
∆G‡
Reaction progress 
∆G‡
Reaction progress 
( 3 ) REACTION ENERGY DIAGRAMS
 REACTION INTERMEDIATE:
- The carbocation which exists momentarily in a multi-step
reaction
- As soon as the intermediate forms, it reacts further to form
the product
- The intermediate cannot be isolated
Intermediate
This reaction has 2 ∆G‡
- It is a 2 step reaction
( 4 ) THE HAMMOND POSTULATE
 HAMMOND POSTULATE:
- The structure of the transition state (TS) resembles the
structure of the nearest stable species
- TS for endergonic steps structurally resemble products
- TS for exergonic steps structurally resemble reactants
Exergonic
Endergonic
transition state
transition state
reactant
Energy
Energy
product
reactant
Reaction progress 
product
Reaction progress 
( 4 ) THE HAMMOND POSTULATE
The importance of the Hammond Postulate:
- We can determine the structure of an intermediate (which is
key to determining how a reaction occurs) by knowing
whether a reaction/ step of a reaction is endergonic or
exergonic
Class Assignment:
Prepare a study guide of at least 8
questions on 1 sheet of paper and an
answer key on a second sheet of paper for
content for tomorrow’s quiz. Be sure that
your study guide and answer key are
neat!  Give it to a neighbor when you
are done!
Be effective! Make sure your classmates
know what they need to know!
Extra Credit Opportunity
• Haiku is an ancient form of Japanese poetry,
best know to most Westerners as three-line
poems with a 5-7-5 syllable count. Here is
your chance to combine your poetic and
organic chemistry skills by writing your very
own organic chemistry haiku for extra credit!
• Present the poem on a piece of paper and
provide an illustration around the poem by
hand! NO CLIP ART!
Energy
Exergonic
Reaction progress 
R
R
C+
R
C+
R
R
H
H
R
H
C+
C+
H
H
H