2061-ch5organic chemistry is governed by only a few key ideas
that determine chemical reactivity.
occur.
Both need to happen in order to fully understand organic
chemistry.
Addition1.
Elimination2.
Substitution3.
Rearrangement4.
Chapter 5: An overview of organic reactions
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Elimination: one molecule splits into two.2.
Substitution: two molecules exchange parts to give
two new products.
4.
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In a clock, we see the hands move but the mechanism
behind the face is what causes the movement.
In an organic reaction, we see the transformation that
has occurred. The mechanism describes the steps that
cause the changes we observe.
Bond-making or bond-breaking
Steps can occur one at a time or at the same time
(concerted)
in a reaction sequence from reactant to product
Bond formation or breakage can be symmetrical or
unsymmetrical.
time - radical reactions
pair - polar reactions
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odd number of electrons (usually 7) in its valence shell
(instead of the stable octet).
Some example radicals:
electron.
species with unpaired electrons - often initiated by
light (hν)
make a new radical.
a stable product.
There are three types of steps in a radical substitution
reaction:
Bond polarity can be increased by acid-base reactions:
Even though electronegativities are similar between C-S
and C-I, these bonds are polar because the electrons in
the large S and I atoms are polarizable - they can easily
respond to other nearby charges.
5.4 Polar reactions
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out and react with electron-poor sites.
Nucleophile: electron-rich (- or δ-) atom that seeks out
an electron-poor atom (nucleus-loving) - Must have
lone pair of electrons! Nucleophiles are Lewis bases.

Electrophiles are Lewis acids.
headed arrow is used to show the movement of a
pair of electrons, from nucleophile to electrophile
(Never the other way around!!)
Nucleophiles and electrophiles
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When the electrophile is an H, it's an acid-base reaction!
CH3O- + H3O+ →
addition reaction.
when there is a strong acid present.

The δ+ H acts as a strong attractor for electrons
from another molecule. The H is the electrophile
in this reaction.
HBr is a strong acid and contains an electron-poor H.
5.5 Addition of HBr to ethylene
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The π electrons from the nucleophilic double bond
attack the electrophilic hydrogen on HBr, forming a
new C-H σ bond.
This leaves the other carbon (formerly of the π bond)
with only 6 electrons and an empty p orbital - a
positively charged carbocation.
move onto bromine, making a bromide ion.
The bromide ion donates an electron pair to the +
carbocation, forming a C-Br σ bond and yielding the
neutral addition product.
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Practice and the knowledge of a few rules will help you
draw correct curved arrows for reaction mechanisms.
Electrons always move from a nucleophile
(Nu: or Nu:-) to an electrophile (E or E+).
Always start with an electron pair!
1.
Negative nucleophiles become neutral products.i.
Neutral nucleophiles become positive products.ii.
The nucleophile can be either negatively charged or
neutral.
2.
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The electrophile can be either positively charged or
neutral.
3.
Carbon never makes more than four bonds!
Hydrogen never makes more than one bond!
4.
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equilibrium.
constant, Keq, is the ratio of product concentrations over
reactant concentrations (each raised to the power of the
balancing coefficient)
For aA + bB cC + dD,
For the reaction we just studied, there is a very large
equilibrium constant.
amount of unreacted starting material will usually be
undetectable. (The product concentration is 1000x the
reactant concentration at equilibrium)
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In order for a reaction to proceed to completion, the
products must be lower in potential energy than the
reactants.
become more stable low-energy molecules.)
For Keq > 1, ΔGo must be negative (the system
releases energy to become more stable).
This is a spontaneous process.

would have to gain potential energy and become
less stable in order to proceed as written)

This change of potential energy during a reaction is called
the Gibbs free energy change (ΔG).
Enthalpy change (ΔH) is related to the strength of
bonds that are broken and formed. A favorable
enthalpy change will have stronger (more stable)
bonds in the product.
change
change
motion and dispersion of energy.

and entropy. ΔG = ΔH - TΔS
Free energy and equilibrium
Whenever a bond is formed, energy is released (like the
sound made when two magnets hit each other).
Whenever a bond is broken, energy is absorbed (like the
force required for you to pull two magnets apart).
Bond dissociation energy (D): amount of energy required
to break a bond to produce two radical fragments:
This energy is mostly dependent of the type of bond, not
the molecule that the bond is in.
Among the weakest bonds are those that can readily
produce radicals:
σ bonds become stronger the more s character they have
CH3-H 439 kJ/mol
H2C=CH-H 464
HC≡C-H 558
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In the reaction of HBr and ethylene, the first step is
formation of the carbocation.
The highest energy, most unstable point in one step of a
reaction is called the transition state.
The transition state occurs when the C-H bond is partially
formed and the H-Br bond is partially broken.
5.9 Energy diagrams and transition states
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state is the activation energy (ΔG‡).
A high ΔG‡ means very few of the molecules in the
sample will have enough energy to reach the
transition state and the reaction will be slow.

already have enough energy to reach the
transition state and the reaction will be fast at
room temperature.
The size of the activation energy determines the rate of
the reaction (whether it will occur quickly or slowly).
The energy difference between the reactant and the
product is the standard free energy difference, ΔGo - as
we saw before, this determines what the equilibrium
constant will be.
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Solution: heat the reaction so more molecules
have enough energy to reach the transition state

Solution: use a catalyst so there is a series of
different, lower-energy transition states.
A reaction with a very high activation energy may never
reach its equilibrium mixture of products because it is
too slow! (The combustion of gasoline has a large
equilibrium constant - the products are favored
energetically - but at room temperature, the reaction is
too slow to observe.)
An intermediate is the product of one step of the
reaction, and reactant of the next step.
In the complete energy