Chemistry, Organic Second Edition
Janice Gorzynski Smith
University of Hawaii
Chapter 6
Lecture Outline
Prepared by Rabi Ann Musah
State University of New York at Albany
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
Understanding Organic Reactions
Writing Equations for Organic Reactions
Equations for organic reactions are usually drawn with a
single reaction arrow () between the starting material and
product.
The reagent, the chemical substance with which an organic
compound reacts, is sometimes drawn on the left side of the
equation with the other reactants. At other times, the reagent
is drawn above the arrow itself.
Although the solvent is often omitted from the equation, most
organic reactions take place in liquid solvent.
The solvent and temperature of the reaction may be added
above or below the arrow.
The symbols h and are used for reactions that require
light and heat respectively.
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Understanding Organic Reactions
Writing Equations for Organic Reactions
Figure6.1
Different ways of writing
organic reactions
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Understanding Organic Reactions
Writing Equations for Organic Reactions
When two sequential reactions are carried out without
drawing any intermediate compound, the steps are usually
numbered above or below the reaction arrow. This convention
signifies that the first step occurs before the second step, and
the reagents are added in sequence, not at the same time.
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Understanding Organic Reactions
Kinds of Organic Reactions
A substitution is a reaction in which an atom or a group
of atoms is replaced by another atom or group of atoms.
In a general substitution, Y replaces Z on a carbon atom.
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Understanding Organic Reactions
Kinds of Organic Reactions
Substitution reactions involve bonds: one bond breaks
and another forms at the same carbon atom.
The most common examples of substitution occur when Z is a
hydrogen or a heteroatom that is more electronegative than
carbon.
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Understanding Organic Reactions
Kinds of Organic Reactions
Elimination is a reaction in which elements of the
starting material are lost and a bond is formed.
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Understanding Organic Reactions
Kinds of Organic Reactions
In an elimination reaction, two groups X and Y are removed
from a starting material.
Two bonds are broken, and a bond is formed between
adjacent atoms.
The most common examples of elimination occur when X = H
and Y is a heteroatom more electronegative than carbon.
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Understanding Organic Reactions
Kinds of Organic Reactions
Addition is a reaction in which elements are added to the
starting material.
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Understanding Organic Reactions
Kinds of Organic Reactions
In an addition reaction, new groups X and Y are added to
the starting material. A bond is broken and two
bonds are formed.
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Understanding Organic Reactions
Kinds of Organic Reactions
Addition and elimination reactions are exactly opposite.
A bond is formed in elimination reactions, whereas a
bond is broken in addition reactions.
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Understanding Organic Reactions
Bond Making and Bond Breaking
A reaction mechanism is a detailed description of how bonds
are broken and formed as starting material is converted into
product.
A reaction can occur either in one step or a series of steps.
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Understanding Organic Reactions
Bond Making and Bond Breaking
Regardless of how many steps there are in a reaction, there
are only two ways to break (cleave) a bond: the electrons in
the bond can be divided equally or unequally between the two
atoms of the bond.
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Understanding Organic Reactions
Bond Making and Bond Breaking
Homolysis and heterolysis require energy.
Homolysis generates uncharged reactive intermediates with
unpaired electrons.
Heterolysis generates charged intermediates. 14
Understanding Organic Reactions
Bond Making and Bond Breaking
To illustrate the movement of a single electron, use a halfheaded curved arrow, sometimes called a fishhook.
A full headed curved arrow shows the movement of an
electron pair.
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Understanding Organic Reactions
Bond Making and Bond Breaking
Homolysis generates two uncharged species with
unpaired electrons.
A reactive intermediate with a single unpaired electron is
called a radical.
Radicals are highly unstable because they contain an
atom that does not have an octet of electrons.
Heterolysis generates a carbocation or a carbanion.
Both carbocations and carbanions are unstable
intermediates. A carbocation contains a carbon
surrounded by only six electrons, and a carbanion has a
negative charge on carbon, which is not a very
electronegative atom.
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Understanding Organic Reactions
Bond Making and Bond Breaking
Figure6.2
Three reactive intermediates
resulting from homolysis and
heterolysis of a C Z bond
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Understanding Organic Reactions
Bond Making and Bond Breaking
Radicals and carbocations are electrophiles because they
contain an electron deficient carbon.
Carbanions are nucleophiles because they contain a carbon
with a lone pair.
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Understanding Organic Reactions
Bond Making and Bond Breaking
Bond formation occurs in two different ways.
Two radicals can each donate one electron to form a twoelectron bond.
Alternatively, two ions with unlike charges can come together,
with the negatively charged ion donating both electrons to
form the resulting two-electron bond.
Bond formation always releases energy.
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Understanding Organic Reactions
Bond Making and Bond Breaking
A number of types of arrows are used in describing organic
reactions.
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Understanding Organic Reactions
Bond Dissociation Energy
The energy absorbed or released in any reaction, symbolized
by H0, is called the enthalpy change or heat of reaction.
Bond dissociation energy is the H0 for a specific kind of
reactionthe homolysis of a covalent bond to form two
radicals.
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Understanding Organic Reactions
Bond Dissociation Energy
Because bond breaking requires energy, bond dissociation
energies are always positive numbers, and homolysis is always
endothermic.
Conversely, bond formation always releases energy, and thus is
always exothermic. For example, the HH bond requires +104
kcal/mol to cleave and releases 104 kcal/mol when formed.
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23
Understanding Organic Reactions
Bond Dissociation Energy
Comparing bond dissociation energies is equivalent to
comparing bond strength.
The stronger the bond, the higher its bond dissociation energy.
Bond dissociation energies decrease down a column of the
periodic table.
Generally, shorter bonds are stronger bonds.
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Understanding Organic Reactions
Bond Dissociation Energy
Bond dissociation energies are used to calculate the enthalpy
change ( H0) in a reaction in which several bonds are broken
and formed.
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Understanding Organic Reactions
Bond Dissociation Energy
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Understanding Organic Reactions
Bond Dissociation Energy
Consider the oxidation of isooctane and glucose to yield CO2
and H2O.
H is negative for both oxidations, so both reactions are
exothermic.
Both isooctane and glucose release energy on oxidation
because the bonds in the products are stronger than the bonds
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in the reactants.
Understanding Organic Reactions
Bond Dissociation Energy
Bond dissociation energies have some important limitations.
Bond dissociation energies present overall energy changes
only. They reveal nothing about the reaction mechanism or
how fast a reaction proceeds.
Bond dissociation energies are determined for reactions in
the gas phase, whereas most organic reactions occur in a
liquid solvent where solvation energy contributes to the
overall enthalpy of a reaction.
Bond dissociation energies are imperfect indicators of energy
changes in a reaction. However, using bond dissociation
energies to calculate H gives a useful approximation of the
energy changes that occur when bonds are broken and
formed in a reaction.
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Understanding Organic Reactions
Thermodynamics
For a reaction to be practical, the equilibrium must favor
products and the reaction rate must be fast enough to form them
in a reasonable time. These two conditions depend on
thermodynamics and kinetics respectively.
Thermodynamics describes how the energies of reactants and
products compare, and what the relative amounts of reactants
and products are at equilibrium.
Kinetics describes reaction rates.
The equilibrium constant, Keq, is a mathematical expression that
relates the amount of starting material and product at
equilibrium.
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Understanding Organic Reactions
Thermodynamics
The size of Keq expresses whether the starting materials or
products predominate once is equilibrium reached.
When Keq > 1, equilibrium favors the products (C and D) and the
equilibrium lies to the right as the equation is written.
When Keq < 1, equilibrium favors the starting materials (A and B)
and the equilibrium lies to the left as the equation is written.
For a reaction to be useful, the equilibrium must favor the
products, and Keq > 1.
The position of the equilibrium is determined by the relative
energies of the reactants and products.
G is the overall energy difference between reactants and
products.
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Understanding Organic Reactions
Thermodynamics
Figure6.3
Summary of the
relationship
between G and Keq
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Understanding Organic Reactions
Thermodynamics
G is related to the equilibrium constant Keq by the following
equation:
When Keq > 1, log Keq is positive, making G negative, and
energy is released. Thus, equilibrium favors the products when
the energy of the products is lower than the energy of the
reactants.
When Keq < 1, log Keq is negative, making G positive, and
energy is absorbed. Thus, equilibrium favors the reactants when
the energy of the products is higher than the energy of the
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reactants.
Understanding Organic Reactions
Thermodynamics
Compounds that are lower in energy have increased stability.
The equilibrium favors the products when they are more stable
(lower in energy) than the starting materials of a reaction.
Because G depends on the logarithm of Keq, a small change in
energy corresponds to a large difference in the relative amount
of starting material and product at equilibrium.
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Understanding Organic Reactions
Thermodynamics
These equations can be used for any process with two states in
equilibrium. As an example, monosubstituted cyclohexanes exist
as two different chair conformations that rapidly interconvert at
room temperature, with the conformation having the substituent
in the roomier equatorial position favored.
Knowing the energy difference between two conformations
permits the calculation of the amount of each at equilibrium.
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Understanding Organic Reactions
Enthalpy and Entropy
G depends on H and the entropy change, S.
Entropy change, S, is a measure of the change in the
randomness of a system. The more disorder present, the higher
the entropy. Gas molecules move more freely than liquid
molecules and are higher in entropy. Cyclic molecules have
more restricted bond rotation than similar acyclic molecules and
are lower in entropy.
S is (+) when the products are more disordered than the
reactants. S is (-) when the products are less disordered than
the reactants.
Reactions resulting in increased entropy are favored.
G is related to H and S by the following equation:
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Understanding Organic Reactions
Enthalpy and Entropy
This equation indicates that the total energy change is due
to two factors: the change in bonding energy and the
change in disorder.
The change in bonding energy can be calculated from bond
dissociation energies.
Entropy changes are important when
The number of molecules of starting material differs
from the number of molecules of product in the
balanced chemical equation.
An acyclic molecule is cyclized to a cyclic one, or a
cyclic molecule is converted to an acyclic one.
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Understanding Organic Reactions
Enthalpy and Entropy
In most other reactions that are not carried out at high
temperature, the entropy term (T S) is small compared to the
enthalpy term ( H0), and therefore it is usually neglected.
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Understanding Organic Reactions
Energy Diagrams
An energy diagram is a schematic representation of the energy
changes that take place as reactants are converted to products.
An energy diagram plots the energy on the y axis versus the
progress of reaction, often labeled as the reaction coordinate, on
the x axis.
The energy difference between reactants and products is H. If
the products are lower in energy than the reactants, the reaction
is exothermic and energy is released. If the products are higher
in energy than the reactants, the reaction is endothermic and
energy is consumed.
The unstable energy maximum as a chemical reaction proceeds
from reactants to products is called the transition state. The
transition state species can never be isolated.
The energy difference between the transition state and the
starting material is called the energy of activation, Ea.
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Understanding Organic Reactions
Energy Diagrams
For the general reaction:
The energy diagram would be shown as:
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Understanding Organic Reactions
Energy Diagrams
The energy of activation is the minimum amount of energy
needed to break the bonds in the reactants.
The larger the Ea, the greater the amount of energy that is
needed to break bonds, and the slower the reaction rate.
The structure of the transition state is somewhere between the
structures of the starting material and product. Any bond that is
partially formed or broken is drawn with a dashed line. Any atom
that gains or loses a charge contains a partial charge in the
transition state.
Transition states are drawn in brackets, with a superscript
double dagger ().
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Understanding Organic Reactions
Energy Diagrams
Figure6.4
Some representative
energy diagrams
Example 1
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Understanding Organic Reactions
Energy Diagrams
Figure6.4
Some representative
energy diagrams
Example 2
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Understanding Organic Reactions
Energy Diagrams
Example 3
Figure6.4
Some representative
energy diagrams
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Understanding Organic Reactions
Energy Diagrams
Figure6.4
Example 4
Some representative
energy diagrams
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Understanding Organic Reactions
Energy Diagrams
Figure6.5
Comparing H and Ea in two
energy diagrams
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Understanding Organic Reactions
Energy Diagrams
Consider the following two step reaction:
An energy diagram must be drawn for each step.
The two energy diagrams must then be combined to form an
energy diagram for the overall two-step reaction.
Each step has its own energy barrier, with a transition state at
the energy maximum.
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Understanding Organic Reactions
Energy Diagrams
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Understanding Organic Reactions
Energy Diagrams
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Understanding Organic Reactions
Energy Diagrams
Figure6.6
Complete energy diagram for
the two-step conversion of
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Understanding Organic Reactions
Kinetics
Kinetics is the study of reaction rates.
Recall that Ea is the energy barrier that must be exceeded
for reactants to be converted to products.
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Understanding Organic Reactions
Kinetics
The higher the concentration, the faster the rate.
The higher the temperature, the faster the rate.
G, H, and Keq do not determine the rate of a reaction.
These quantities indicate the direction of the equilibrium and
the relative energy of reactants and products.
A rate law or rate equation shows the relationship between
the reaction rate and the concentration of the reactants. It is
experimentally determined.
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Understanding Organic Reactions
Kinetics
Fast reactions have large rate constants.
Slow reactions have small rate constants.
The rate constant k and the energy of activation Ea are inversely
related. A high Ea corresponds to a small k.
A rate equation contains concentration terms for all reactants in
a one-step mechanism.
A rate equation contains concentration terms for only the
reactants involved in the rate-determining step in a multi-step
reaction.
The order of a rate equation equals the sum of the exponents of
the concentration terms in the rate equation.
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Understanding Organic Reactions
Kinetics
A two-step reaction has a slow rate-determining step, and a fast
step.
In a multi-step mechanism, the reaction can occur no faster than
its rate-determining step.
Only the concentration of the reactants in the rate-determining
step appears in the rate equation.
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Understanding Organic Reactions
Catalysts
Some reactions do not proceed at a reasonable rate unless a
catalyst is added.
A catalyst is a substance that speeds up the rate of a reaction. It
is recovered unchanged in a reaction, and it does not appear in
the product.
Figure6.7
The effect of a catalyst
on a reaction
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Understanding Organic Reactions
Enzymes
Enzymes are biochemical catalysts composed of amino acids
held together in a very specific three-dimensional shape.
An enzyme contains a region called its active site which binds an
organic reactant, called a substrate. The resulting unit is called
the enzyme-substrate complex.
Once bound, the organic substrate undergoes a very specific
reaction at an enhanced rate. The products are then released.
Figure6.8
A schematic representation
of an enzyme at work
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