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*Organic Chemistry, Second EditionJanice Gorzynski
SmithUniversity of HawaiiCopyright The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
Prepared by Rabi Ann MusahState University of New York at
AlbanyChapter 6Organic Reactions
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*Understanding Organic ReactionsWriting organic reaction
equations and use of arrows.Types of reactions: substitution,
elimination, addition.Bond breaking: homolytic and
heterolyticReactive species: radicals, carbocations and
carbanions.Calculation of H of reaction (enthalpy) from bond
dissociation energies.Thermodynamics: free energy (G), enthalpy
(H), entropy (S) and the equilibrium constant Keq.Energy diagrams:
single step concerted and multistep.Kinetics: 1st & 2nd order
rates, activation energy, catalysis.Chapter 6 Topics:
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*Understanding Organic ReactionsEquations for organic reactions
are usually drawn with reagents to the left of a reaction arrow ()
and products to the right.A reagent, the chemical substance with
which an organic compound reacts, may be placed to the left of the
arrow or on the arrow itself. The solvent and temperature are often
omitted but at times is also placed above or below the arrow.The
symbols h and are placed above or below the arrow for reactions
that require light and heat, respectively.Writing Equations for
Organic Reactions:
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*Figure 6.1 Different ways of writing organic
reactionsUnderstanding Organic ReactionsWriting Equations for
Organic Reactions:Here, all are needed together.
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*When two sequential reactions are carried out without drawing
any intermediate compound, the steps are 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. Omitting the numbers means they are present
together.Understanding Organic ReactionsWriting Equations for
Organic Reactions:
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*Substitution Reactions:A substitution is a reaction in which an
atom or a group of atoms is replaced by another atom or group of
atoms.Below, Y replaces Z on a carbon atom.Kinds of Organic
Reactions
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*Substitution reactions involve breaking one bond and forming
another at the same carbon atom.In a nucleophilic attack, the group
that departs needs to have some stability to exist by itself (a
weak base).Kinds of Organic ReactionsSubstitution Reactions:
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*Elimination is a reaction in which elements of the starting
material are lost and a bond is formed.Water is frequently an
eliminated component.Kinds of Organic ReactionsElimination
Reactions:
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*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.Kinds of Organic ReactionsElimination Reactions:
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*Addition is a reaction in which elements are added to the
starting material. (Opposite of elimination.)Kinds of Organic
ReactionsAddition Reactions:
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*In an addition reaction, new groups X and Y are added to the
starting material. A bond is broken and two bonds are formed.Kinds
of Organic ReactionsAddition Reactions:
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*Addition and elimination reactions are exactly opposite. A bond
is formed in elimination reactions, whereas a bond is broken in
addition reactions.Kinds of Organic ReactionsAddition and
Elimination Reactions:
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*Bond Making and Bond Breaking are the essence of chemical
reactions: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.Understanding Organic Reactions
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*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.Understanding Organic ReactionsHomolytic Bond Breaking:
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*Homolysis and heterolysis require energy.Homolysis generates
uncharged reactive intermediates with unpaired electrons
(radicals).Heterolysis generates charged
intermediates.Understanding Organic ReactionsHeterolytic Bond
Breaking:
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*To illustrate the movement of a single electron, use a
half-headed curved arrow, sometimes called a fishhook.A full headed
curved arrow shows the movement of an electron pair.Understanding
Organic ReactionsBond Making and Bond Breaking:
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*Homolysis: 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: 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.Understanding Organic
Reactions
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*Figure 6.2 Three reactive intermediates resulting from
homolysis and heterolysis of a C Z bondUnderstanding Organic
ReactionsRadicals, Carbocations and Carbanions
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*Radicals and carbocations are electrophiles because they
contain an electron deficient carbon (sp2 carbon atoms).Carbanions
are nucleophiles because they contain a carbon with a lone pair
(sp3 carbon atoms).Understanding Organic ReactionsRadicals,
Carbocations and Carbanions
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*Bond formation occurs in two different ways.Two radicals can
each donate one electron to form a two-electron 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.Understanding Organic ReactionsBond Making:
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*A number of types of arrows are used in describing organic
reactions.Understanding Organic ReactionsUse of Arrows:
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*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 reaction, the homolysis of a covalent bond to form two
radicals.Understanding Organic Reactions
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*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.Understanding Organic ReactionsBond Dissociation Energy:
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*
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*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.Understanding Organic ReactionsBond Dissociation Energy:
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*Bond dissociation energies are used to calculate the enthalpy
change (H0) in a reaction in which several bonds are broken and
formed.Understanding Organic ReactionsBond Dissociation Energy:
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*Understanding Organic ReactionsBond Dissociation Energy:
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*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.Understanding Organic ReactionsBond Dissociation
Energy:
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*Thermodynamics and Kinetics: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. Understanding
Organic Reactions
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*G is the term for standard state free energy and represents the
overall energy difference between reactants (R) and products (P).H
is the term for enthalpy in the standard state.S is the term for
entropy in the standard state.
Standard state conditions have reactants and products at 1 M
concentrations or 1 atmosphere pressure.
The overall free energy at any concentrations of R and P is:G =
G + RT ln [P]/[R]Where R is the constant 8.314 joules /mol-oK and T
= oK (R also = 1.987 cal /mol-oK)Understanding Organic
ReactionsThermodynamic Terms:
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*At equilibrium the overall free energy of reaction is zero. G =
0 so the equation: G = G + RT ln [P]/[R]
Becomes: G = - RT ln Keq
This provides a relationship between the standard state free
energy and the equilibrium constant.
For the reaction: R P Keq = 1000 at 25oCG = -(8.314)(298) ln
1000G = -17118 cal/molUnderstanding Organic
ReactionsThermodynamics:
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*The size of Keq expresses whether the starting materials or
products predominate once equilibrium is reached.When Keq > 1,
equilibrium favors the products (C and D) and the equilibrium lies
to the right as the equation is written. This is a useful
reaction.When Keq < 1, equilibrium favors the starting materials
(A and B) and the equilibrium lies to the left as the equation is
written.The position of the equilibrium is determined by the
relative energies of the reactants and products.Understanding
Organic ReactionsThermodynamics:
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*Figure 6.3 Summary of the relationship between G and Keq. Note
the use of base a 10 log vs a natural log in these equations.
Understanding Organic Reactions G = - 2.3 RT log Keq = - RT ln
KeqThermodynamics:
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*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.Understanding Organic
ReactionsThermodynamics:
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*Understanding Organic ReactionsThermodynamics:
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*These equations can be used for any process with two states in
equilibrium. E.g., monosubstituted cyclohexanes exist as two
different chair conformations that rapidly interconvert at room
temperature and equatorial position favored.Knowing the energy
difference between two conformations permits the calculation of the
amount of each at equilibrium.Understanding Organic
ReactionsThermodynamics:
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*Enthalpy and Entropy:G depends both on H and S.Entropy change,
S, is a measure of the change in the randomness of a system. The
more disorder present, the higher the entropy. The value of S is
(+) when the products are more disordered than the reactants and
(-) when the products are less disordered than the reactants.
Reactions resulting in increased entropy contribute to a negative
G.G is related to H and S by the following equation:Understanding
Organic Reactions
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*The previous equation indicates that the total free energy
change is due to two factors: the change in bonding energy (H) and
the change in disorder (S).The change in bonding energy can be
calculated from bond dissociation energies.Entropy changes are
important whenThe 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. Understanding
Organic ReactionsEnthalpy (H) and Entropy (S) :
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*In most other reactions that are not carried out at high
temperature, the entropy term (TS) is small compared to the
enthalpy term (H0), and therefore it is usually
neglected.Understanding Organic ReactionsEnthalpy and Entropy:
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*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 bond energy difference between reactants and
products is H. If the products have lower bond energy than the
reactants, the reaction is exothermic and energy is released. If
the products have higher bond energy than the reactants, the
reaction is endothermic and energy is consumed.Understanding
Organic Reactions
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*Energy Diagrams:A diagram can also be made by plotting free
energy. Then energy difference between reactants and products is G.
If the products are lower in energy than the reactants, the
reaction is exergonic and energy is released. If the products are
higher in energy than the reactants, the reaction is endergonic and
energy is required.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.Understanding Organic Reactions
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*For the general reaction:The energy diagram would be shown
as:Understanding Organic ReactionsEnergy Diagrams:
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*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 ().Understanding
Organic ReactionsEnergy Diagrams:
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*Example 1Figure 6.4 Some Representative energy
diagramsUnderstanding Organic ReactionsEnergy Diagrams:
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*Example 2Understanding Organic ReactionsFigure 6.4 Some
Representative energy diagramsEnergy Diagrams:
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*Example 3Understanding Organic ReactionsFigure 6.4 Some
Representative energy diagramsEnergy Diagrams:
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*Example 4Understanding Organic ReactionsFigure 6.4 Some
Representative energy diagramsEnergy Diagrams:
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*Figure 6.5 Comparing H and Ea in two energy
diagramsUnderstanding Organic ReactionsEnergy Diagrams:
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*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.Understanding Organic ReactionsEnergy
Diagrams:
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*Understanding Organic ReactionsEnergy Diagrams:
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*Understanding Organic ReactionsEnergy Diagrams:
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*Figure 6.6 Complete energy diagram forthe two-step conversion
of
Understanding Organic ReactionsEnergy Diagrams:
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*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.Understanding Organic Reactions
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*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.Understanding Organic
ReactionsKinetics:
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*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 but 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.Understanding Organic ReactionsKinetics:
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*A two-step reaction has a slow rate-determining step, and a
fast step.The reaction can occur no faster than its slow step. Only
the concentration of the reactants in the rate-determining step
appears in the rate equation.Understanding Organic
ReactionsKinetics:
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*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.Figure 6.7 The effect of a
catalyst on a reactionUnderstanding Organic Reactions
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*Organic Chemistry, Second EditionJanice Gorzynski
SmithUniversity of HawaiiCopyright The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
Prepared by Rabi Ann MusahState University of New York at
AlbanyEnd Chapter 6