1 Chapter 6 Understanding Organic Reactions. 2 Writing Equations for Organic Reactions Equations for...

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Chapter 6

Understanding Organic Reactions

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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.

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• 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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• 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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Kinds of Organic Reactions

• substitution reaction : 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.

• Acid-Base reaction, Oxidation-Reduction

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• 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 an atom that is more electronegative than carbon.

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• Elimination reaction : elements of the starting material are “lost” and a bond is formed.

• 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.

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Kinds of Organic Reactions• The most common examples of elimination occur when X = H

and Y is a heteroatom more electronegative than carbon.

dehydrohalogenation

dehydration

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• Addition reaction : elements are added to the starting material.

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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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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.

• Rearrangement (sigmatropic rearrangement) :

change of carbon skeleton

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Bond Making and Bond Breaking

• reaction mechanism : 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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• Two ways to break a bond (two ways to deal with bonding electron pair.) : homolytic cleavage heterolytic cleavage

Mostly depend on bond strength (bonding energy)

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• Polarity of the bond plays bigger role.• Generally, more electronegative part takes the pair of electrons.

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Radicals, Carbocations, Carbanions

• A full headed curved arrow shows the movement of an electron pair.

• To illustrate the movement of a single electron, use a half-headed curved arrow, sometimes called a fishhook.

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• Radical : reactive intermediate with a single unpaired electron.

generated from homolysis• 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 ReactionsRadicals, Carbocations, Carbanions

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Understanding Organic Reactions• 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.

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• A number of types of arrows are used in describing organic reactions.


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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.

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Understanding Organic ReactionsBond Dissociation Energy• bond dissociation energies are always positive numbers, and

homolysis is always endothermic.• bond formation always releases energy, and thus is always

exothermic. • For example, the H—H bond requires +104 kcal/mol to cleave and

releases –104 kcal/mol when formed.

24Comparing bond dissociation energies is equivalent to comparing bond strength.

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• Bond dissociation energies decrease down a column of the periodic table.

• Generally, shorter bonds are stronger bonds.

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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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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 in the reactants.

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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.

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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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The Equilibrium Constant

At equilibrium, DG = 0 and Q = K

If we replace Qeq by K , equilibrium constant

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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.

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Understanding Organic ReactionsThermodynamics

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Energy Changes and Conformational Isomerism

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Understanding Organic ReactionsEnthalpy and Entropy• G° depends on H° and the entropy change, S°.• S°(Entropy change) : 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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• 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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Enthalpy and Entropy

• 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.

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Energy Diagrams• For the general reaction of a single step reaction

• The energy diagram would be shown as:

• An energy diagram is a schematic representation of the energy changes that take place as reactants are converted to products.

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Understanding Organic ReactionsActivation energy and Transition state• Ea : the minimum amount of energy needed to break the bonds in

the reactants.• The transition state : 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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Energy Diagrams

Example 1 Example 2

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Energy Diagrams

Example 3 Example 4

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Comparison of two pathways

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Energy Diagrams for a two-step reaction mechanism• Consider the following two step reaction:

• An energy diagram must be drawn for each step.

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Energy Diagrams

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Energy Diagrams

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Energy Diagrams for a two-step reaction mechanism• 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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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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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.

• 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 ReactionsKinetics• 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.

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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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Catalysts

• A catalyst is a substance that speeds up or slows down the rate of a reaction.

• It is recovered unchanged in a reaction, and it does not appear in the product.

C

CH3(CH2)2

O

OHC

CH3(CH2)2

O

OCH2CH3

+ CH3CH2OH + H2OH2SO4

Catalyst

H2+

Butanoic acidEthanol

Ethyl butanoate

Cyclohexene

Pd

Cyclohexane 53


Common catalysts in organic chemistry

• acids

C

CH3(CH2)2

O

OHC

CH3(CH2)2

O

OCH2CH3

+ CH3CH2OH + H2OH2SO4

Catalyst

H2+

Butanoic acidEthanol

Ethyl butanoate

Cyclohexene

Pd

Cyclohexane

• metals Lowers activation energy by creating intermediates!

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Understanding Organic ReactionsEnzymes• Enzymes are biochemical catalysts.• 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.

• Through stabilization of the transition state

6.2, 6.10, 6.18, 6.22, 6.23, 6.26, 6.27, 6.35, 6.38, 6.43,

6.46, 6.49, 6.50

Homework

Preview of Chapter 7

Alkyl Halides and Nucleophilic substitution

1. What is Hammond postulation?2. What is the product and stereochemical outcome

of the reaction of (S)-2-bromobutane with -OH a) through SN1 mechanism? b) through SN2 mechanism?

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