1. 5. An Overview of Organic Reactions

2. 5.1 Kinds of Organic Reactions
Addition reactions – two molecules combine
examples: HBr+ethylene; HBr + methyl cyclohexene; Hydrogenation of ethene: CH2=CH2+H2→ CH3CH3
II. Elimination reactions – one molecule splits into two
Examples: acid-catalyzed dehydration of an alcohol; dehydrohalogenation:CH3CH2Br →CH2=CH2+HBr
III. Substitution – parts from two molecules exchange
Examples: Halogenation of alkanes; CH3Br+KOH→CH3OH+KBr
IV. Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected
Examples: Primary carbocation converted by hydride shift to Secondary or tertiary carbocation; 1-butene→trans-2-butene

3. 5.2 How Organic Reactions Occur: Mechanisms
A reaction mechanism consists of the sequence of individual steps by which the reactants are converted to products
A step involves either the formation or breaking of a covalent bond
Steps can occur in individually or in combination with other steps
Steps that occur simultaneously are said to be concerted.
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

4. Types of Steps in Reaction Mechanisms
Formation of a covalent bond
Homogenic or heterogenic
Breaking of a covalent bond
Homolytic or heterolytic
Oxidation of a functional group
Reduction of a functional group
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

5. Homogenic & Heterogenic Formation of a Bond
Homogenic bond formation combines 2 fragments, each contributing 1 electron to the bond pair
This type of bond formation does not involve ions but radicals.
Heterogenic bond formation combines 2 fragments with both electrons of the bond pair contributed by 1 fragment. That is,
One fragment supplies two electrons
One fragment supplies no electrons
This type of bond formation generally involves ions

6. Homolytic and Heterolytic Breaking of Covalent Bonds
In a homolytic breaking, each fragment gets one electron from the bond.
In Heterolytic breaking, one fragment gets both electrons of the bond pair
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

7. Indicating Steps in Mechanisms
Curved arrows indicate breaking and forming of bonds
Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’)
Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

8. McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003
Radicals
Alkyl groups are abbreviate “R” for radical
Example: Methyl iodide = CH3I, Ethyl iodide = CH3CH2I, Alkyl iodides (in general) = RI
A “free radical” is an “R” group on its own:
CH3 is a “free radical” or simply “radical”
Has a single unpaired electron, shown as: CH3.
Its valence shell is one electron short of being complete
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

9. 5.3 Radical Reactions and How They Occur
A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule
A radical can add to an alkene to give a new radical, causing an addition reaction
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

10. Steps in Radical Substitution
Three types of steps
Initiation – homolytic formation of two reactive species with unpaired electrons
Example – formation of Cl atoms form Cl2 and light
Propagation – reaction with molecule to generate radical
Example - reaction of chlorine atom with methane to give HCl and CH3.
Termination – combination of two radicals to form a stable product: CH3. + CH3.  CH3CH3
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

11. 5.4 Polar Reactions and How They Occur
Molecules will have polar bonds when atoms with different electronegativities are bonded
This causes a partial negative charge on the more electronegative atom and a compensating partial positive charge on the less electronegative atom
The more electronegative atom has the greater electron density
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

12. Electronegativity of Some Common Elements
The relative electronegativity is indicated
Higher numbers indicate greater electronegativity
Carbon bonded to a more electronegative element (N, O, F, Cl) has a partial positive charge (+)
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

13. McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003
Polarizability
The electron cloud in a larger atom such as Br or I can be distorted. This distortion is called polarization. Polarizability is the tendency to undergo polarization. Larger atoms have greater polarizability than smaller atoms. When an electron cloud is distorted, regions of differing electron density result.
Polar reactions occur between regions of high electron density and regions of low electron density
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

14. Generalized Polar Reactions
An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species
An electrophile is a Lewis acid; electron pair acceptors
A nucleophile is a Lewis base; electron pair donors
The combination is indicated with a curved arrow from nucleophile to electrophile
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

15. 5.5 An Example of a Polar Reaction: Addition of HBr to Ethylene
HBr adds to the  part of C-C double bond
The  bond is electron-rich, allowing it to function as a nucleophile
H-Br is electron deficient at the H since Br is more electronegative, making HBr an electrophile
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

16. Mechanism of Addition of HBr to Ethylene
HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion
Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond
The result is that ethylene and HBr combine to form bromoethane
All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

17. 5.6 Using Curved Arrows in Polar Reaction Mechanisms
Curved arrows are a way to keep track of changes in bonding in polar reaction
The arrows track “electron movement”
Electrons always move in pairs in polar reactions
Charges change during the reaction
One curved arrow corresponds to one step in a reaction mechanism
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

18. Rules for Using Curved Arrows
The arrow goes from the nucleophilic reaction site to the electrophilic reaction site
The nucleophilic site can be neutral or negatively charged
The electrophilic site can be neutral or positively charged
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

19. 5.7 Describing a Reaction: Equilibria, Rates, and Energy Changes
Reactions can go either forward or backward to reach equilibrium. At equilibrium, both forward & reverse reactions occur at the same speed. However, the concentrations of the reactants and products need not be equal.
The multiplied concentrations of the products divided by the multiplied concentrations of the reactants is the equilibrium constant, Keq
Each concentration is raised to the power of its coefficient in the balanced equation.
The value of Keq determines whether product concentrations are larger (Keq > 1) or reactant concentrations are larger (Keq < 1)
Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

20. Numeric Relationship of Keq and Free Energy Change
The standard free energy change at 1 atm pressure and any T is DGº
The relationship between free energy change and an equilibrium constant is:
DGº = - RT ln Keq where
R = cal K-1mol-1; K-1mol-1
T = temperature in Kelvin
ln = natural logarithm of Keq
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

21. Changes in Energy at Equilibrium
Free energy changes (DGº) can be divided into
a temperature-independent part called entropy (DSº) that measures the change in the amount of disorder in the system
a temperature-dependent part called enthalpy (DHº) that is associated with heat given off (exothermic) or absorbed (endothermic)
Overall relationship: DGº = DHº - TDSº
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

22. 5.8 Describing a Reaction: Bond Dissociation Energies
Bond dissociation energy (D): Heat change that occurs when a bond is broken by homolysis
The energy is mostly determined by the type of bond, independent of the molecule
The C-H bond in methane requires a net heat input of 105 kcal/mol to be broken at 25 ºC.
Table 5.3 lists energies for many bond types
Changes in bonds can be used to calculate net changes in heat
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

23. Calculation of an Enthalpy Change from Bond Dissociation Energies
Addition of Cl-Cl to CH4 (Table 5.3)
Breaking: C-H D = 438 kJ/mol
Cl-Cl D = 243 kJ/mol
Making: C-Cl D = 351 kJ/mol
H-Cl D = 432 kJ/mol
Energy of bonds broken = = 681 kJ/mol
Energy of bonds formed = = 783 kJ/mol
DHº = 681 – 783 kJ/mol = -102 kJ/mol
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

24. 5.9 Describing a Reaction: Energy Diagrams and Transition States
The highest energy point in a reaction step is called the transition state
The energy needed to go from reactant to transition state is the activation energy (DG‡)
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

25. McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003
First Step in Addition
In the addition of HBr the (conceptual) transition-state structure for the first step
The  bond between carbons begins to break
The C–H bond begs to form
The H–Br bond begins to break
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

26. 5.10 Describing a Reaction: Intermediates
If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product
These are called reaction intermediates or simply “intermediates”
Each step has its own free energy of activation
The complete diagram for the reaction shows the free energy changes associated with an intermediate
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

27. Formation of a Carbocation Intermediate
HBr, a Lewis acid, adds to the  bond
This produces an intermediate with a positive charge on carbon - a carbocation
This is ready to react with bromide
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

28. Carbocation Intermediate Reactions with Anion
Bromide ion adds an electron pair to the carbocation
An alkyl halide produced
The carbocation is a reactive intermediate
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003

29. Reaction Diagram for Addition of HBr to Ethylene
Two separate steps, each with a own transition state
Energy minimum between the steps belongs to the carbocation reaction intermediate.
McMurry Organic Chemistry 6th edition Chapter 5 (c) 2003