Chapter 5 An Overview of Organic Reactions

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Chapter 5 An Overview of Organic Reactions
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5.1 Kinds of Organic Reactions

• In general, we look at what occurs and try to
  learn how it happens
• Common patterns describe the changes
• Addition reactions two molecules combine
• Elimination reactions one molecule splits into
  two
• Substitution parts from two molecules exchange
• Rearrangement reactions a molecule undergoes
  changes in the way its atoms are connected

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5.2 How Organic Reactions Occur Mechanisms

• In a clock 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 behind the changes that we can observe
• Reactions occur in defined steps that lead from
  reactant to product

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Steps in Mechanisms

• We classify the types of steps in a sequence
• A step involves either the formation or breaking
  of a covalent bond
• Steps can occur in individually or in combination
  with other steps
• When several steps occur at the same time they
  are said to be concerted

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Types of Steps in Reaction Mechanisms

• Formation of a covalent bond
• Homogenic or heterogenic
• Breaking of a covalent bond
• Homogenic or heterogenic
• Oxidation of a functional group
• Reduction of a functional group

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Homogenic Formation of a Bond

• One electron comes from each fragment
• No electronic charges are involved
• Not common in organic chemistry

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Heterogenic Formation of a Bond

• One fragment supplies two electrons
• One fragment supplies no electrons
• Combination can involve electronic charges
• Common in organic chemistry

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Homolytic Breaking of Covalent Bonds

• Each product gets one electron from the bond
• Not common in organic chemistry

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Heterolytic Breaking of Covalent Bonds

• Both electrons from the bond that is broken
  become associated with one resulting fragment
• A common pattern in reaction mechanisms

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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)

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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

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5.3 Radical Reactions and How They Occur

• Note Polar reactions are more common
• Radicals react to complete electron octet of
  valence shell
• 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

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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

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5.4 Polar Reactions and How They Occur

• Molecules can contain local unsymmetrical
  electron distributions due to differences in
  electronegativities
• This causes a partial negative charge on an atom
  and a compensating partial positive charge on an
  adjacent atom
• The more electronegative atom has the greater
  electron density

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Electronegativity of Some Common Elements

• The relative electronegativity is indicated
• Higher numbers indicate greater electronegativity
• Carbon bonded to a more electronegative element
  has a partial positive charge (?)

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Polarizability

• Polarization is a change in electron distribution
  as a response to change in electronic nature of
  the surroundings
• Polarizability is the tendency to undergo
  polarization
• Polar reactions occur between regions of high
  electron density and regions of low electron
  density

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Generalized Polar Reactions

• An electrophile, an electron-poor species,
  combines with a nucleophile, an electron-rich
  species
• An electrophile is a Lewis acid
• A nucleophile is a Lewis base
• The combination is indicate with a curved arrow
  from nucleophile to electrophile

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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
  much more electronegative, making HBr an
  electrophile

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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

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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
• Charges change during the reaction
• One curved arrow corresponds to one step in a
  reaction mechanism

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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

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5.7 Describing a Reaction Equilibria, Rates, and
Energy Changes

• Reactions can go either forward or backward to
  reach equilibrium
• The multiplied concentrations of the products
  divided by the multiplied concentrations of the
  reactant is the equilibrium constant, Keq
• Each concentration is raised to the power of its
  coefficient in the balanced equation.

aA bB
cC dD
Keq Products/Reactants Cc Dd /
AaBb
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Magnitudes of Equilibrium Constants

• If the value of Keq is greater than 1, this
  indicates that at equilibrium most of the
  material is present as products
• If Keq is 10, then the concentration of the
  product is ten times that of the reactant
• A value of Keq less than one indicates that at
  equilibrium most of the material is present as
  the reactant
• If Keq is 0.10, then the concentration of the
  reactant is ten times that of the product

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Free Energy and Equilibrium

• The ratio of products to reactants is controlled
  by their relative Gibbs free energy
• This energy is released on the favored side of an
  equilibrium reaction
• The change in Gibbs free energy between products
  and reacts is written as DG
• If Keq gt 1, energy is released to the surrounding
  (exergonic reaction)
• If Keq lt 1, energy is absorbed from the
  surroundings (endergonic reaction)

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Numeric Relationship of Keq and Free Energy Change

• The standard free energy change at 1 atm pressure
  and 298 K is DGº
• The relationship between free energy change and
  an equilibrium constant is
• DGº - RT ln Keq where
• R 1.987 cal/(K x mol)
• T temperature in Kelvin
• ln natural logarithm of Keq
• (See the example in the book for a calculation)

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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

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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

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Calculation of an Energy 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 438 243 681 kJ/mol
• Energy of bonds formed 351 432 783 kJ/mol
• DHº 681 783 kJ/mol -102 kJ/mol

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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)

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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 CH bond begs to form
• The HBr bond begins to break

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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

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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

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Carbocation Intermediate Reactions with Anion

• Bromide ion adds an electron pair to the
  carbocation
• An alkyl halide produced
• The carbocation is a reactive intermediate

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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.

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Biological Reactions

• Reactions in living organisms follow reaction
  diagrams too
• They take place in very controlled conditions
• They are promoted by catalysts that lower the
  activation barrier
• The catalysts are usually proteins, called
  enzymes
• Enzymes provide an alternative mechanism that is
  compatible with the conditions of life