Reaction Mechanisms of Organic Reactions

1
Reaction Mechanisms of Organic Reactions
25.1 Types of Reactive Species in Organic
Chemistry 25.2 Formation of Reactive
Species 25.3 Types of Organic Reactions 25.4 Conve
ntions for Writing Reaction Mechanisms 25.5 Induc
tive Effect and Resonance Effect
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Types of Reactive Species in Organic Chemistry
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25.1 Types of Reactive Species in Organic
Chemistry (SB p.98)

• Three classes of reactive species commonly
  encountered in organic reactions
• Free radicals
• Electrophiles
• Nucleophiles

4
25.1 Types of Reactive Species in Organic
Chemistry (SB p.98)
Free Radicals

• Electrically neutral atoms or groups of atoms
  possessing an unpaired electron
• Highly reactive (? unstable electronic
  configuration)
• Tend to seek extra electron(s) in order to attain
  the stable octet

5
25.1 Types of Reactive Species in Organic
Chemistry (SB p.98)
Free Radicals
e.g.
6
25.1 Types of Reactive Species in Organic
Chemistry (SB p.98)
Electrophiles

• Electron-deficient species that tend to accept
  electron(s)
• Possess an empty orbital to receive electrons
• Usually cations or free radicals
• Tend to seek an electron-rich centre for reaction

7
25.1 Types of Reactive Species in Organic
Chemistry (SB p.98)
Electrophiles
e.g. Cations
Br , Cl , NO2 , R , RCO
e.g. Free radicals
8
25.1 Types of Reactive Species in Organic
Chemistry (SB p.99)
Nucleophiles

• Electron-rich species that tend to seek an
  electron-deficient site for reaction
• Possess lone pairs of electrons
• Usually anions or molecules with lone pair(s) of
  electrons

9
25.1 Types of Reactive Species in Organic
Chemistry (SB p.99)
Nucleophiles
e.g. Anions
Cl- , Br- , I- , RO- , CN- , OH- ,
RCOO-
e.g. Molecules with lone pairs of electrons
H2O , ROH , ROR , NH3 , RNH2 , R2NH ,
R3N
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Formation of Reactive Species
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25.2 Formation of Reactive Species (SB p.99)
Bond Fission

• Breaking of bonds
• Reactions of organic compounds always involve the
  formation and breaking of covalent bonds
• Two ways of breaking of bonds
• 1. Homolysis
• 2. Heterolysis

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25.2 Formation of Reactive Species (SB p.99)
Homolysis

• Each fragment takes away one of the two bonding
  electrons
• Free radicals are produced

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25.2 Formation of Reactive Species (SB p.99)
Homolysis
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25.2 Formation of Reactive Species (SB p.100)
Homolysis

• Energy must be supplied to cause homolysis of
  covalent bonds
• Usually accomplished in two ways
• ? by heating
• ? by irradiation with light

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25.2 Formation of Reactive Species (SB p.100)
Homolysis
e.g. chlorine undergoes homolysis readily when
heated, or when irradiated with light of a
particular wavelength
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25.2 Formation of Reactive Species (SB p.100)
Homolysis

• Organic reactions always involve the breaking and
  formation of covalent bonds of carbon
• General equation of homolysis of a bond to carbon

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25.2 Formation of Reactive Species (SB p.100)
Homolysis
e.g. Methane undergoes homolysis to form a
methyl radical and a hydrogen radical
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25.2 Formation of Reactive Species (SB p.100)
Heterolysis

• One fragment takes away both bonding electrons
• Leaving the other fragment with an empty orbital
• Produces charged fragments or ions

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25.2 Formation of Reactive Species (SB p.100)
Heterolysis
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25.2 Formation of Reactive Species (SB p.100)
Heterolysis

• Normally requires the bond to be polarized

• Results from the difference in electronegativity
  between the atoms joined by the bond

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25.2 Formation of Reactive Species (SB p.100)
Heterolysis

• The greater the difference in electronegativity
  between the bonded atoms
• ? the greater the polarization of the bond
• Atom B is more electronegative than atom A

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25.2 Formation of Reactive Species (SB p.100)
Heterolysis

• Heterolysis of a bond to carbon can lead to the
  formation of
• ? Carbon cation (known as carbocation or
  carbonium ion), or
• ? Carbon anion (known as carbanion)
• Depend on the electronegativity of the atom that
  is bonded to the carbon atom

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25.2 Formation of Reactive Species (SB p.101)
Heterolysis
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Types of Organic Reactions
25
25.3 Types of Organic Reactions (SB p.101)
Categories of Organic Reactions
1. Substitution reactions 2. Addition
reactions 3. Elimination reactions 4. Condensation
reactions 5. Rearrangement reactions
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25.3 Types of Organic Reactions (SB p.101)
Substitution Reactions

• Characteristic reactions of saturated compounds
  (such as alkanes, haloalkanes) and aromatic
  compounds
• An atom or a group of atoms of the reactant
  molecule is replaced by another atom or group of
  atoms

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25.3 Types of Organic Reactions (SB p.101)
Substitution Reactions
e.g. Chloromethane reacts with sodium hydroxide
to form methanol and sodium chloride
28
25.3 Types of Organic Reactions (SB p.101)
Addition Reactions

• Characteristic reactions of compounds with
  multiple bonds
• Two products react to give a single product

29
25.3 Types of Organic Reactions (SB p.101)
Addition Reactions
e.g. Reaction of ethene with bromine
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25.3 Types of Organic Reactions (SB p.101)
Elimination Reactions

• Opposite of addition reactions
• Atoms or groups of atoms are removed from two
  adjacent atoms (usually carbon atoms) of the
  reactant molecule
• A method for preparing compounds with double and
  triple bonds

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25.3 Types of Organic Reactions (SB p.102)
Elimination Reactions
e.g. In dehydrohalogenation of bromoethane,
ethene and hydrogen bromide are formed
32
25.3 Types of Organic Reactions (SB p.102)
Condensation Reactions

• Two or more molecules join together
• A small molecule is removed in the process

33
25.3 Types of Organic Reactions (SB p.102)
Condensation Reactions
e.g. The esterification of ethanoic acid and
ethanol gives ethyl ethanoate, with a water
molecule being eliminated
34
25.3 Types of Organic Reactions (SB p.102)
Rearrangement Reactions

• A molecule undergoes reorganization of its
  constituent atoms or groups of atoms
• e.g.

35
Conventions for Writing Reaction Mechanisms
36
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Reaction Mechanism

• Describes the actual sequence of bond breaking
  and bond forming during a reaction
• Cannot be determined by experiments and predicted
  by theory

37
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Reaction Mechanism

• The movement of electron pairs is emphasized in
  the reaction mechanisms
• A curly arrow is used to show the movement of an
  electron pair during the reaction

38
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Reaction Mechanism

• The beginning of the arrow shows where the
  electron pair starts from
• The arrow-head shows where the pair ends up

39
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Reaction Mechanism
e.g. The heterolytic bond fission of hydrogen
chloride can be illustrated below
40
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Reaction Mechanism

• A curly arrow with half an arrow head is used to
  indicate the movement of a single electron

41
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Reaction Mechanism
e.g. The homolytic bond fission of a chlorine
molecule can be illustrated below
42
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
43
25.4 Conventions for Writing Reaction Mechanisms
(SB p.104)
Substitution Reactions
1. Nucleophilic substitution reaction
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25.4 Conventions for Writing Reaction Mechanisms
(SB p.104)
Substitution Reactions
2. Electrophilic substitution reaction
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25.4 Conventions for Writing Reaction Mechanisms
(SB p.104)
Substitution Reactions
3. Free radical substitution reaction
46
25.4 Conventions for Writing Reaction Mechanisms
(SB p.105)
Addition Reactions
1. Nucleophilic addition reaction
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25.4 Conventions for Writing Reaction Mechanisms
(SB p.105)
Addition Reactions
2. Electrophilic addition reaction
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25.4 Conventions for Writing Reaction Mechanisms
(SB p.105)
Addition Reactions
3. Free radical addition reaction
49
25.4 Conventions for Writing Reaction Mechanisms
(SB p.105)
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Inductive Effect and Resonance Effect
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25.5 Inductive Effect and Resonance Effect (SB
p.106)
Inductive Effect and Resonance Effect

• The availability of electrons in bonds or at
  atoms has significant effects on the type of
  organic reaction that occurs
• Two types of electronic effect
• 1. Inductive effect
• 2. Resonance effect

52
25.5 Inductive Effect and Resonance Effect (SB
p.106)
Inductive Effect

• If a carbon atom is joined to an atom X of higher
  electronegativity than carbon
• ? the bonding electrons of the C ? X bond will
  be displaced away from the carbon atom
• ? the bonding electrons of the C ? X bond will
  be displaced towards atom X

53
25.5 Inductive Effect and Resonance Effect (SB
p.106)
Inductive Effect

• Carbon atom will exhibit a partial positive
  charge
• Atom X will exhibit a partial negative charge

54
25.5 Inductive Effect and Resonance Effect (SB
p.106)
Inductive Effect

• If a carbon atom is joined to an atom Y of lower
  electronegativity than carbon
• ? the bonding electrons of the C ? Y bond will
  be displaced away from atom Y
• ? the bonding electrons of the C ? Y bond will
  be displaced towards the carbon atom

55
25.5 Inductive Effect and Resonance Effect (SB
p.106)
Inductive Effect

• Carbon atom will exhibit a partial negative
  charge
• Atom Y will exhibit a partial positive charge

56
25.5 Inductive Effect and Resonance Effect (SB
p.106)
Inductive Effect

• Represented by an arrow head in the middle of the
  covalent bond pointing in the direction of the
  displacement of electrons

57
25.5 Inductive Effect and Resonance Effect (SB
p.107)
Inductive Effect

• When an electron-withdrawing group (X) is linked
  to carbon
• ? the group develops a partial negative charge
• ? exert a negative inductive effect (-I)

58
25.5 Inductive Effect and Resonance Effect (SB
p.107)
Inductive Effect

• When an electron-releasing group (Y) is linked to
  carbon
• ? the group develops a partial positive charge
• ? exert a positive inductive effect (I)

59
25.5 Inductive Effect and Resonance Effect (SB
p.107)
Inductive Effect
1. Groups which exhibit negative inductive effect
(i.e. electron-withdrawing groups)
NO2? gt F? gt COOH? gt Cl? gt Br? gt I?
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25.5 Inductive Effect and Resonance Effect (SB
p.107)
Inductive Effect
2. Groups which exhibit positive inductive effect
(i.e. electron-releasing groups)
Alkyl groups like CH3?, C2H5?, C3H7?
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25.5 Inductive Effect and Resonance Effect (SB
p.107)
Inductive Effect

• The stabilities of the carbocations in decreasing
  order

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25.5 Inductive Effect and Resonance Effect (SB
p.107)
Why is tert-Butyl Carbocation the Most Stable
among the Four Carbocations?

• Three electron-releasing methyl groups
  surrounding the central carbon atom
• Help reduce the positive charge on the central
  carbon atom by exerting positive inductive effects

63
25.5 Inductive Effect and Resonance Effect (SB
p.107)
Inductive Effect

• The greater the number of alkyl groups attached
  to the central carbon atom
• ? the more dispersion the charge
• ? the more stable the carbocation

64
25.5 Inductive Effect and Resonance Effect (SB
p.107)
Resonance Effect

• Electronic effect involving ? electrons or
  electrons present in unhybridized p orbitals

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25.5 Inductive Effect and Resonance Effect (SB
p.107)
Resonance Effect
e.g. the carboxylate ion is stabilized by
resonance effect ? represented by two resonance
structures
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25.5 Inductive Effect and Resonance Effect (SB
p.108)
Resonance Effect

• The actual structure of the carboxylate ion is
  the resonance hybrid of the two resonance
  structures

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25.5 Inductive Effect and Resonance Effect (SB
p.108)
Resonance Effect

• The negative charge is dispersed over two oxygen
  atoms
• Delocalized ? electrons over the whole ?COO-
  group
• Extra stability to the ion

68
25.5 Inductive Effect and Resonance Effect (SB
p.108)
Another Example of Resonance Stabilization
e.g. the carbocation with the positively charged
carbon atom directly bonded to a benzene
ring ? represented by four resonance
structures
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25.5 Inductive Effect and Resonance Effect (SB
p.108)
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The END
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25.1 Types of Reactive Species in Organic
Chemistry (SB p.99)
Back
Check Point 25-1
Identify the following chemical species as
electrophiles, nucleophiles, or one that could
act as both an electrophile and a
nucleophile. (a) Cl (d) CH3Cl (b) C2H5 (e)
NO2 (c) NH3 (f) CH3OH
Answer

• Nucleophile (e) Electrophile
• Electrophile (f) Electrophile and nucleophile
• Nucleophile
• Electrophile and nucleophile

72
25.2 Formation of Reactive Species (SB p.101)
Back
Check Point 25-2
Which type of bond fission, homolysis or
heterolysis, is most likely to occur in (a) a
bond between identical atoms? (b) a bond between
atoms having widely different electronegativities
? (c) a bond between atoms having similar
electronegativities?
Answer

• Homolysis
• Heterolysis
• Homolysis

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25.3 Types of Organic Reactions (SB p.102)
Check Point 25-3
Classify the following reactions into
substitution, addition, elimination, condensation
or rearrangement. (a) C2H4 H2O ??
CH3CH2OH (b) CH3CH2CH2OH ?? CH3CH CH2
H2O (c) CH3CH2CH2OH ?? CH3CHOHCH3 (d) CH3CH3
Cl2 ?? CH3CH2Cl HCl (e) CH3CH2COOH CH3CH2NH2
?? CH3CH2CONHCH2CH3 H2O
Answer
74
25.3 Types of Organic Reactions (SB p.102)
Check Point 25-3

• Addition reaction
• Elimination reaction
• Rearrangement reaction
• Substitution reaction
• Condensation reaction

Back
75
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Example 25-4A
Answer
76
25.4 Conventions for Writing Reaction Mechanisms
(SB p.103)
Example 25-4A
Back
77
25.4 Conventions for Writing Reaction Mechanisms
(SB p.104)
Example 25-4B
The mechanism for the reaction between ethene and
bromine in tetrachloromethane is as
follows (a) Is the bond fission in Br2
homolytic or heterolytic? Explain your
answer. (b) State which is the nucleophile and
which is the electrophile. (c) What type of
organic reaction is this?
Answer
78
25.4 Conventions for Writing Reaction Mechanisms
(SB p.104)
Example 25-4B
(a) It is a heterolytic bond fission because the
shared bonding electrons are displaced to one
atom and ions are formed. (b) Ethene is the
nucleophile and bromine is the electrophile. (c) A
ddition reaction
Back
79
25.4 Conventions for Writing Reaction Mechanisms
(SB p.105)
Let's Think 1
Why are substitution and addition reactions the
most common types of organic reactions?
Answer
Substitution and addition reactions are the most
common types of organic reactions because they
take place under relatively mild conditions.
Back
80
25.4 Conventions for Writing Reaction Mechanisms
(SB p.105)
Back
Check Point 25-4

• Free radical substitution reaction
• Nucleophilic substitution reaction
• Electrophilic addition reaction
• Nucleophilic addition reaction

Answer
81
25.5 Inductive Effect and Resonance Effect (SB
p.108)
Check Point 25-5
(a) Draw the two resonance structures for
propanoate ion (CH3CH2COO).
Answer
82
25.5 Inductive Effect and Resonance Effect (SB
p.108)
Back
Check Point 25-5
(b) State whether the following species exhibit
positive or negative inductive effects. (i) I
(ii) NO2 (iii) COOH (iv) C2H5

• Negative inductive effect
• Negative inductive effect
• Negative inductive effect
• Positive inductive effect

Answer