Ch.5 An Overview of Organic Reactions
5.1 Kinds of Organic Reactions
Four Types of Organic Reaction
Addition Reaction:
A
+
B
C
These reactants
add together ...
example
... to give this
single product
H
H
H
H
+
H-Br
HH
H
H
BrH
Ch.5 An Overview of Organic Reactions
Elimination Reaction:
C
A
HH
H
B
... splits apart to give
these two products
This one
reactant ...
example
+
H
BrH
base
H
H
H
H
+
H-Br
Ch.5 An Overview of Organic Reactions
Substitution Reaction:
A B
+
A C
C D
These two reactants
exahange parts ...
example
H
H C H
H
+
+
B D
... to give these
two new products
Cl-Cl
UV
H
H C Cl
H
+
H-Cl
Ch.5 An Overview of Organic Reactions
Rearrangement Reaction:
A
This single reactant ...
example
B
... gives the isomeric product
acid catalyst
Ch.5 An Overview of Organic Reactions
5.2 How Organic Reactions Occur: Mechanisms
Reaction Mechanism: an overall description of how a reaction occurs
Bond cleavage
A
B
A
+
B
radical
reaction
Homolytic bond breaking (radical):
one electron stays with each fragment
A
B
A
+
B
Heterolytic bond breaking (polar):
two electrons stay with one fragment
polar
reaction
Ch.5 An Overview of Organic Reactions
Bond making
A
B
A
B
Homolytic bond making (radical):
one electron donated by each fragment
A
B
A
radical
reaction
B
Heterolytic bond making (polar):
two electrons donated by one fragment
polar
reaction
Ch.5 An Overview of Organic Reactions
5.3 Radical Reactions and How They Occur
• radical reactions are not as common as polar reactions, but important
reaction
• radicals are highly reactive because one electron is deficient: try to
achieve a valence-shell octet
Substitution reaction: abstract an atom from another molecule
R
A
B
unpaired electron
R A
substitution
product
+
B
unpaired electron
Ch.5 An Overview of Organic Reactions
Addition reaction: addition to multiple bonds
unpaired electron
R
A
B
R A B
unpaired electron
addition
product
radical
Elimination reaction: eliminate another radical species
B A R
unpaired electron
B
A
R
unpaired electron
Ch.5 An Overview of Organic Reactions
Radical Chain Reaction
radical substitution reaction: three kinds of steps
H3C H
+
Cl Cl
hv
H3C Cl
+
H Cl
Step 1. Initiation: production of a small number of reactive radicals
Cl
Cl
hv (UV)
2
Cl
Ch.5 An Overview of Organic Reactions
Step 2. Propagation: repetition of chain reaction
+
Cl
(a)
H3C H
(b)
CH3 + Cl Cl
CH3
+
H Cl
H3C Cl
+
Cl
(c) Repeat steps (a) and (b) until termination
Cl H
H3C H
CH3
Cl
Cl
Cl
Cl CH3
Ch.5 An Overview of Organic Reactions
Step 3. Termination: two radicals combine to form a stable product;
such termination steps occur infrequently becacuse the concentration
of radicals in the reaction at any given moment is very small.
possible termination steps:
Cl
+
Cl
Cl2
CH3
+
Cl
H3C Cl
CH3
+ CH3
H3C CH3
All radical reactions involve odd number of electrons: bonds are
broken and formed by reaction of species that have unpaired electrons.
Ch.5 An Overview of Organic Reactions
5.4 Polar Reactions and How They Occur
• polar reactions occur because of the attraction between positive and
negative charges on different functional groups in molecules
• most organic molecules are electrically neutral; they have no net positive
or negative charge
• Certain bonds, particularly the bonds in functional groups, are polar due
to the electronegativity differences
Ch.5 An Overview of Organic Reactions
H
2.1
Li
1.0
Na
0.9
K
0.8
Cs
0.7
Be
1.6
Mg
1.2
Ca
1.0
C
N O
F
2.5 3.0 3.5 4.0
Si P
S Cl
1.8 2.1 2.5 3.0
Br
2.8
I
2.5
Y δ-
δ+
M
δ+
C
C δ-
Y = O, N, Cl, Br
M = a metal
Ch.5 An Overview of Organic Reactions
Polarity Patterns in Some Functional Groups
nonpolar groups
C C
Alkane
C C
Alkene
C C
Alkyne
Arene
(aromatic ring)
Ch.5 An Overview of Organic Reactions
polar
groups
δ+ δC X
(X= F, Cl, Br, I)
Halide
δ+ δC SH
Thiol
δO +
δ
C C OH
δCarboxylic
acid
δ+ δC OH
δ+ δ- δ+
C O C
Alcohol
Ether
δ+ δ- δ+
C S C
Sulfide
δO
+
δ
C C O-C
δ
Ester
δ+ δC NH2
Amine
O +
δ
C C H
δO +
δ
C C C
Aldehyde
Ketone
δ-
δ
O
+ δ
C C Nδ
Amide
δO
+ δ
C C Clδ
Carboxylic
acid chloride
Ch.5 An Overview of Organic Reactions
Polar bonds can also result from the interaction of functional groups
with solvents and with Lewis acids or bases (activation).
For example, the polarity of C-O bond in methanol is greatly enhanced
by protonation of the oxygen atom with an acid; much more reactive CO bond
H
O
CH3
wealky polar C-O bond
H-A
H
H
O
CH3
+
protonated methanol
(strongly polar C-O bond)
A-
Ch.5 An Overview of Organic Reactions
Polarizability is another factor of bond reactivity
• As the electric field around a given atom changes because of changing
interactions with solvent or with other polar molecules, the electron
distribution around that atom also change. The measure of this response
to an external influence is called the polarizability of the atom.
• Larger atoms with more loosely held electrons are more polarizable than
smaller atoms with tightly held electrons. (polarizability; C-I > C-F)
I δδ+
C
Because of iodine's high polarizability,
the C-I bond behaves as if it were polar.
Ch.5 An Overview of Organic Reactions
Because unlike charges attract, the fundamental
characteristic of all polar organic reactions is that electronrich sites in one molecule react with electron-poor sites in
another molecule.
Bonds are made when an electron-rich atom donates a pair
of electrons to an electron-poor atom, and bonds are broken
when one atom leaves with both electrons from the former
bond.
Ch.5 An Overview of Organic Reactions
Electron Movements
A curved arrow shows where two electrons move when reactant bonds are
broken and product bonds are formed.
A generalized polar reaction
This curved arrow shows that
electrons move from B- to A+
A
Electrophile
(electron-poor)
+
B
Nucleophile
(electron-rich)
A B
The electrons that moved
from B- to A+ end up here in
this new covalent bond
Ch.5 An Overview of Organic Reactions
• nucleophile: a substance that is 'nucleus-loving' (Remember that a
nucleus is positively charged); electron-rich atom; neutral or negatively
charged
• electrophile: a substance that is 'electron-loving'; electron-poor;
neutral or positively charged
some nucleophiles (electron-rich)
H3N
H2O
Br
HO
some electrophiles (electron-poor)
-
H
δ+ δ
H3C Br
δ-
O
+
δ
C
Ch.5 An Overview of Organic Reactions
Sometimes, a species could be both nucleophilic and electrophilic depending
on the circumstances
H3C-OH
AlCl4- CH3+
H
water as a
nucleophile
O
H3C-MgBr
H
CH4
water as an
electrophile
Lewis bases are electron donors and behave as nucleophiles, whereas Lewis
acids are electron acceptors and behave as electrophiles.
Therefore, much of organic chemistry is explainable in terms of acid-base
reactions.
Ch.5 An Overview of Organic Reactions
5.5 An Example of a Polar Reaction: Addition of HBr
to Ethylene
• Electrophilic addition:
H
+
H
HH
H
H-Br
H
Ethylene
(nucleophile)
H
Hydrogen bromide
(electrophile)
H
BrH
Bromoethane
• C=C double bond: σ-bond + π-bond
;electron rich and more accessible electrons → nucleophilic
Ch.5 An Overview of Organic Reactions
• HBr is H+ donor: electrophile
C
C
C-C σ-bond: stronger; less
accessible bonding electrons
C-C π-bond: weaker; more
accessible bonding electrons
Ch.5 An Overview of Organic Reactions
electrophilic addition:
H-Br
H
H
Br-
H
H
H
H
Br
H
H
H
H
H
H
H
H
carbocation
intermediate
The electrophile HBr is attacked by the
π-electrons of the double bond, and a
new C-H σ-bond is formed. This leaves
the other carbon atom with a + charge
and a vacant p-orbital
Br- donates an electron pair to the
positively charged carbon atom,
forming a C-Br σ-bond and yielding the
neutral addition product.
Ch.5 An Overview of Organic Reactions
All polar organic reactions take place between electron-rich
sites and electron-poor sites and involve the donation of an
electron pair from a nucleophile to an electrophile
Ch.5 An Overview of Organic Reactions
5.6 Using Curved Arrows in Polar Reaction Mechanisms
Electron movements in polar reaction mechanisms
rule 1 Electrons move from a nucleophile source (Nu:) to an electrophilic
sink (E).
The nucleophile source must have an electron pair available, usually
either in a lone pair or a multiple bond.
O
E
C
E
N
E
E
Ch.5 An Overview of Organic Reactions
The electrophilic sink must be able to accept an electron pair, usually
because it has either a positively charged atom or a positively
polarized atom in a functional group.
Nu:
Nu:
C
Nu:
δ
δ
C Halogen
+
-
δ+ δ
H O
Nu:
δ+ δC O
Ch.5 An Overview of Organic Reactions
rule 2 The nucleophile can be either negatively charged or neutral.
If the nucleophile is negatively charged, the atom that gives away an
electron pair becomes neutral.
H3C O
negatively
charged
+
H Br
H3C O-H
neutral
+
Br
negatively
charged
Ch.5 An Overview of Organic Reactions
If the nucleophile is neutral, the atom that gives away an electron
pair acquires a positive charge.
H-Br
H
H
neutral
H
H
H
H
H
H
Br-
H
positive
negative
Ch.5 An Overview of Organic Reactions
rule 3
The electrophile can be either positively charged or neutral.
If the electrophile is positively charged, the atom bearing that charge
becomes neutral after accepting an electron pair.
O
C
H
positive
OH
+
CN
negative
CN
neutral
Ch.5 An Overview of Organic Reactions
O
C
O
H
+
CN
H
H
CN
O
unstable
OH
CN
CN
neutral
Ch.5 An Overview of Organic Reactions
If the electrophile is neutral, the atom that accept an electron pair
acquires a negative charge. For this to happen, the negative charge
must be stabilized by being on an electronegative atom such as a
oxygen or a halogen.
H
H
H
H
+
H-Br
neutral
H
H
H
H
+
Br-
H
positive
stable, negatively
charged ion
Ch.5 An Overview of Organic Reactions
rule 4
H
H
The octet rule must be followed.
H
H
+ H Br
H
H
H
H
+
Br-
H
This hydrogen already has two electrons. When another
electron pair moves to the hydrogen from the double
bond, the electron pair in the H-Br bond must leave.
Ch.5 An Overview of Organic Reactions
O
C
H
OH
+
C N
CN
This carbon already has eight electrons. When another
electron pair moves to the carbon from -CN, an electron
pair in the C=Obond must leave.
Ch.5 An Overview of Organic Reactions
Practice
Electron Movements
O
H3C
O
CH2
+
H3C Br
H3C
O
H3C
CH2
O
H3C
CH2
H3C Br
C
H2
CH3
+
Br-
Ch.5 An Overview of Organic Reactions
Practice
Electron Movements
O
O
Cl
OCH3
H3C
H3C
+
Cl-
H2O
+
OCH3
HOHH
H
H
H
BrH
H
H
H
+
Br-
Ch.5 An Overview of Organic Reactions
5.7 Describing a Reaction: Equilibria, Rates, and
Energy Changes
Chemical reactions can go in either forward or reverse direction. The
position of the equilibrium is expressed by Keq (equilibrium constant)
aA + bB
[Products]
Keq =
[Reactants]
Keq > 1: forward reaction
Keq < 1: backward reaction
cC + dD
=
[C]c[D]d
[A]a[B]b
Ch.5 An Overview of Organic Reactions
What determine the magnitude of the equilibrium constant?
Gibbs free-energy change, ∆G: the energy change that occurs during a
chemical reaction
standard Gibbs free-energy change, ∆Go: 1 atm, 298 K, 1 M reactant
concentrations
• Keq > 1: forward reaction : ∆Go negative
Keq < 1: backward reaction: ∆Go positive
∆Go = - RT ln Keq
∆Go = ∆Ho + Τ∆So
∆H, enthalpy
∆G, entropy
Ch.5 An Overview of Organic Reactions
• The enthalpy term is frequently larger and more dominant than the
entropy term.
H2C CH2
+
HBr
CH3CH2Br
∆Go = - 44.8 kJ/mol
∆Ho = - 84.1 kJ/mol
∆So = - 0.132 kJ/(K.mol)
T = 298 K
∆H (heat of reaction): measure of the change in total bonding energy
during a reaction
Ch.5 An Overview of Organic Reactions
• ∆H < 0: exothermic: the bonds in the products are stronger (more
stable) than the bonds in the reactants, heat is released
• ∆H > 0: endothermic: the bonds in the products are weaker (less
stable) than the bonds in the reactants, heat is absorbed
∆S (entropy change): measure of the change in the amount of molecular
disorder, or freedom of motion, that accompanies a reaction.
A
A
B
+
B
+
C
∆So > 0
C
∆So < 0
Ch.5 An Overview of Organic Reactions
Explanation of Thermodynamic Quantities: ∆Go = ∆Ho - T∆So
Term
Name
∆ Go
Gibbs free-energy change
The energy difference between reactants and products. When ∆Go is negative,
the reaction is exergonic, has a favorable equilibrium constant, and can occur
spontaneously. When ∆Go is positive, the reaction is endergonic, has an
unfavorable equilibrium constant, and cannot occur spontaneously.
Ch.5 An Overview of Organic Reactions
∆Ho
Enthalpy change
The heat of reaction, or difference in strength between the bonds broken
in a reaction and the bonds formed. When ∆Ho is negative, the reaction
releases heat and is exothermic. When ∆Ho is positive, the reaction
absorbs heat and is endothermic.
∆So
Entropy change
The change in molecular disorder during a reaction. When ∆So is
negative, disorder decreases; when ∆So is positive, disorder increases.
Ch.5 An Overview of Organic Reactions
• The equilibrium constant tells only the position of the equilibrium, or
how much product is theoretically possible. It doesn't tell the rate of
reaction, or how fast the equilibrium is established.
• Some reactions are extremely slow even though they have favorable
equilibrium constants.
For example,
Gasoline reacts with oxygen slowly at room temperature (stable)
but, at higher temperature such as occur in contact with a lighted match,
gasoline reacts rapidly with oxygen and undergoes complete conversion
to the equilibrium products H2O and CO2.
Rate → Is the reaction fast or slow?
Equilibrium → In what direction does the reaction proceed?
Ch.5 An Overview of Organic Reactions
5.8 Describing a Reaction: Bond Dissociation Energies
Bond dissociation energy (D): the amount of energy required to break
a given bond to produce two radical fragments; in the gas phase at 25oC
A
B
A
+
∆H = bond dissociation energy (D)
B
Ch.5 An Overview of Organic Reactions
Table 5.3 Bond dissociation energy (D)
366 kJ/mol
H3CO H
HO OH
437 kJ/mol
213 kJ/mol
H I
298 kJ/mol
PhCH2 H
368 kJ/mol
Cl Cl
243 kJ/mol
H2C=CHCH2 H
361 kJ/mol
Br Br
193 kJ/mol
H3C Br
293 kJ/mol
151 kJ/mol
H3C I
234 kJ/mol
H Cl
H Br
I
I
432 kJ/mol
Ch.5 An Overview of Organic Reactions
• bond dissociation energies can be used for the rough calculation of ∆Ho
H
H C H
H
+
Cl Cl
H
H C Cl
H
+
H Cl
reactant bonds broken
product bonds formed
C-H
Cl-Cl
C-Cl
H-Cl
total
D= 438 kJ/mol
D= 243 kJ/mol
D= 681 kJ/mol
total
∆Ho= 681 - 783 = - 102 kJ/mol
; exothermic by -102 kJ/mol
D= 351 kJ/mol
D= 432 kJ/mol
D= 783 kJ/mol
Ch.5 An Overview of Organic Reactions
but, limitations with the calculations:
- no information on ∆So and thus no information on ∆Go
- no information about the rate of reaction even if ∆Go is favorable
- measured in the gas phase; but most organic reactions are carried in
solution
Solvation effect:
solvent molecules can surround and interact with dissolved reactants
- can weaken bonds and cause large deviation from the gas-phase value
of ∆Ho
- the entropy term ∆So also can be different in solution because the
solvation of a polar reactant by a polar solvent causes a certain amount
of orientation in the solvent and thereby reduces the amount of disorder
Ch.5 An Overview of Organic Reactions
5.9 Describing a Reaction:
Energy Diagrams and Transition States
For a reaction to take place, reactant molecules must collide, and
reorganization of atoms and bonds must occur.
BrH
H
H
H
H-Br
H
H
H
Br
H
H
carbocation
H
H
H
H
H
Ch.5 An Overview of Organic Reactions
reaction energy diagram:
describes energy changes a reaction
transition state (TS)
Energy
carbocation product
CH3CH2 Br
activation
energy
∆G
∆Go controls the
position of the
equilibrium
∆Go
∆G‡ controls the
reactants
H2C CH + HBr
Reaction progress
(reaction coordinate)
reaction rate
Ch.5 An Overview of Organic Reactions
transition state: the highest energy structure, unstable, can't be isolated
H
H
H
Br
H
H
A hypothetical transition state structure for the first step.
The C-C p bond is just beginning to break, and C-H bond is just beginning
to form, and the H-Br bond is just beginning to break
Ch.5 An Overview of Organic Reactions
Activation energy, ∆G‡ : the energy difference between reactants and
transition state
- determine how rapidly the reaction occurs at a given temperature
- collisions with energies greater than the activation energy can form
products
- the activation complex can proceed to the products or revert back to
the reactants
• typical organic reactions: ∆G‡ ~10-35 kcal/mol (40-150 kJ/mol)
• Reactions with activation energy less than 20 kcal/mol (80 kJ/mol) take
place at or below room temperature, whereas reactions with higher
activation energies normally require a higher temperature.
Ch.5 An Overview of Organic Reactions
- a large activation energy: slow reaction because few collisions occur
with enough energy for the reacting molecules to reach the transition
state.
Energy
a slow exergonic reaction
∆G
∆Go
Reaction progress
(reaction coordinate)
Ch.5 An Overview of Organic Reactions
Energy
a slow endergonic reaction
∆G
∆Go
Reaction progress
(reaction coordinate)
Ch.5 An Overview of Organic Reactions
- a small activation energy: fast reaction
Energy
a fast exergonic reaction
∆G
∆Go
Reaction progress
(reaction coordinate)
Ch.5 An Overview of Organic Reactions
Energy
a fast endergonic reaction
∆G
∆Go
Reaction progress
(reaction coordinate)
Ch.5 An Overview of Organic Reactions
Energy
What kind of a reaction can have a symmetric reaction energy diagram?
Reaction progress
(reaction coordinate)
Ch.5 An Overview of Organic Reactions
5.10 Describing a Reaction: Intermediates
reaction intermediate: species exist momentarily during the course of the
multi-step reaction
BrH
H
H
H
H-Br
H
H
H
Br
H
H
reaction intermediate
H
H
H
H
H
Ch.5 An Overview of Organic Reactions
can't be isolated, unstable but more stable than TS1 and TS2
TS1
TS2
∆G2
Energy
CH3CH2
Br
∆G1
reactants
H2C CH2+ HBr
∆Go
Reaction progress
(reaction coordinate)
CH3CH2Br
Ch.5 An Overview of Organic Reactions
hypothetical reaction energy diagrams for some two-step reactions:
exergonic
Energy
∆G2
∆G1
∆Go
Reaction progress
(reaction coordinate)
Ch.5 An Overview of Organic Reactions
Energy
hypothetical reaction energy diagrams for some two-step reactions:
endergonic
∆G2
∆G1
∆Go
Reaction progress
(reaction coordinate)
Ch.5 An Overview of Organic Reactions
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 condi
tions of life
uncatalyzed
Energy
•
•
•
•
•
enzyme catalyzed
Chemistry @ Work
Explosives
• spontaneous break down of molecules into fragments- usually stable gases such
as N2, H2O, CO2
• instantaneouse release of large quantaties of hot gases, which set up a devasting
shock wave as they expand
• primary explosives: highly sensitive, ex) Pb(N3)2
• secondary explosives: less sensitive to heat and shock, detonated by primary
initiators
HO
OH + 3 HNO3
H2SO4
O2NO
ONO2
OH
ONO2
Glycerin
Nitroglycerin
(highly unstable)
+
3 H2O
1865, Alfred Nobel
commercial dynamite: a mixture of ammonium nitrate and nitroglycerin absorbed onto
diatomaceous earth (stabilized)
Explosives
Chemistry @ Work
• military explosives: stable
CH3
O2N
NO2
CH2ONO2
O2NOH2C
CH2ONO2
CH2ONO2
NO2
Trinitrotoluene
(TNT)
Pentaerythritol tetranitrate
(PETN)
O2N
N
N
NO2
N
NO2
RDX
(research department explosive)
• plastic explosive: PETN and RDX compounded with waxes or synthetic
polymers