Transcript Document

Addition and elimination reactions are exactly opposite. A
 bond is formed in elimination reactions, whereas a 
bond is broken in addition reactions.
Bond Making and Bond Breaking
• A reaction mechanism is 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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• Regardless of how many steps there are in a reaction, there
are only two ways to break (cleave) a bond: the electrons in
the bond can be divided equally or unequally between the two
atoms of the bond.
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• Homolysis and heterolysis require energy.
• Homolysis generates uncharged reactive intermediates with
unpaired electrons.
• Heterolysis generates charged intermediates.
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• To illustrate the movement of a single electron, use a halfheaded curved arrow, sometimes called a fishhook.
• A full headed curved arrow shows the movement of an
electron pair.
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• Homolysis generates two uncharged species with
unpaired electrons.
• A reactive intermediate with a single unpaired electron is
called a radical.
• 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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Three reactive intermediates resulting from homolysis and heterolysis of a C – Z bond
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• Radicals and carbocations are electrophiles because they
contain an electron deficient carbon.
• Carbanions are nucleophiles because they contain a carbon
with a lone pair.
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Heterolytically cleave each of the carbon-hetratom
bonds and label the organic intermediate as a
carbocation or carbanion
a)
+ OH
OH
carbocation
b)
H3CH2C
Li
H3C
CH2
+
Li
carbanion
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• Bond formation occurs in two different ways.
• Two radicals can each donate one electron to form a twoelectron 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.
• Bond formation always releases energy.
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Relative stabilities of carbocations
Relative stability of radicals
Dielectric constant (Debye), 25 oC
İncreasing polarization
Polar
Protic
Aprotic
HCN
123
HCONH2
110
H2SO4
110
H2O
81
HCO2H
59
CH3OH
49
(CH3)2SO
38
CH3CN
37
(CH3)2NCHO
33
30
CH3CH2OH
25
23
(CH3)2CHOH
18
(CH3)3COH
11
7
CH3COOH
Apolar
[(CH3)2N]3PO
(CH3)2CO
Tetrahydrofuran (THF)
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4.3
(CH3CH2)2O
2.3
C6H6
2
CCl4
1.8
n-C5H12
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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• Because bond breaking requires energy, bond dissociation
energies are always positive numbers, and homolysis is always
endothermic.
• Conversely, 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.
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• Comparing bond dissociation energies is equivalent to
comparing bond strength.
• The stronger the bond, the higher its bond dissociation energy.
• Bond dissociation energies decrease down a column of the
periodic table.
• Generally, shorter bonds are stronger bonds.
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Which has the higher bond dissociation energy?
a) H-Cl or H-Br
b)
c)
H3C
(H3C)2C
OH
O
H3C
H3C
SH
OCH3
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• 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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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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Calculate H for each of the following reactions, knowing
H of O2 and O-H = 119 kcal/mol, H of C-H = 104 kcal/ml
and H of one C=O = 128 kcal/mol.
a)
CH4 + 2O2
Bonds Broken
CO2 + 2H2O
Bonds Formed
C-H = 4 x 104 kcal/mol
= 416 kcal/mol
C-O = 2 x -128 kcal/mol
= -256 kcal/mol
O-O = 2 x 119 kcal/mol
O-H = 4 x -119 kcal/mol
= -476 kcal/mol
= 238 kcal/mol
H = 416 + 238 =
+654 kcal/mol
H = -256 + -476 =
-732 kcal/mol
H = 654 + -732 kcal/mol = -78 kcal/mol
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