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Addition and elimination reactions are exactly opposite. A bond is formed in elimination reactions, whereas a bond is broken in addition reactions.
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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 half-headed 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 two-
electron 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
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Relative stability of radicals
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Dielectric constant (Debye), 25 oC
Polar Protic Aprotic
İncreasing
polarization
HCN 123
HCONH2 110
H2SO4 110
H2O 81
HCO2H 59
49 (CH3)2SO
38 CH3CN
37 (CH3)2NCHO
CH3OH 33
30 [(CH3)2N]3PO
CH3CH2OH 25
23 (CH3)2CO
(CH3)2CHOH 18
(CH3)3COH 11
7 Tetrahydrofuran (THF)
CH3COOH 6
4.3 (CH3CH2)2O
2.3 C6H6
2 CCl4
Apolar 1.8 n-C5H12
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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)H3C OH H3C SH
(H3C)2C O H3C OCH3c)
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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
CO2 + H2O2
Bonds Broken
C-H = 4 x 104 kcal/mol = 416 kcal/mol
O-O = 2 x 119 kcal/mol
= 238 kcal/mol
H = 416 + 238 = +654 kcal/mol
Bonds Formed
C-O = 2 x -128 kcal/mol = -256 kcal/mol
O-H = 4 x -119 kcal/mol = -476 kcal/mol
H = -256 + -476 = -732 kcal/mol
H = 654 + -732 kcal/mol = -78 kcal/mol