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.

Relative stabilities of carbocations

Relative stability of radicals

http://img684.imageshack.us/i/carbanions.png/

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