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Organic Chemistry Chapter 6: Chemical Reactivity and Mechanisms

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Chapter 6: Chemical Reactivity and Mechanisms

Enthalpy

Enthalpy (H) is a thermodynamic quantity that measures the total heat content of a system. In organic chemistry, it is often used to describe the energy changes associated with making or breaking chemical bonds.

  • Bond Formation: Releases energy (exothermic).

  • Bond Breaking: Requires energy (endothermic), usually considered in homolytic or heterolytic cleavage.

  • Homolytic Cleavage: Each atom retains one electron, forming radicals.

  • Heterolytic Cleavage: One atom retains both electrons, forming ions.

  • Enthalpy Change ():

Example: The reaction of an alkane with bromine () to form a bromoalkane involves bond breaking and formation, and the enthalpy change can be calculated using bond dissociation energies.

Bond Dissociation Energies

Bond dissociation energy is the energy required to break a specific bond in a molecule homolytically.

Bonds to H

Kcal/mol

Bonds to methyl

Kcal/mol

X–X bonds

Kcal/mol

H–H

104

CH3–H

105

Br–Br

46

H–CH2CH3

98

CH3–Br

70

Cl–Cl

58

H–C6H5

113

CH3–Cl

84

I–I

36

H–OH

111

CH3–OH

95

F–F

38

H–C≡C–H

125

CH3–F

110

O–O

35

C–C

85

CH3–CH3

88

O–H

95

C=C

146

CH3–CH2CH3

84

Additional info: Table includes more bonds and energies relevant for calculations.

Entropy

Entropy (S) is a measure of the randomness, disorder, or freedom of motion in a system. It is a key factor in determining the spontaneity of chemical reactions.

  • Positive Entropy Change (): More disorder, such as gas expansion or increased molecular freedom.

  • Negative Entropy Change (): Less disorder, such as when molecules become more ordered.

  • Boltzmann Equation: where is the number of microstates.

Example: Expansion of a gas from a closed to an open container increases entropy.

Gibbs Free Energy

Gibbs Free Energy (G) combines enthalpy and entropy to predict whether a reaction is spontaneous.

  • Equation:

  • Spontaneous Reaction: (exergonic)

  • Non-spontaneous Reaction: (endergonic)

  • Enthalpy and Entropy Contributions: ,

Example: The conversion of reactants to products with a negative is spontaneous.

Equilibria

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products.

  • Equilibrium Constant (): for the reaction

  • Relationship to Free Energy:

  • R (Gas Constant): kcal/(mol·K)

(kcal/mol)

K

Conversion to Products (%)

+1.36

0.1

9%

0

1

50%

-1.36

10

91%

-2.72

100

99%

-4.08

1000

99.9%

-5.44

10,000

99.99%

-6.8

100,000

99.999%

-8.16

1,000,000

99.9999%

-9.52

10,000,000

99.99999%

-13.6

3,200,000,000

~100%

Example: A large negative results in almost complete conversion to products.

Kinetics

Kinetics is the study of the rates and mechanisms of chemical reactions. It focuses on how quickly reactions proceed and the factors that influence reaction rates.

  • Rate Law:

  • Order of Reaction:

  • Rate Constant (): Depends on temperature and activation energy.

  • Activation Energy (): The minimum energy required for a reaction to occur.

  • Effect of Temperature: Higher temperature increases the fraction of molecules with sufficient energy to react.

Example: A reaction with a lower proceeds faster than one with a higher .

*Additional info: These notes are based on lecture slides and include inferred academic context for completeness.*

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