Thermodynamics in Organic Chemistry: Enthalpy, Entropy, and Bond Dissociation

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Enthalpy (ΔH) in Organic Chemistry

Definition and Molecular Orbitals

Enthalpy (ΔH or q) is the heat energy exchange between a chemical reaction and its surroundings. It is a central concept in understanding the energy changes that occur during bond formation and bond breaking in organic reactions.

Bond Formation: Making a bond releases energy, stabilizing the system.
Bond Breaking: Breaking a bond requires the system to absorb energy.
Molecular Orbitals (MO): When two atomic orbitals combine, they form bonding and antibonding molecular orbitals. Electrons in the bonding MO are lower in energy and stabilize the bond, while electrons in the antibonding MO are higher in energy.
Kinetic Energy: Electrons must absorb kinetic energy to overcome the stability of the bond during bond breaking.

Example: The diagram shows atomic orbitals combining to form bonding and antibonding MOs. Electrons in the bonding MO are stabilized, while those in the antibonding MO are destabilized.

Bond Cleavage: Homolytic vs. Heterolytic

Bonds can break in two primary ways:

Homolytic Cleavage: Each atom retains one electron from the bond, forming radicals.
Heterolytic Cleavage: One atom retains both electrons, forming ions.

Bond Dissociation Energy (BDE) or ΔH for bond breaking typically refers to homolytic bond cleavage.

Example: X–Y → X• + Y• (homolytic, radicals) X–Y → X⁺ + Y⁻ (heterolytic, ions)

Bond Dissociation Energy Table

BDE values indicate the energy required to break specific bonds. Higher BDE means a stronger bond.

Bonds to H	KJ/MOL	KCAL/MOL	C—C bonds	KJ/MOL	KCAL/MOL	X—X bonds	KJ/MOL	KCAL/MOL
CH₃—H	435	104	CH₃—CH₃	368	88	F—F	159	38
CH₂CH₂—H	410	98	CH₂CH₂—CH₃	356	85	Cl—Cl	243	58
CH₃—Br	293	70	CH₃—OH	381	91
CH₃—I	234	56

Additional info: Table values are typical for organic chemistry and help predict reaction energetics.

Exothermic vs. Endothermic Reactions

Most reactions involve multiple bonds breaking and forming. The overall enthalpy change determines if the reaction is exothermic or endothermic.

Exothermic Reaction: Energy gained by bonds formed exceeds energy needed for bonds broken. Products are more stable than reactants.
Endothermic Reaction: Energy needed for bonds broken exceeds stability gained by bonds formed. Products are less stable than reactants.

Example: Combustion reactions are typically exothermic.

Energy Diagrams for Exothermic vs. Endothermic Reactions

Exothermic: Products are lower in energy; energy is released as heat; ΔH is negative; temperature of surroundings increases.
Endothermic: Products are higher in energy; energy is consumed; ΔH is positive; temperature of surroundings decreases.

Additional info: Energy diagrams visually represent the enthalpy changes during reactions.

Entropy (ΔS) in Organic Chemistry

Definition and Volume Effects

Entropy (ΔS) is a measure of the disorder or randomness in a system. It increases as the number of possible arrangements (microstates) of molecules increases.

When a gas expands into a larger volume, the number of states the molecules can occupy increases, leading to greater entropy.
More volume for gas to occupy = greater entropy.

Example: Gas expanding from a closed to an open container increases entropy.

Factors Affecting Entropy

Entropy is most significantly affected by:

Number of Moles: Entropy increases when there are more moles of product than reactant (e.g., A → B + C).
Cyclic vs. Acyclic Compounds: Entropy increases when a cyclic compound becomes acyclic, as there are more possible arrangements.

Example: Conversion of cyclohexane to hexane increases entropy.