5.mm

Hydration of an Aldehyde

• Process

  • Water adds reversibly to the carbonyl group of an aldehyde.

  • Equilibrium is established between the aldehyde and its hydrate.

• Equilibrium Constant (KKK)

  • Formula:
    K=[hydrate]\[aldehyde]

  • Example (Isobutyraldehyde at 25°C):

    • K=0.5: Twice as much aldehyde as hydrate at equilibrium.

• Measuring Concentrations

  • Method:

    • Measure UV absorption of aldehyde in water.

    • Compare with UV absorption in cyclohexane (where no hydrate forms).

• Free Energy Change (ΔG\Delta GΔG)

  • Formula:
    ΔG=−RTln⁡K

    • R=8.314 J K−1mol−1

    • T=298 

  • Example (Isobutyraldehyde at 25°C):
    ΔG=−8.314×298×ln(0.5)=+1.7kJ mol−1

    • The hydrate is 1.7 kJ mol⁻¹ higher in energy than the aldehyde.

• Energy Profile Diagram

  • Shows:

    • Relative energy levels of the aldehyde and hydrate.

    • Hydrate is higher in energy than the aldehyde.