mod06lec40 - Active methylene group

Understanding Active Methylene Groups

Acidity and pKa Concepts

• The lecture introduces the concept of Active Methylene Group, focusing on its acidity and pKa values.

• An acid (AH) in water dissociates to produce H3O+ and A-, with equilibrium dictated by the stability of the conjugate base (A-).

• The stability of the conjugate base is crucial for determining acid strength; a higher concentration of A- indicates a stronger acid.

• The acidity constant (Ka) is defined as the ratio of concentrations: [H3O+][A-]/[HA], where a higher Ka signifies a stronger acid.

• pKa is introduced as the negative logarithm of Ka; thus, lower pKa values indicate stronger acids.

Comparison of Acid Strength

• A table illustrates various pKa values, starting from alkanes with an unmeasurable value (~50), moving up to carboxylic acids at around 5.

• Nitromethane's structure features a nitro group attached to CH3, making it more acidic due to resonance stabilization in its conjugate base after deprotonation.

Stability and Resonance Forms

• Nitriles are discussed next; their sp hybridization allows for resonance forms that stabilize the resulting anion upon deprotonation, making them more acidic than methane but less so than ketones.

• Sulfones are noted for their acidity with pKa values between 28 and 31; they can also form stable anions through delocalization involving electronegative oxygen atoms.

Active Methylene Groups Explained

• Active methylene groups possess strong electron-withdrawing groups that enhance acidity; diketones have significantly lower pKa (~9), compared to simple ketones (~20).

• Deprotonation leads to enolate formation, which can delocalize charge across multiple atoms, contributing to increased acidity in active methylene compounds.