Question

Examples Benzocaine pka = SON HEN CHER Cephalexin NH3 What is the most dominant species at pH 7.4? рка = -> H+ TV For each: •Give the functional group name, hybridization and acidity (Acidic, Basic or Neutral) of the circled atoms •Give the most likely basic form in water •Estimate the pka of circled atoms •Which one is the stronger acid (blue circle) 118

Answer 1

The functional group, acidity, Pk_a and hybridisation for the given two compounds are shown in the image below:

BEN ZOCAINE > Ester Chybridisation - spl) pka = 2:51 h + group Amine Chybridisation=sp3) I of NH3 group CE PHALEXIN marine pa - sulfhydryl Amine group þka = W. Vi _ H + 2 0 on carboxylic Amide group acid Acidity Molecule Hybridisation Basic Basic Amine (NH₂) Amide (G NH) cameo role (-grou) Ester (-90-) Acid Neutral sulfhydryl Rs-H) group Basu

The most dominant species at pH= 7.4 for both compounds are shown in the above image formed after removal of a proton H⁺ and the most likely basic form in water is when the molecules possess the basic ammonium ion (RNH3)⁺ in the structure which is an indication of basicity for a given molecule, thus the reactant molecules are the most likely basic forms as shown above.

Out of the two blue circled functional groups: amine group in benzocaine and carboxylic acid in cephalexin, the stronger acid is the carboxylic acid because of resonance (migration of lone pair of electrons from hydroxyl group to the carbonyl group) in the molecule of carboxylic acid which is absent for amines.