W4.2_Factors Affecting pKa

Question 1

What are the general factors affecting pKa and the principle in acids and bases.

Answer 1

• *Acid: anion and H+ on same side of equation
• *Bases: cation and H+ on different side of equation
• Acids: Better stabilisation of anion from partial neutralisation/sharing of charge -> shift eqm to the right -> higher [H+] -> stronger
• Bases: Better stabilisation of cation/worse stabilisation of molecule from partial neutralisation/sharing of charge -> shift eqm to the left -> lower [H+] -> stronger

Question 2

What are the factors that affect pKa in acids?

Answer 2

• Higher electronegativity of atom carrying the charge
• More nearby electron withdrawing groups (halogen/OX/NX2)
• More resonance
• Both of above two reasons combined
• Character of orbital carrying the charge (more s character -> closer to nucleus -> more stable, thus sp2>sp3)

Question 3

What are the factors that affect pKa in alkali?

Answer 3

• Lower electronegativity of atom carrying the charge
• Absence/increased distance from electron withdrawing groups
• Absence of resonance
• Character of orbital carrying the charge (more s character -> closer to nucleus -> less stable, thus sp3>sp2)
• Availability of lone pair of electrons/electron releasing groups like cyclohexane ring

Question 4

Explain how the value of pKa can be infered in determining acids or bases. In compounds with multiple pKas, contrast zwitterion and unionsed environment. Explain the different graphs illustrating the changes of compounds in different pH.

Answer 4

• pKa 0-5: strong acids/weak bases
• 5-9: infer from structure
• 9-14: weak acids/strong bases
• Multiple pKas (caused by multiple ionisable environments)
• Zwitterion: molecules with positive and negative charges -> no net charge
• Unionised: no charge
• Graphs: percentage of different species, overall net charge across pH spectrum