Lecture Outline pKas and pH

Name ______________________________________ 1 Benzoic acid (below) can be deprotonated by NaHCO3 (baking soda). Write the acid-base equilibrium for this reaction. (The following pKas may be useful, or not. Think carefully about – which one you need. H2CO3 pKa = 6; HCO3 pKa = 10) CO2H (redraw this to show the structure!) 2 In a liquid-liquid extraction, a compound partitions itself between an organic solvent, e.g. diethyl ether, "Et2O", and water, based on relative solubilities. (a) In which solvent do you expect benzoic acid to be more soluble, Et2O or H2O? (b) In which solvent do you expect sodium benzoate to be more soluble, Et2O or H2O? (c) Suppose you had a mixture of benzoic acid and ethyl benzoate (below), dissolved in ether. You want to separate them. What will you do? CO2H CO2CH2CH3 Lecture outline pKas and pH — The Henderson-Hasselbalch equation relates pKa, pH, and the concentrations of conjugate acid and conjugate base. Consider an + – acid HA dissociating to H and A . The math is at the right... Ka = – log Ka [A– ] [H+] [HA] = + – log[H ] – If pH = pKa, then log([A ]/[HA]) = 0 – – This means ([A ]/[HA]) = 1, so [A ] = [HA]. This is a good reference point. pKa = pH – log – log [A– ] [HA] [A– ] [HA] When pH is lower than pKa, which form dominates? When pH is higher than pKa, which form dominates? If you dissolve a bit of acetic acid, CH3CO2H, in an aqueous solution at pH 10, which form is present in greater concentration, acetic acid or its conjugate base? How about a bit of ammonium chloride, NH4Cl, in a solution at pH 4? Separations based on acidity/basicity — Organic chemists frequently separate compounds based on their relative solubilities in water and an immiscible organic solvent, by using a technique called liquid-liquid extraction. You will be doing this in the lab next week. For example, if we have a mixture of two compounds, say NaCl and naphthalene (C10H8), we could sepate them by dissolving them in a mixture of diethyl ether, "Et2O" and water. All of the NaCl would dissolve in the aqueous layer, and all of the naphthalene would dissolve in the Et2O layer. Separating the two liquid layers accomplishes the separation of the solutes. But what do we do if we need to separate several organic compounds that all have appreciable solubility in organic solvents? In many cases we can take advantage of acidity and basicity to modify the solubility of one or more compounds in the mixture and accomplish the separation. For example, how would you use an extraction to separate benzoic acid (Ph–CO2H) and biphenyl (Ph–Ph)? ("Ph" is the standard abbreviaiton for a phenyl group — it's benzene minus an H, connected to something) Your solvents are Et2O ("ether") and water. Both compounds are far more soluble in ether than in water. How do we separate them? Illustrate the separation procedure with a flow chart. Suppose we wanted to separate aniline (Ph–NH2) from biphenyl (Ph–Ph). Write a flow chart. How about a mixture of these four compounds? For problems like this we'll normally assume that our lab is stocked with common acids and bases — aqueous HCl, aqueous NaOH, and aqueous NaHCO3. You already know most of the pKa values that you need. Some others that + may be useful are H2CO3, 6.3; Ph–OH, 10; Ph–NH3 , 4.6. O OH OH NH2 OH