Acid-Base Catalyzed Reactions

1. Acid-Base Catalyzed Reactions • In most reactions that are acid- or base- catalyzed, there is no reaction at all in the absence of at least a trace of catalyst. • sequences involve proton transfer reactions between catalyst and reagent(s) • Experiment shows: K1 k2 log kobs slope = 1 constant -pH

2. Definitions of Acids and Bases • Bronsted-Lowry Definition: According to this theory, an acid is a proton donor and a base a proton acceptor: • Lewis Definition: A more general definition of acids and bases refers to the capability of coordinating with unshared electron pairs • Lewis acids have a vacant orbital which permits coordination of molecules with unshared electron pairs • Lewis bases have unshared electron pairs available for donation.

3. Review of Bronsted Acidity and Basicity • The relative strengths of acids are determined by how well they transfer a proton to a standard base (traditionally water): • The equilibrium constant for transfer of a proton from an acid HA to water is called the dissociation constant, Ka: • in dilute • aqueous solution • Due to large variations in Ka for different compounds, the log form is commonly employed:

4. Relative Strengths of Some Acids and Bases

5. Biodiesel Production • Transesterification of natural oils yields lower molecular weight esters of appropriate viscosity for fuel applications. • Triglyceride transesterification with methanol can be acid or base catalyzed. • Free fatty acids within some feedstocks can only be esterified under acidic conditions. • Carboxylate salts create foaming problems, and adversely affect product viscosities

6. Biodiesel Production • The Biox plant in Hamilton, Ontario has a nameplate capacity of 67 million litres per year for converting a wide range of feedstocks into clear, acid-free biodiesel.

7. Biodiesel Production • Patented U of T process uses THF to yield a miscible, homogeneous catalytic process. • FFA’s in the feedstock are esterified under acidic conditions, then the whole mixture is transesterified under more efficient base catalysis. • Equilibrium is driven by excess methanol, and the phase-separation of glycerol from the reaction mixture

8. Acid Catalysis in Concentrated Solutions • Protonation of the substrate is most often the first step of acid-catalyzed reactions. • The extent to which the substrate is protonated influences the reaction rate, making the prospect of increasing [H3O+] quite attractive.

9. Standard Measure of Acidity - pH • The most familiar measure of the acidity (tendency to protonate a base) of a solution is pH: • For the protonation of a base B, we are interested in this equilibrium: • We relate the extent of reaction to the pH through the acid dissociation constant, Ka: • or • For concentrated solutions of strong acids, we find two problems: • Measuring the pKa of strong acids with respect to H2O protonation. • Accounting for the non-ideality of concentrated acid solutions that are much more “acidic” than their pH would suggest.

10. Hammett Indicators • Given that we are interested in knowing the extent to which our substrate (S) is protonated (SH+) in a given acid solution, a relevant question is: • For a range of acidic solutions (0 mol% H2SO4 to anhydrous H2SO4), to what extent is a neutral base protonated? • Hammett and coworkers have addressed this issue by measuring the tendency of an acidic solution to protonate various neutral bases, called Hammett Indicators. For example, • The concentrations of BH+ (nitroanilinium ion) and B (nitroaniline) can be measured by a spectrophotometric technique.

11. Hammett Acidity Function • The Ka for this reaction is known: • Taking logs yields: • We define a new parameter, Ho, the Hammett Acidity Function: • which reduces our equilibrium relationship to:

12. Hammett Acidity Function • To a series of acid solutions of varying concentration, an indicator of known pKa is added, and the ratio of [B] to [BH+] measured. • The Hammett acidity function is easily calculated for each solution by: • The acidity function accounts for solution non-ideality by lumping the activity of H+ (an essentially indeterminable quantity) with the activity coefficients gB and gBH+. • In dilute solutions, aH+[H+], gB1 and gBH+1, leaving

13. Hammett Acidity Function • The acidity function is by no means a universal indication of the tendency of an acid solution to protonate a base. • gB and gBH+ relate to the Hammett indicator, and may not relate to your substrate. • H0 measures the tendency of a solution to protonate a neutral base, not to a base of any other electrical charge. • Other Hammett measures (H-, derived from p-toluene sulfonate) can be used for anionic substrates.

14. Ho and Acid Catalyzed Reaction Kinetics • For those acid catalyzed reactions in which protonation of a neutral substrate is a kinetically significant step, there may exist a relationship between the reaction rate and the acidity function. • Consider a reaction proceeding by the following mechanism: • r1: • r2: • where reaction 2 is rate limiting: • and reaction1 is at equilibrium:

15. Ho and Acid Catalyzed Reaction Kinetics • If we assume that our neutral substrate has great chemical similarity to the Hammett indicator used to determine Ho of the acid: • The rate of the reaction becomes: • The observed rate constant for the reaction, • relates to the acidity function according to:

16. Ho and Acid Catalyzed Reaction Kinetics Compare this to the dilute solution example summarized in Slide 1