AcidBase 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.
• FFAs 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