Proton Transfer Reactions Bronsted-lowry Theory Chemistry Lab Report

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In this module you learned about contaminant distribution between air-water, organic-aquatic and organic acid/base-aquatic phases. For the module discussion, pose a question that you have about one of the topics.

Use this as an opportunity to extend your knowledge or to clarify a concept. You will be assessed on a) how well you are able to put the question in context, b) your answer to this question, and c) your replies to others.

  • Make an Initial Post: Provide the question and provide a satisfactory answer citing relevant literature. This should not be a simple yes or no question. Initial posts should be at least 300 words.


Module Reading material from Chapter 10 of Environmental Organic Chemistry. Rene P. Schwarzenbach, Philip M. Gschwend, and Dieter M. Imboden. 3rd Edition. Wiley Interscience (2016). ISBN: 978-1-118-76723-8.

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CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING 5.0 ADK 1 ORGANIC SOLVENT-WATER PARTTIONING  In this section we are concerned with partitioning of organic compounds between two immiscible liquids.  This type of equilibrium is particularly useful when considering distribution of nonpolar organics between water and natural solids (i.e. soils, sediments, biosolids etc.).  Substantial information exists in pharmacology studies of nonpolar drug uptake by organisms and cellular material.  n-Octanol has been used traditionally as a surrogate for organic matter.  Kow values of organic compounds may differ by as much as 10 order of magnitudes (see Fig. 1)  Partitioning between an organic solvent and water is described by the organic solvent-water distribution coefficient defined as: Kosw  5.1 Cos mol / lorg. solvent C w mol / lwater [1] THERMODYNAMIC BASIS  EXPERIMENT: Consider the following experiment (see Fig. 2):  Add an immiscible solvent (i.e. n-octanol) in water.  Mix the two liquids well.  Allow some time for the system to reach equilibrium and for the phases to separate.  Add a known amount of a compound in the aqueous phase and allow the system to reach equilibrium. CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 2 CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING Figure 2: Partitioning of a Chemical Compound between Immiscible Organic Solvent-Water Phases ADK 3 CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 4 OBSERVATIONS:  It is obvious that some water will dissolve in octanol ( 1 molecule H2O/4 molecules octanol) and some octanol will dissolve in water ( 8 molecules octanol/100,000 molecules H2O).  The molar volume of the solvent may be altered if appreciable amounts of water dissolve in the organic solvent. For example, the molar volume of pure n-octanol Vos = 0.16 l/mol whereas the molar volume of octanol saturated with water is 0.12 l/mol.  The molar volume effect is less pronounced for highly nonpolar solvents.  The compound dissolved in the aqueous phase will gradually dissolve in the organic solvent phase until equilibrium is reached.  At equilibrium the fugacities of the organic compound in the aqueous and organic solvent phase will be equal: f os  f w [2]  os x os   w x w [3]  In terms of molar concentrations:  osCosVos   wCwVw K osw  [4] C os  wV w  C w  osVos ln K osw  ln  w  ln  os  ln [5] Vw Vos e Gosw   cons tan t RT [6] CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 5  COMMENTS (on Eq. [6]):  Obviously the partition constant is related to the partial molar excess free energy of the compound in the two phases (RTln) expressing the difference between the nonideality of the solution of the compound in the two phases  For nonpolar compounds w is quite large (102-1011) due to the cost of “cavity” formation and thus w dominates the magnitude of Kosw for nonpolar compounds.  Since the organic solvent (os) is likely to be more chemically related to the organic compound (oc) dissolved, the os-oc molecule interactions are not dominant thus os = pure compound = 1.  ASSUMPTIONS:  The activity coefficient of the partitioning compound is independent of its concentration in the aqueous phase.  Organic solvent molecules present in the aqueous solution have no effect on the activity coefficient of the compound. In other words os-oc molecule interactions in the aqueous phase are neglected.  Thus the term w can be substituted by wsat: Kosw  wsatVw 1 1 1   sat    sVs Cw  s Vs [7]  CONCLUSIONS: From the above relationship and values of Table 1 we can conclude that:  Nonpolar compounds of low water solubility strongly favor the organic over the aqueous phase.  Nonpolar organic compounds feel equally comfortable in the highly nonpolar n-hexane and in the more polar n-octanol, as CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 6 shown by the similar values of the partition constants (benzene, toluene etc.). Also note that nonpolar solvents have molar volumes close to that of n-octanol (0.12 l/mol).  Polar substances (with oxygen or nitrogen functional groups) feel more comfortable in octanol (-OH moieties create a friendly environment for polar groups) than in nonpolar hexane. Note the similar values of activity coefficients for benzene and water in octanol (2.7, 3.6) and their difference in hexane (2.0, 2600) CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 7 Table 1: n-Octanol-Water and n-Hexane-Water Partition Coefficients1 1 2 Partitioning Compound Kow Khw Cwsat mol/l o h n-Hexane 13,000 52,000 1.510-4 4.4 1.0 Benzene 130 170 2.310-2 2.7 2.0 Toluene 490 560 5.610-3 3.0 2.4 Chlorobenzene 830 810 4.510-3 2.2 2.1 Naphthalene 2,300 2,400 8.710-4 4.2 3.7 Benzaldehyde 30 13 3.110-2 8.9 19.0 Nitrobenzene 68 29 1.710-2 7.3 16.0 1-Hexanol 34 2.8 1.310-1 1.9 21.0 Aniline 7.9 0.8 3.910-1 2.7 25.0 Phenol 28 0.1 8.910-1 0.32 61.0 Water 0.04 0.00005 5.510-1 3.6 2600 Schwarzenbach R. P. et al., “Envronmental Organic Chemistry”, Wiley Interscience, 1993. Value probably incorrect since wsat w because of intermolecular interactions of the phenol species at saturation concentrations CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING 5.2 ADK 8 EXPERIMENTAL DETERMINATION OF Kow  “Shaker Flask” Method: The method yields direct measurements of Kow. The procedure is similar to that described in section 4.1 above. The method however is only applicable to compounds of low-tomedium hydrophobicity (Kosw < 105). Data for more hydrophobic compounds are very limited. 5.3 Kow ESTIMATION TECHNIQUES  Aqueous Solubility: The octanol-water partition coefficient has been related to the aqueous solubility of organic compounds by empirical relationships such as the following equation: log K ow   log C wsat  log  o  log Vo [8]  The above equation is based on the following assumptions:  o is set equal to 1, 10, 100, 1000 (diagonal lines in Figure 3).  the activity coefficient of the compound is independent of concentration in both phases  the influence of octanol presence in water on w is negligible thus: w  1 C wsatVw the concentration of the compound is low enough so that the molar volume of water-saturated octanol is not affected (Vo  0.12 l/mol)  The deviations between the -lines and the actual experimental points in Fig. 2 represent estimates of log for each compound.  From the experimental data plotted on Fig. 2 we conclude that for most liquid compounds having water solubility greater than about 10-6 mol/l CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 9 the estimated values of  are between 1 and 10, indicating nearly ideal solution behavior of these compounds in water-saturated octanol.  If all else fails estimate Kow using web-based programs  Virtual Computational Chemistry Labs ALOGPS Program  Syracuse Research Corporation Interactive LogKow (KowWin) Demo CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 10 CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 11 EXAMPLE 1 10-6 mole of lindane is added to 100 mL separatory funnel containing 10 mL of octanol and 90 mL of water. Determine the concentration (mg/l) of lindane at equilibrium and 25C. Kow of lindane is 6.025103 IUPAC name (1r,2R,3S,4r,5R,6S)-1,2,3,4,5,6hexachlorocyclohexane Formula Mol. mass Chemical data C6H6Cl6 290.83 g/mol Production and agricultural use is banned the Legal 169 countries that parties to the Stockholm status Convention, but pharmaceutical use is allowed until 2015. SOLUTION 1. Mass balance for lindane nT  no  nw  106 mol 2. The fractional distribution of lindane in the aquatic and organic phases can be defined as: fw  nw n moles in water   w6 moles in octanol  moles in water no  nw 10 fw  CwVw CoVo  CwVw 3. From the definition of Kow: K ow  Co Cw Co  K owCw 4. Combination of the two equations above gives: CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING fw  CwVw K owCwVo  CwVw fw  Vw K owVo  Vw fw  90ml  0.0015 6.025  103 10ml   90ml ADK 12 5.Determine the number of lindane moles in water nw  f wnT  0.0015  106 mol nw  1.5  109 mol 6. Determine the molar concentration of lindane in water: Cw  nw 1.5  109 mol   1.667  108 mol / l 3 Vw 90ml  10 l / ml      Cw  1.667  108 mol / l 290.83  103 mg / mol  4.848  10 3 mg / l  0.5 ppm EXAMPLE 2 Use web-based programs to estimate Kow for the following compounds 1. caffeine 2. theobromine 3. androstenedione 4. testosterone SOLUTION 1. Google caffeine 2. Go to the Wikipedia entry and pickup CAS number for caffeine: 58-08-2 3. Go to the Virtual Computational Chemistry Labs ALOGPS Program by following this link 4. On the first line of the dialog box key in the CAS number for caffeine 58-08-2 and click the button submit on the right hand corner. Allow the program to run calculations and voila: 5. It returns a logKow = 0.16 CONTAMINANT FATE AND TRANSPORT ORGANIC SOLVENT-WATER PARTITIONING ADK 13 6. Similarly go to the Syracuse Research Corporation Interactive LogKow (KowWin) Demo enter the CAS number and get 7. This returns a logKow = 0.16 8. Repeat for the other compounds. CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING 4.0  ADK 1 AIR-WATER PARTITIONING This section discusses equilibrium partitioning of organic chemicals between the gas phase and an aqueous solution.  For neutral compounds at dilute solute concentrations in pure water the air-water distribution ratio is referred to as the Henry’s Law constant.  Henry’s Law constant (KH) represents a measure of the relative abundance of a chemical in the gas phase (partial pressure Pi) to that in the aqueous phase (molar concentration, Cw), thus: KH   Pi atm  l / mol Cw [1] If the abundance of a compound in the gaseous phase is expressed as  mol/lair (Ca) then a dimensionless Henry’s Law constant, K H is defined:  C K H  a mol / l air / mol / l w Cw  [2]  K H and KH are related by applying the ideal gas law to convert partial pressure ( Pi   n i V  RT ):  K KH  H RT  [3] The air-water partition coefficient quantifies the relative escaping tendency of a compound to exist as vapor molecules rather than in the water solution.  Compounds with high vapor pressures (low fugacity in the gas phase) and high activity coefficient in water (high fugacity in aqueous solution), partition is favorable for the gaseous phase. (see Fig. 1) CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 2 CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING 4.1  ADK 3 THERMODYNAMIC BACKGROUND The fugacity of a compound in an ideal gas and in aqueous solution has been defined as: f g  Pi fw   w xw P 0 ( l ) [4] where Pi = partial pressure of compound Po(l) = vapor pressure of the pure liquid compound  At equilibrium: Pi   w x w P 0 ( l )  [5] After expressing the concentration of the compound in the liquid phase as C w  x w Vw , the partition constant is defined as: 0 Pi  w x w P  l  KH     wVw P 0  l  [6] Cw x w Vw  The above equation shows that KH is directly proportional to both the activity coefficient of the compound in water (w) and the vapor pressure of the pure organic compound [P0(l)]. 4.2  EFFECT OF CONCENTRATION ON KH As the concentration of the compound in the liquid phase increases towards saturation is KH affected?  Suppose that a pure compound in its gaseous phase comes into contact with pure water. If the system is allowed to reach equilibrium, then the fugacity of the compound in the gas phase is equal to its vapor pressure thus: P 0   w x w P 0  l  [7] Also at equilibrium the liquid phase will be saturated with the compound, thus: CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING x  sat w  1  sat w P0  0 P  l [9] If the natural state of the compound is gas then: P 0  1 atm  [8] Note that if the natural state of the compound is liquid then: P 0  P 0  l  ADK 4 [10] Thus it can be stated that the equilibrium partial pressure in air above a saturated aqueous solution of a compound is equal to the vapor pressure of the pure compound at the same temperature.  Assuming that the molar volume of the aqueous solution is not altered considerable by the organic compound, under aqueous phase saturation conditions the distribution ratio is: K Hsat   P0   sat Vw P 0  l  w sat Cw [11] So the question of how different KH is from Khsat comes down to how w differs from wsat.  Research indicates that for several compounds studied (benzene, toluene, chlorinated solvents, organosulfur compounds) up to 3% molar solutions solute-solute interactions are negligible thus KH is not affected by concentration. 4.3  EFFECT OF TEMPERATURE Assuming that over small temperature ranges:  the molar volume of water remains constant  vapor pressure dependence on temperature is given by the equation: CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ln P 0    H vap RT for a liquid compound that  ADK 5  cons tan t sat w  [12] 1 the temperature dependence x wsat is: ln   sat w   [13] Thus the temperature dependence of KHsat is given by the equation: ln K H  ln K 4.4 H se   cons tan t RT sat H  H vap  H es      cons tan t RT   H Henry RT [14]  cons tan t EFFECT OF SALTS Dissolved salts affect the activity coefficient in aqueous solutions but not the fugacity of the compound in the gaseous phase. Thus air-water partitioning varies directly with changes in w.  In general values of the air-water partition coefficient are greater in seawater than in pure water (see Table 1) CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 6 Table 11 Dimensionless Henry’s Constants for Distilled Water and Seawater Compound KH ’ KH’ KH’(sw)/ (dist. water) (seawater) KH’(dw) CCl3F 3.6 5.0 1.4 CCl4 0.98 1.5 1.5 CH3CCl3 0.53 0.94 1.8 Hexachlorobenzene 0.054 0.07 1.3 2,4’-Dichlorobiphenyl 0.00713 0.079 1.4 2,4,4’-Trichlorobiphenyl 0.00595 0.00885 1.5 2,5,3’,4’-Tetrachlorobiphenyl 0.00357 0.00461 1.3 Dimethyl sulfide 0.075 0.089 1.2 Thiophene 0.095 0.11 1.2 1 Cited by Schwarzenbach R. P. et al. “Environmental Organic Chemistry”, Wiley, (1996). CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 7 EXAMPLE 1 (Air-Water Partitioning) The atmospheric concentration of the herbicide atrazine is 75.9 ng/m3, what is the partial pressure of atrazine in air at 27.5 oC? Atrazine, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine Properties C8H14ClN5 215.68 g mol−1 colorless solid 1.187 gcm−3 Molecular formula Molar mass Appearance Density Melting point 175 °C Boiling point 200 °C Solubility in water 7 mg/100 mL SOLUTION 1. Convert atrazine concentration to mol/l: 75.9ng / m3 C  109 g / ng  103 m3 / l  3.52  1013 mol / l 215.68 g / mol     2. Apply the ideal gas law: Pi  CRT  3.52  1013 mol / l 0.0821atm  l / mol  K 300.5 K   8.68  1012 atm   EXAMPLE 2 The Henry's law constant for atrazine at 25 oC is 6.2 X 10-6 atm-L/mol, estimate the air/water partition coefficient, KAW, for atrazine at 25 oC. SOLUTION KAW is the dimensionless Henry’s constant thus: KH 6.2  106 atm  l / mol  K AW  K H    2.53  10 7 RT 0.0821atm  l / mol  K 298 K  CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 8 EXAMPLE 3 Estimate the Henry's law constant for the herbicide metolalchlor (FW = 283.8 g/mol; mp < 25 oC; Sw = 530 mg/L; Pol = 1.7 X 10-8 atm). IUPAC name (RS)-2-Chloro-N-(2-ethyl-6-methyl-phenyl)-N-(1methoxypropan-2-yl)acetamide Empirical names Dual, Pimagram, Bicep, CGA-24705, Pennant. Molecular formula Molar mass Appearance Properties C15H22ClNO2 283.79 g mol−1 Off-white to colorless liquid Boiling point 100 °C at 0.001 mmHg Solubility in water 530 ppm at 20 °C SOLUTION 1. Convert aquatic solubility on a molar basis: 530mg / l  103 g / mg  1.87  103 mol / l Cwsat  283.8 g / mol 2. Estimate KH from solubility and vapor pressure: Po 1.7  108 atm K H  lsat   9.1  10 6 atm  l / mol 3 Cw 1.87  10 mol / l   CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 9 EXAMPLE 4 200 mL of water containing 10-10 mol of compound X dissolved is added into a sealed container with 9.5 L of headspace. The KH of X is 0.369 atm-L/mol. What is the final aquatic concentration of X in the container and in the headspace at 25 oC after equilibrium is established? NOTE: Ignore water evaporation. SOLUTION 1. Determine the initial molar concentration of compound X in the aqueous phase: 1010 mol i Cw   5  1010 mol / l 0.2l 2. Write a mass balance for X after equilibrium is established: Cwi Vw  Cwf Vw  CaVa [1] where Cwf  molar concentration of X in the aqueous phase at equilibrium, mol/l Ca = molar concentration of X in the air phase at equilibrium, mol/l Vw = volume of water, l Va = volume of headspace, l 3. Calculate the dimensionless Henry’s constant: 0.369atm  l / mol K C [2] K H  H   0.015  af RT 0.0821atm  l / mol  K 298K  Cw 4. Solve Eq. [2] for Ca, substitute in Eq. [2] and solve Cwf   Cwi Vw 5  1010 mol / l 0.2l    2.92  1010 mol / l K H Va  Vw 0.015  9.5l  0.2l 5. Determine Ca Ca  0.015Cwf  0.015 2.92  1010 mol / l  4.38  1012 mol / l Cwf    CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING 4.5 ADK 10 ESTIMATION METHODS OF KH  Most data in the literature are based on estimation methods  The most common estimation method involves calculation of KH from capor pressure and solubility data.  Table 2 shows experimental and estimated Henry’s constants.  An alternative method has been proposed by Hine and Mookerjee2 based on structural contributions.  The underlying idea of the method is that every subunit of organic compounds has a constant effect on air-water partitioning regardless of the compound in which it occurs.  Table 3 shows the various subunit contributions.  Calculation of Henry’s constant is based on linear algebraic equations of the following form: log K H  a 1 subunits of type a 1  a 2 subunits of type a 2  ... [16] EXAMPLE 5 Estimate KH for bromodichloromethane and phenol using the Hine and Mookerjee method. SOLUTION 1. For bromodichloromethane (CHBrCl2) we have the bollowing subunit contributions: C-H = +0.11 C-Br = -0.87 C-Cl = -0.30. Thus: log K H  1 0.11  1 0.87  2 0.30  1.36 K  K  2 H estimated H observed  0.044  0.085 Hine, J. and Mookerjee P. K., “The intrinsic hydrophilic character of organic compounds. Correlations in terms of structural contributions”, J. Org. Chem., 40, 292-298, 1975. CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 11 CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 12 CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING ADK 13 2. For Phenol we have the following contributions: Car=Car = -0.33 Car-H = +0.21 Car-O = +0.74 O-H = -3.21 Thus the overall contributions are: log K H  6 0.33  5 0.21  1 0.74  1 3.21  3.40 K  K  4.6  H estimated H estimated  0.00040  0.00041 based on estimation from P 0 and S data EXPERIMENTAL DETERMINATION OF KH The following is the classical method used:  Equal volumes of a solution (eg. toluene) and nitrogen gas are introduced in a syring  Vigorously shake syring to establish gas-water equilibrium  Determine the concentration of the compound of interest in both the gaseous and aqueous phase   Refill with nitrogen and repeat the above procedure The fraction of the compound remaining each time in the aqueous phase is: Fraction In Water = C wVw C gVg  C wVw [17] where Cg = concentration of compound in gas, mol/l Vg = volume of gas, l Cw = conc. of the compound in aqueous phase, mol/l Vw = volume of water, l  Using the dimensionless gas-water partition ratio Dgw = Cg/Cw the above equation becomes: Fraction in Water = Vw DgwVg  Vw [18] CONTAMINANT FATE & TRANSPORT AIR-WATER PARTITIONING  For pure water Dgw = KH’. ADK 14 The concentration of the compound measured in the gas after the nth equilibration is: C g ,n  Dgw  fraction in water  C w ,0 n   Vw log C g ,n  n  log    log C w ,0 Dgw   D V V gw g w    [19] A plot of logCg,n vs. number of successive equilibrations has a slope of   Vw log   and an intercept of log C w ,0 Dgw  . Since Vg and Vw  D V V gw g w   are known the value of Dgw can be computed. CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES 6.0 ADK 1 PARTITIONING BEHAVIOR OF ORGANIC ACIDS/BASES  The discussion up to this point was limited to neutral compounds. The question is: “what happens when charged species are involved?”  Many organic chemicals undergo proton transfer reactions resulting in formation of charged species, cations or anions.  Charged species may exhibit different properties and reactivity compared to their parent neutral compounds.  A proton transfer reaction occurs when an acid (proton donor, HA) reacts with a base (proton acceptor, B) as shown below: HA  A   H  H   B  BH  [1]  HA B   BH A       Acid Base Conjugate Conjugate Acid of B Base of HA  Proton transfer reactions are fast and reversible, and are treated as equilibrium processes. 6.1 ORGANIC ACIDS  Consider the reaction of an organic acid with water acting as a base: HA  H 2 O  H 3O   A  [2]  For dilute solutions the infinite dilution state is used as a reference state thus for water the pure liquid reference state is appropriate: H   H 2 O  H 3 O  K = 1 or G 0  0 [3]  Taking into account Eq. 3, Eq. 2 can be rewritten as: HA  H   A   Thus at equilibrium: [4] CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES    H    A   G  ln   ln K a ADK 2 H  A    HA 0 RT HA 0 with G 0    HA   A0   H0 but by convention  H0  0 thus :    0 G 0    HA   A0   Thus the equilibrium or dissociation constant is defined as:   H    A    Ka H  A [5]  HA  HA  For dilute aqueous solutions of moderate to low ionic strength:  A   HA   1, thus:   A   log K log   HA a     pH  pK  log  H H   a [6] where pK a   log K a  pKa is a measure of the relative strength of an organic acid compared to the H3O+/H2O pair. Low pKa’s signify a strong organic acid (see Tables 1-2).  Also when pKa = pH we have equal amounts of the dissociated (A-) and undissociated (HA) species present.  A   HA at pH = pK  6.2 [7] a ORGANIC BASES  Similarly to acids the base dissociation constant is defined for the reaction of a base with water: B  H 2 O  OH   BH     OH    BH     Kb OH   BH     B B [8] CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 3 CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 4 CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 5  For the sake of comparison based on a uniform scale, it is convenient to consider the dissociation constant of the conjugate acid BH+: BH   H   B    H     B      BH   Ka H [9] B  BH   The two dissociation constants are related through the autodissociation constant of water (*at 25 oC):    K w  K a K b   H  H    OH   OH    1.01  10 14   [10]  Or using the p-notation: pK a  pK w  pK b [12]  In summary:  The stronger an acid is (low pKa), the weaker the basicity of its conjugate base (high pKb).  The stronger a base is (low pKb), the weaker its conjugate acid (high pKa). 6.3 ORGANIC ACID/BASE FUNCTIONAL GROUPS  Certain functional groups present in the molecule of organic compounds contribute proton donor/acceptor properties (see Fig. 1).  In the pKa range of 3-11 the most important functional groups are:  hydroxyl groups bound to aromatic rings  aliphatic and aromatic carboxyl groups  aliphatic and aromatic amino groups  nitrogen bound to aromatic rings (pyridines  aliphatic or aromatic thiols CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 6 CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 7  Also note that organic compounds may contain more that one acid/base groups.  Aminoacids (building blocks of proteins) possess both acid and base functional groups and proton transfer may follow one of two pathways: R-CH-COOH K a1 NH3  R-CH-COOH NH2 (neutral species) K a2 R-CH-COO K a1 NH3  R-CH-COO R-CH-COOH NH3 NH2 R-CH-COO K a2 NH2 (“zwitterion”)  Although four acidity constants are defined above only two of constants can be determined experimentally: K a 1  K a1  K a1 Ka2  K a2 K a2 K a2  K a2 [13]  For compounds with acid and base functional groups (e.g. amino-acids, hydroxy-isoquiolines) there exists a pH value for which the average net cherge of all species is zero, called isoelectric point, defined as: pH isoelectric  1  pK a1  pK a 2  2 [14]  Speciation of organic acids/bases at a given pH is expressed by the distribution ratio defined as: CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES  HA  1  HA   A  1   A   HA     1   pH  pK 1  10 BH    1  a    BH     B 1   B   BH    a  6.4 ADK 8   a  [15] EFFECT OF CHEMICAL STRUCTURE ON Ka  Inductive Effects: The relative position of electronegative groups with respect to the acid function group affects Ka  More specifically the closer the electronegative group to the acid function group the lower the pKa.  This is a result of a more stable electron distribution in the molecule that makes the ionized species more stable thus lowering its pKa.  See example of chlorobutyric acid in Table 3.  Delocalization Effects: Delocalization of electrons observed in conpounds with mobile -bonds leads to stability and decrease of pKa of a given functional group  For example compare an aliphatic alcohol with a phenol see Table 4.  Proximity Effects: Presence of substituents in the proximity of acid/base functional groups may lead to formation of intramolecular hydrogen bonds resulting in additional stability and consequently lowering of pKa. (see Figure 2). CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 9 Table 3: Negative Inductive Effect on pKa for Chlorobutyric Acid Compound pKa CH3CH2CH2COOH 4.81 CH2CH2CH2COOH 4.52 Cl 4.05 CH3CHCH2COOH Cl 2.86 CH3CH2CHCOOH Cl Table 4: Delocalization Effects on pKa for Aliphatic Alcohol and Phenol Compound pKa R CH2  OH R CH2  O > 14 OH O - O 9.92 O O Figure 2: Proximity Effects in para- and ortho- Hydroxybenzoic Acid CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES 6.5 ADK 10 DETERMINATION OF Ka  Experimental Determination: There is a large data base of pKa values for organic acids and bases1.  The most common procedure icludes titration and determination of the concentration ratio of acid-base pairs at various pH values using conductance, electrometric and spectrophotometric methods.  Estimation: Estimation is based on linear free energy relationships (LFERs) to quantify the effects of structural entities (moities) on pKa. The procedure is tedious. 6.6 AQUEOUS SOLUBILITY  The water solubility of the ionic form of organic acids and bases is generally several orders of magnitude higher than the solubility of the neutral species ( C wsat  HA .  The total concentration ( C wsat,total ) of the compound (sum of the dissociated and undissociated forms) is strongly pH-dependent. At low pHs the total concentration equals the concentration of the undissociated species HA. However, at higher pHs, C wsat,total is given by: C wsat,total  C wsat a [16]  Eq. 16 is valid up to the solubility product of the salt(s) possibly formed due to presence of counterions.  For organic bases the equation is: C sat w ,total C wsat  B   1 a [17] CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 11  Solubility data for organic acids/bases are extremely scarce. 6.7 AIR-WATER PARTITIONING  Generally ionized species are assumed not to be present in the gaseous phase  The air-water distribution ratio (not partition constant), Daw(HA, A-) is defined as:  HA  HA   A   HA  HA    HA   A   HA Daw  HA,A   a  w w w a [18]  w w   a K H   a w KH RT  Similarly for a base: Daw  B , BH     1   a  K H   1   a  6.8 KH RT [19] ORGANIC SOLVENT-WATER PARTITIONING  Organic acid/base solvent-water partitioning is more complex than air- water partitioning since both the dissociated and undissociated species are soluble in both phases.  The octanol-water distribution coefficient is defined as: Dow  HA, A    HA  HA   A  o ,total w where [20]  w Dow(HA,A-) = octanol-water distribution ratio for organic acids [ ] = all terms in brackets denote concentrations. 1 CRC Handbook of Chemistry and Physics, Lewis Publishers, 1985. CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 12  For organic acids/bases Kow(HA) of the undissociated species is more than 2 orders of magnitude larger that the corresponding Dow(A-) of the respective ion. Thus at: pH   pK a  2 for acids pH   pK a  2 for bases the neutral species is the dominant species on the octanol-water distribution ratio of the compound.  At these pH values analogously to the air-water distribution ratio, the octanol-water distribution ratio is defined by the equations: Dow  HA, A     a  K ow  HA Dow  B , BH     1   a   K ow  B CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES ADK 13 EXAMPLE 1 For 3,4,5-trichlorophenol at pH 8.0 determine the: 1) aqueous solubility, 2) air-water distribution ratio, and 3) octanol-water distribution coefficient The following information is given: Cwsat  64.49 mg/l at 25 oC, : pKa = 7.84, KH = 2.2810-7 atm·m3/mol, and KOW = 10230 3,4,5-trichlorophenol CAS Number or ID: 609-19-8 Molecular Weight:197.44 Abbreviation: 345TCPHL Chemical Formula: C6H3Cl3O Apearance: off-white crystalline solid Melting Point: 101 oC Boiling Point: 275 oC SOLUTION 1. The aqueous solubility can be determined by: C wsat,total  C wsat a a  where a  1 HA  1    pH  pK  HA  A  1  A  1  10 HA 1  0.409 1  108.0  7.84  a  CONTAMINANT FATE & TRANSPORT ORGANIC ACIDS/BASES Cwsat,total  64.49  157.7 mg / l 0.409 ADK 14 Ans. 1 2. Determine the dimensionless Henry’s constant K H  KH 2.28  107 atm  m3 / mol   9.32 10 6 -5 3 RT 8.205  10 m  atm/K  mol 298 K     HA, A   3.8110   Daw HA, A   a K H  0.409 9.32  106 Daw   6 Ans. 2 3. Since pH=8.0
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Running head: PROTON TRANSFER REACTIONS

PROTON TRANSFER REACTIONS
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PROTON TRANSFER REACTIONS

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PROTON TRANSFER REACTIONS
Question: How does the Bronsted-Lowry Theory demonstrate the proton transfer reactions
in organic acids/bases?
According to the Bronsted-Lowry theory, an acid is a substance that donates a proton
during its reaction with another substance. A compound that gives a proton to another is
there...


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