Orgnic Acids & Bses pk Vlues ESE/CH 176 Environmentl Physicl Orgnic Chemistry 23-27 April 2018
Wter Dissocition Rection: K + - H2O ¾¾ ¾¾ H + OH K + - HO 2 + HO 2 ¾¾ ¾¾ HO 3 + OH Mss Action Lw K = g H + - [ H ] g [ OH ] + - ( ) [ HO] K = K [ H O] = = g [ H ] g [ OH ] + - w 2 + - + - H OH H OH OH 2 K» [ H ][ OH ] = 10 w + - -14
Acid Dissocition For generic monoprotic cid, HA HA ¾¾ ¾¾ H + A K + - K = the chrcteristic equilibrium constnt K determines the strength of n cid lrge K vlues = strong cid o nitric, hydrochloric, sulfuric cids smll K vlues = wek cid CO 2 H 2 O, cetic, hypochlorous cids K normlly given in terms of pk number pk º - log K
Acids nd Bses Acid Specition s function of ph some terms defined [HA] T = the totl mount of the cid A [ ] [ ] - HA = HA + [A ] T! i = the frction of [HA] T tht hs lost i protons [ HA] [ HA] [ HA] 0 = = = - + [ HA] [ HA] + [ A ] [ HA] + K [ HA]/[ H ] T - - - [ A ] [ A ] [ A ] 1 = = = - + - - [ HA] [ HA] + [ A ] [ H ][ A ]/ K + [ A ] T
Acids nd Bses Solving for! 0 nd! 1 = 0 = 1 K K + [ H ] + + [ H ] K + + [ H ] for given K, specition s function of ph cn be plotted in the following wy:
1.0 lph! 0.8 0.6 0.4 HA A - [HA or BH+ ] [A- or B] 0.2 0.0 0 2 4 6 8 10 12 14 ph
0 H + - OH -2 log C -4-6 -8 HA A - [H+ ] [OH -] [HA or BH+ ] [A- or B] -10-12 -14 0 2 4 6 8 10 12 14 ph
Acid Dissocition Diprotic cid H 2 A Acids nd Bses H 2 A = H + + HA - HA - = H + + A 2- K 1 K 2 Triprotic cid H 3 A H 3 A = H + + H 2 A - H 2 A - = H + + HA 2- HA 2- = H + + A 3- K 1 K 2 K 3
Acids nd Bses Acid Specition s function of ph [H i A] T the totl mount of the cid [H 2 A] T = [H 2 A] + [HA - ] + [A 2- ] [H 3 A] T = [H 3 A] + [H 2 A - ] + [HA 2- ] + [A 3- ] α i = the frction of TOTA tht hs lost i protons 2- [ HA 3 ] [ HA ] 0 2 [ HA 3 ] T [ HA 3 ] T = = - 3- [ HA 2 ] [ A ] 1 4 [ HA 3 ] T [ HA 3 ] T = =
0-2 log C vs. ph for Diprotic Acid Oxlic Acid [H 2 A] T = 10-3 M pk 1 = 0.90 pk 2 = 4.20 log C -4-6 -8-10 [H+ ] [OH -] [H 2A] [H A-] [A^ 2-] -12 HO O -14 O OH 0 2 4 6 8 10 12 14 ph
O OH OH 0 log C vs. ph for Diprotic Acid [H 2 A] T = 10-4 M pk 1 = 2.97 pk 2 = 13.70-2 Slicylic Acid -4 log C -6-8 -10-12 -14 [H+ ] [OH -] [H 2A] [H A-] [A^ 2-] 0 2 4 6 8 10 12 14 ph
log C vs. ph for Triprotic Acid O OH O 0-2 Citric Acid HO HO O OH -4 [H+] [OH-] log C -6-8 [H3A] [H2A-] [HA^2-] [A^3-] -10-12 -14 0 2 4 6 8 10 12 14 ph [H 3 A] T = 10-2 M pk 1 = 3.13 pk 2 = 4.72 pk 3 = 6.33
Acids nd Bses Solubility ionized form re much more soluble Henry s Lw constnt ionized species re much less voltile Orgnic-Wter prtition coefficients ionized species re less hydrophobic
Orgnic Acids nd Bses Wht is the ph of 0.1 M pentchlorophenol solution? pk = 4.75 OH Cl Cl Cl Cl Cl
Wht is the ph of 0.1 M pentchlorophenol solution? pk = 4.75 ph = 2.88 HA K!!! 1 Cl Cl!!! H + + A K 1 = 10 4.75 (0.1 M x) = x + x K 1 = x 2 (0.1 x) x 2 + K 1 x 0.1 K 1 = 0 x = 1.325 10 3 log x = 2.88 OH Cl Cl Cl
Rnge of pk vlues (cids) 2,4,6-trinitrophenol 0.38 phenol 9.92 2,4,6-trimethylphenol 10.39 OH Rnge of pk vlues (bses) 2,4,6-trichloroniline 0.09 niline 4.63 2,4,6-trimethylniline 5.24
OH Inductive Effects on pk depend on position electron-withdrwing groups hlogens, nitro, phenyl lowers pk pk = 9.92 pk = 8.56 OH OH Cl electron-donting groups lkyl increse pk pk = 9.12 pk = 9.41 OH Cl Cl
OH 9.92 Resonnce Effects on pk delocliztion; mobile electrons negtive resonnce (lower pk ) 7.23 OH NO 2 -OH, -NO 2, -NH 3 + OH positive resonnce (higher pk ) electrons my be shred with substituent only t certin positions 8.28 7.15 OH NO 2 NO 2
Proximity Effects on pk Hydrogen Bonding (intrmoleculr) ortho positioning of two H-bond groups lowers first pk, rises second pk Steric Effects inhibition of H 3 O + ccess hindrnce of resonnce
Quiz Rnk the following compounds by order of pk, from lowest (most cidic) to highest. 2,3-dichlorophenol 2,4-dichlorophenol 2,6-dichlorophenol 3,4,5-trichlorophenol
Oxlic Acid A Diprotic Acid pk 1 = 1.27 ; pk 2 = 4.28 kidney stones of clcium oxlte clcium oxlte, CC 2 O 4 K sp = 1.5 10-8
Clcium Oxlte Kidney Stones
Answer to Quiz Rnk the following compounds by order of pk, from lowest (most cidic) to highest. pk 2,6-dichlorophenol 6.79 2,3-dichlorophenol 7.70 3,4,5-trichlorophenol 7.84 2,4-dichlorophenol 7.85
pk Estimtion Need to estimte pk vlues mesured dt is vilble, but mesured t vrible ionic strength mesured vlues vry by up to 0.3 ph units
pk Estimtion Hmmett (1940) liner free energy reltionship between pk nd structure effect of nerby substituents on cid or bse functionl group pk å = pk -r s X H i i
pk Estimtion O Benzoic Acid induction 5 C OH resonnce proximity intrmoleculr H-bonding pk 4 3 steric hindrnce 2 --NO 2 (ortho) --Cl (ortho) --NO 2 (pr) --Cl (met) --Cl (pr) --OCH 3 (met) --H (benzoic cid) --CH 3 (pr)
Hmmett Constnts
σ met substituent in the 3 or 5 position σ pr substituent in the 4 position σ pr - inductive nd resonnce effects of substituents in the 4 position of phenols, nilines, nd pyridines σ ortho phenol substituent in 2 or 6 position for phenol σ ortho niline substituent in 2 or 6 position for niline
pk Estimtion Hmmett constnts σ i For this dt: 5 4 --CH 3 (pr) --OCH 3 (met) --H (benzoic cid) pk = 1.1 F i +4.23 ρ ~ 1.1 ρ = 1.00 for benzoic cid pk 3 --Cl (pr) --Cl (met) --NO 2 (pr) intercept is 4.23 2 pk (benzoic) = 4.19 F i -0.2 0.0 0.2 0.4 0.6 0.8 1.0
pk Estimtion H 2 C COOH Other Acids phenol phenylcetic cid 3-phenylpropionic cid phenoxycetic cid Bses niline benzylmine pyridine H 2 C O H 2 C C H 2 COOH CH 2 COOH NH 2 N
Vlues of ρ for orgnic cids phenol benzoic phenylcetic 3-phenylpropionic phenoxycetic
Vlues of ρ for orgnic bses niline benzylmine pyridine Extr o-nitrophenol pyridine drwn incorrectly
pk pk Estimtion 3-butyl-4-formyl-5-bromobenzoic cid pk = 4.20-1.00 [ s ( butyl) + s ( formyl) + s ( bromo)] pk pk å = pk -r s H i i met pr met = 4.20-1.00 [(- 0.07) + (0.22) + (0.39)] = 3.66 O C OH Br CH 3 HC O
pk Estimtion 2,4,6-trinitrophenol pk = pk -r s phenol - pk = 9.92-2.25[2 s ( nitro) + s ( nitro)] pk pk pk = 9.92-2.25[2(1.24) + (1.25)] = 1.53 = å H i i ortho pr 0.42 (experimentl) O 2 N OH NO 2 NO 2
pk pk Estimtion 3,4-dichloroniline = pk -r s - pk = 4.63-2.89 [ s ( chloro) + s ( chloro)] pk pk pk = 4.63-2.89 [(0.37) + (0.23)] = = 2.90 å H i i met pr 2.97 (exp.) NH 2 Cl Cl
Acids, Bses, nd Prtitioning Effect of cid/bse chemistry on C w st C st w - ( HA, A ) = st + C ( HB, B) w = C st w C ( HA) st w 1- ( B) 1 = 1 + 10 ( ph - pk )
Acids, Bses, nd Prtitioning Effect of cid/bse chemistry on K H - w w K ( HA, A ) = K ( HA) + w w K ( HB, B) = (1- ) K ( B)
Acids, Bses, nd Prtitioning Effect of cid/bse chemistry on K ow - ow ow K ( HA, A ) = K ( HA) + ow ow K ( HB, B) = (1- ) K ( B)
Octnol-Wter Prtition Coefficients K Compound log K OW Acetmide -1.16 Methnol -0.82 Formic cid -0.41 Diethyl Ether 0.83 p-dichlorobenzene 3.37 Hexmethylbenzene 4.61 2,2',4,4',5-Pentchlorobiphenyl 6.41
K ow of 3,4,5-trichlorophenol t ph 8.0 pk = 7.84; K ow (HA)= 10 4.01 1 1 = = = 0.41 ( ph - pk ) (8-7.84) 1+ 10 1+ 10 K HA A K HA - 4.01 (, ) ow = ow( ) = (0.41)(10 ) K ow - ( HA, A ) = 10 3.62