Chemical Structure and AcidBase Relationship Proton Affinity on Acid-base force: Proton affinity (PA) is The proton affinity is a quantitative measure of the base strength of a chemical species in the gas phase. -Big PA strong base and its conjugate acid is weak -Acidity and bacicity of a chemical species are effected than İE, EA, BE, Inductive effect, rezonance and charge of chemical species. -The main factor that makes the acid-base strength order in the gas phase different from that in the solution phase is solvation. (water for hydration). While the solvation of the acid or base molecule reduces the acid-base strength, the solvation of the formed ions increases. -The acidity of hydrides, which are binary compounds of hydrogen, increases from left to right in a period and from top to bottom in a group. Acid Base Strength Proton affinity: In gase phase, Enthalpy of reaction of an chemical species with H+. IE I<Br<Cl<F PA HF>HCl>HBr>HI Acid strenght HF<HCl<HBr<HI Inductive effect and Rezonans Inductive effect is property of an atom or atom groups to change electron density of neighbouring atom due to electronegativity difference. Inductive effect cause to chance IE of central atom. PA increasing Lto R NF3 NCl3 NH3 NH3 CH3NH2 CF3CH2OH H2O<OH-<O2- (CH3)2NH (CH3)3N CCl3CH2OH PA decreasing Lto R NO- NO2- NO3- CH3CH2OH Hydration Effect: In gase phase; HA(g) H+(g) + A-(g) In water solution; HA(aq) H+(aq) + A-(aq) HA(aq) HA(g) -DHh(HA) HA(g) H+(g) + A-(g) –PA (A-) H+(g) + A-(g) H+(aq) + A-(aq) ) -DHh(ion) DH= -DHh(HA)-PA(A-) -+DHh(ion) H2S and HF in gase phase H2S is more stronger acid but HF is more stronger in water solution. Hidration effect on Acid-base force: DH= -DHh(HA)-PA(A-) -+DHh(ion) In water solution, hidration of acid molecule decrease acid forse also for base . Hidration of ions increase acid forse also for base Examples: H2S and HF, (CH3)3N and CH3NH2 Acidity resulting from hydration of metal cation For Mn+ cation in water; If Mn+ is a transition metal cation or q/r is big, coordinative covalent bond form. Other presentation of this reaction In case of q/r increasing this reaction moves to more left and the solution becomes more acidic. As a result; -In IA and IIA metal cations (they interact water with only ion-dipol interaction), q/r increase as linear. With increasing q/r ratio, cation make more positive oxygen and facilitate to leave proton. -Transition metal cations are more asidic than metal cations with simmilar q/r ratio. Because these metal cations form more high kovalent character bond with water and proton easily leave. For example Co2+, Fe2+ and Cd2+ is mopre acidic than Mg2+. -Different cations of same element, such as Fe3+, Fe2+’den ve Co3+, Co2+den more acidic. Q7r bigger cation become more positive to oxygen. -Metal cations with +4 and bigger than +^+ oxidation step become more positive to oxgen thus all of the H leave and Cr+4 and Mn7+ form CrO42- and MnO4-. Their water solutions are quite asidic. Oxides Acidity or basicity of an oxide can be explain with Lux-Flood definition. The higher the tendency of an oxide to accept oxide ions, the stronger acid and the higher its tendency to give oxide ions, the stronger base. Oxide includes at least one X-O bond (X: metal, semimetal or ametal), Leaving oxide ion in this structure; Leaving of oxide ion depending on electronegativity of X atom; increasing of X atom make leaving of oxide ion diffucult. Thus this inreases the acidity. Oxy acids are formed from the hydrolysis of acidic oxides and metal hydroxides are formed from the hydrolysis of basic oxides. Therefore, acidic oxides are acid anhydrides and basic oxides are base anhydrides. Basicity of Nitrogen Compound and Steric Effect Nitrogen atom in their compounds inclu lone pair and these compounds bahave as LB. Steric effect both decrease acidity and basicity of the compounds. Reacted acid and base don’t close to each other. This is called as pre-tension In same molecule, repulsion of substituent each other, molecule geometry and hybridization type can be change. Oxyacids [OpA(OH)q] p: Oxo number q: number of OH A: Ametal An oxyacid includes at least one X-O-H. Some oxiacids include X-H Bond. Pauling Rules 1. pKa1 8-5p 2. q> 1 ise pKa2 = pKa1 + 5 pKa3 = pKa2 + 5 For H2SO4 1. pKa1 8-5x2 -2 2. pKa2 -2 +5 +3 Force of oxyacid depends on leaving of H+ from X-O-H Easy H O H leaving of proton, more strong acid. O S O H O H İncreasing of oxygen number increase acidity HClO4 > HClO3 > HClO2 > HClO Increasing of electronegativity of ametal increase. H2SO4 > H2SeO4 > H2TeO4 O S O O ÖRNEK: H2CO3 asitin denel pKa değeri 6.4 dür. 1. Pauli kuralına göre beklenen pKa değeri nedir? 2. Deneysel veri ile uyumlu mudur? Nedenini izah ediniz. 1. pKa1 8 – 5x1 3 2. CO2 + H2O H2CO3 Uyumlu değildir. Çözünmüş CO2 nin yaklaşık %1 i H2CO3 halinde bulunur. O C O O O O + H O OH C C H - OH2 OH ÖRNEK : Aşağıda bazı asitlerin denel PKa değerleri verilmiştir. H3PO4 2.1 H3PO3 1.8 H3PO2 2.0 Bu asitlerin açık formülünü tahmin ediniz. pKa 8 – 5 x 1 = 2 Hepsinde 1 okso grubu bulunur. pKa 8 – 5 x 2 = -3 O O O P P P HO OH HO H OH OH Fosforik asit Fosforöz asit HO H H Hipofosforöz asit Inductive Effect Electronegative groups Cl C F, Cl, Br, NO2 Electronegative atom remove electron density from C atom. Electropositive groups CH3 C R, CH3 Alkyl groups direct electron density to carbon atom. Lewis acidity BF3>BH3>BMe3 İncreasing electronegativity increase acidity İncreasing methyl number decrease acidity. Cl atom move away decrease Acidity. Electronegativity and Size Effect In hydro acids across a group Acidity increases as atomic diameter increases. Large anions have lower charge density and attract the H atom more weakly. In hydro acids over a period, as electronegativity increases, acidity increases. As the electronegativity of the conjugate base increases H+ ion dissociates more easily. . Hybridyzation Effect sp3 H pKa H C H 50 H İncreasins s character increase electronegativity and increase acidity. H H sp2 H C C H 35 sp H C C H 25 H C 10-45 C sp2 : H+ + C C 10-26 C C H H+ + C C : sp Steric and Solvatation CH 3 COOH 4.75 CH 3CH 2 COOH 4.87 CH 3CH 2CH 2 COOH 4.81 …. Close to each other pKa CH 3 CH 3 C COOH CH 3 …. More high pKa Solvatation decrease ion energy.. H O H H O H O H O C O H Branched H - steric effect H O O H H H O C Not solvatation. O H H O 5.02 H - H H O O H H O O H H Decrease acidity Polarization, Hard and Soft Acid-Base Polarization, , is the distortion that occurs as a result of the migration of electrons from one region to another under the influence of an external electric field. As increases, the electron cloud becomes more easily polarized. Polarization - If the charge of the cation is large and its diameter is smal Fe(III) > Fe(II) -If the anion has a large charge and a large diameter S-2 > O-2 -The electron configuration of the cation determines the polarizing power.* (n-1)dxns0 > (n-1)s2(n-1)p6ns0 Hg2+ (116 pm) > Ca2+ (114 pm) * Shielding effect of d electrons small and Z* is more HARD: Small atoms and ions, small polarization, HOMO-LUMO energy difference is large SOFT: Larger atoms and ions, larger polarization, smaller HOMO-LUMO energy difference Verici atom : N, O, F Verici atom: P ve S Pearson Principle Hard acids prefer hard bases, soft acids prefer soft bases. Reference Asit Basicity HARD H+, Mg2+, Sc3+ F- > Cl- > Br- > I- SOFT Hg2+ F- < Cl- < Br- < I- Li-I + Ag-F Li-F + Ag-I HS HS HH SS ΔH = -17 kcal/mol HgF2 + BeI2 HgI2 + BeF2 SH HS SS HH ΔH = - 95 kcal/mol The hard-hard interaction is more dominant in terms of energy. HA HB SA SB HA SB HOMO-LUMO farkı büyük Covalent interaction Ionic interaction Weak Strong Strong Weak Weak Weak Stable Stable Unstable AgI(k) SA-SB (Covalent) İnsoluble in water. AgF(k) SA-HB (Ionic compounds) Soluble in water. NaF < NaCl <NaBr<NaI (Ionic compound) As the cation-anion radius difference increases, the solubility in water increases. HA-HB Ionic contribution and water solubility increases. SUMMARY • Hard Lewis acids: • Atomic centres of small ionic radius • High positive charge • Species do not contain electron pairs in their valence shells • Low electron affinity • Likely to be strongly solvated • High energy LUMO • Soft Lewis acids: • Large radius • Low or partial (delta+) positive charge • Electron pairs in their valence shells • Easy to polarise and oxidise • Low energy LUMOs, but large magnitude LUMO coefficients • Hard Lewis bases: • Small, highly solvated, electronegative atomic centres: 3.0-4.0 • Species are weakly polarisable • Difficult to oxidise • High energy HOMO • Soft Lewis bases: • Large atoms of intermediate electronegativity: 2.5-3.0 • Easy to polarise and oxidise • Low energy HOMOs but large magnitude HOMO coefficients.