Acid and Base

Acid-Base Theory

Acid-Base Theory

Contents I Background 1 Arrehnius Theory 2 Brnsted-Lowry Theory II Acid-Base Theories 1 Lewis Acid-Base Theory 2 Lux-Flood Theory 3 Hard-Soft Acid-Base Theory III Measurements of Acidity & Basicity IV Descriptive Chemistry of Acids & BasesI Background What is Arrhenius (Ostwald) Theory? What is Arrhenius acid and Arrhenius base? Acid - proton donor, Base hydroxide donor What is the core concept of Arrhenius Theory? What is ionization theory? Why is water special to Arrhenius Theory? What is self-ionization? Are there other substances that can have self ionization? What is the Brnsted-Lowry Theory? What is the difference between the Arrhenius Theory and the Brnsted-Lowry Theory? Acid proton donor, Base proton acceptor Whats the physical meaning of Brnsted-Lowry Theory?1 competition for protons rather than proton & hydroxide donors2 A-B conjugate3 insight of the atom transfer processes 4 useful in non-aqueous systemsSubstances that have auto-dissociation (self-ionization)Aqueous system:Kw = 1.0x10-14 at 24.8 C, 1 atmNeutral point: (1/2)pKw = 7.0leveling effect The acidity and basicity in the aqueous solutions are limited by H+ and OH. The acidity of any substance is stronger than H+ or the basicity is stronger than OH will be leveled off. This is called leveling effect. KAB = 10-20 KAB = 10-29 (Cl transfer)(phosphoryl halides)Solvolysis A process in which the solute can combine with the ionic component of the solvent or have the solvent ionized. 1 Hydrolysis combination of H+ in water 2 Non-aqueous solvolysis Alcoholysis Gutman elaborates the reaction of Me4N+Cl and ferric chloride in phosphoryl chloride as solvolysis via chloride-transfer pathway. As for the amphoteric substance, such as Al(OH)3 is insoluble in water, but dissolves in strong acid and strong base. Similarly, Al2(SO3)3 is insoluble in liquid sulfur dioxide, but dissolves by adding SOCl2 or CsSO3. Question: Does the reaction of thionyl chloride with sulfite undergo solvolysis in SO2?n A clever experiment of isotope labeling shows no exchange between 32SOCl2 and 35SO2. n No oxygen exchange observed for the reverse reaction either.n However, the halide exchange between SOCl2 and SOBr2 was observed.Lewis Acid-Base TheoryIn water: B(OH)3+H2OH++B(OH)4, so B(OH)3 is an Arrehnius acid.NH3+H2ONH4++OH, so NH3 is an Arrehnius base.Boric acid being an acid is not only because it is ionized to give proton, but for it decreases [OH] B(OH)3 + OH B(OH)4Ammonia being a base is not only because it is ionized to give proton, but for it decreases [H+] NH3 + H+ NH4+Lewis Base electron pair donor, Lewis Acid electron pair acceptorFor the reactions in which adducts are formed.Me3N(g) + BF3(g) R3N-BF3(s) (m.p. > 200 C)

Inductive effect Base strength: Me3N>NH3>NF3Acid strength: Me3B As > Sb O >> S > Se > Te F > Cl > Br > IDonors having tendency to bind with Class (b) metal ions N As > Sb O Se ~ Te F < Cl < Br < I Ralph G. Pearson suggested the term hard for the metal ions of Class (a) and their corresponding bases; soft for the metal ions of Class (b) and their corresponding bases.Hard acids prefer to bind to hard bases; and soft acids prefer to bind to soft bases. MeHgF + HSO3 MeHgSO3 + HF Keq ~ 103MeHgOH + HSO3 MeHgSO3 + H2O Keq > 107 Transmetallation used in the Synthesis of Organometalsmetathesis MXn + n RLi MRn + n LiX MXn + n RMgX MRn + n MgX2 Carbon or hydride nucleophiles: LiR, NaH, Na(C5H5), NaBH4, KHB(SitBu)3, LiAlH4 (LAH), AlR3, AlRn(OR)3-n, GaMe3, R2Mg, R2Zn, RZnX, LiCuR(CN), NaNH2, radical donor: RHgX, R4Sn Electrophiles: proton sauce, RX, R3SiX, CO2, ROTs, ROTf, Ph3C+BF4-,Anions: X, BF4, BArf4, PF6, ClO4, NO3Coupling Reactions: Heck reaction, Stille reaction (AX/RnSnX4-n), Suzuki reaction (ArX/RB(OH)2), etc.

Physical meanings behind HSAB1 ionization energy & electronegativity 2 polarizability3 HOMO-LUMO gap: large for hard species, small for soft species4 soft-soft interactions imply covalency; and hard-hard interactions are for ionic bonding

III Measurements of Acidity and Basicity Aqueous basicity & aciditypH = log[H+]; pH + pOH = 14.0 at 24.8 C, 1 atmKa, DG = -RTlnK; DS = klnEither the logarithm of an equilibrium constant or of a measurement for the arrangement of the states means a thermodynamic function. Gase-Phase basicity & acidityProton affinity of a base B is defined as the negative value of the enthalpy for the reaction: B(g) + H+(g) BH+(g)Using Born-Haber cycle, PA may be determined by measuring ionization energies of H and BH as well as bond enthalpy of BH.

Some determined PA (kJ/mol): For N3, 3084; NH2, 2565; NH2, 1689 (charge effect) NH3, 872; NF3, 604; Me3N, 974 (inductive effect)Difference between NH3 and NF3 is 268 kJ/mol; between PH3 and PF3 is only 106, but DEN is larger for PF. This is attributed to the resonance effect, thus the charge in PF3H+ is substantially delocalized.Gase-phase acidity for a conjugate acid of B will be the enthalpy of dissociation of BH+. Gase-phase acidity for the Lewis acids of metal cations parallels the electron affinities.Acidity of OxidesAn acidity scale for the oxides is the measurement of the reaction enthalpy, and make aBaA = (DHrxn)1/2.For instance CaO + SiO2 CaSiO3 DH = 86 kJ/mol aBaA = 9.5The a parameter is calibrated with awater = 0.0. Therefore, the negative values for the a parameter indicates high basicity, e.g. 15.2 for Cs2O; and the positive values for the nonmetal oxides, e.g. 11.5 for Cl2O7, an anhydride of orthochloric acid. Drago-Wayland Equation for the HSABDH = EAEB + CACB for the reaction: A + B ABwherein E stands for electroststic and C for covalent.IV Descriptive Chemistry of Acids & Bases1 general guidelines Basicity of metal oxides increases while going down the periodic table. BeO is amphoteric. With increasing covalency, oxides become more acidic or less basic. The acidity of nonmetal oxides increases when the values of (aB-aA) increases. The acidity of the hydrated metal ions of high positive charge density increases when the value of z2/r increases. Hydration and hydrolysis are more feasible with the same trend. The metal ions of high charge to size density are more acidic, e.g. Fe3+ pKa 2.19; Fe2+ pKa 10.1. The EN of the central atom is directly influence the acidity of oxyacids. Acidity: HClO4 ~ HNO3 > H2SO4 >> H3PO4 > H2CO3 >> H3BO3 The more oxygens around the central atom, the larger acidity is. Acidity: HClO4 > HClO3 > HClO2 > HClO Change the inductive effect of the substituents of amines, change the basicity. NH2OH < NH2NH2 < NH3 < MeNH2; NF3 is not a base; 2 sulfuric acid hydrogen sulfate ion sulfate ionsulfuric acidwater

b.p.m.p.ddielectric constantspecific conductanceviscosityion product constant300 C10.371 C1.83 g cm-3 (25 C)110 e0 (20 C)1.04x10-2 W-1cm-1 (25 C)24.54 g cm-1s-1 (20 C)2.7x10-4 (25 C)100 C0 C1.00 g cm-3 (4 C)81.7 e0 (18 C)4x10-8 W-1cm-11.01 g cm-1s-1 (20 C)1.008x10-14 (25 C)

3 ammoniaammoniawater

b.p.m.p.ddielectric constantspecific conductanceviscosityion product constant-33.38 C-77.7 C0.725 g cm-3 (-70 C)26.7 e0 (-60 C)1x10-11 W-1cm-10.254 g cm-1s-1 (-33 C)5.1x10-27100 C0 C1.00 g cm-3 (4 C)81.7 e0 (18 C)4x10-8 W-1cm-11.01 g cm-1s-1 (20 C)1.008x10-14 (25 C)

Ammonia is a poor solvent for highly charged ions, including carbonates, sulfates, phosphates, etc., but nonpolar molecules, or large polarizable ions such as iodide, thiocyanate are soluble in ammonia.Sovolysis takes place in the presence of chlorine or phosphoryl chloride. The former reaction may also be considered as disproportionation.4 oxoacidsOxoacids are customarily written as HmXO(m+n). The functional formula is XOn(OH)m. Hydrated oxometals such as uranyl UO22+, vanadyl VO2+.Acidity: HClO4 > HClO3 > HClO2 > HClO > HBrO > HIOHClO4 > H2SO4 > H3PO4 or OP(OH)3 > H2CO3 > H3BO3CrO3 > Cr2O3 > CrO (basic)CrO3 + H2O H2CrO4 (chromic acid)5 binary oxidesOxides of the metals of Groups 1 and 2 are generally basic, except BeO. Al2O3 is amphoteric. Nonmetal oxides are generally acidic.Acidity: Cl2O7 > SO3 > P4O10 > SiO25 solid acids:Clays and Zeolites are porous solids. The clays have layer structures, and the zeolites are aluminosilicates with channels. They have replaceable cations and may be exchanged with H+, thereby forming solid acids. The open channels of zeolites vary in size, thus may serve as catalysts for specific selectivity. For instance, ZSM-5 (Na3(AlSi31O64)316H2O) is used to produce gasoline from methanol. 6 superacids: The protonated solvent molecules of the already strong acids.Examples: SbF5 produces extremely strong proton donor in HF or HSO3F. SbF5 + 2 HF H2F+ + SbF6 SbF5 + 2 HSO3F H2SO3F+ + FSO3SbF5