Models Of Acid-Base Chemistry - Department Of Chemistry

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Models of Acid-Base ChemistryChapter 6Friday, October 30, 2015

Models of Acid-Base ChemistrySeveral models have been developed to classify, rationalize andpredict the reactivity of acid-base pairs (or donor-acceptor pairs). Arrhenius Model – acids give hydrogen ions in aqueous solutionand bases give hydroxide in aqueous solution Brønsted-Lowry Model – acids are hydrogen ion donors and basesare hydrogen ion acceptors Lewis Model – acids are electron pair acceptors and bases areelectron-pair donors Electrophile-Nucleophile Model – acids are electrophilic reagentsand bases are nucleophilic reagents Lux-Flood Model – acids are oxide (O2–) acceptors and bases areoxide donors Usanovich Model – an acid-base reaction is a reaction leading toformation of a salt

Arrhenius Acid-Base ModelAcids form hydrogen ions in aqueous solution and bases formhydroxide in aqueous solutionHCl aq NaOH aq NaCl aq H 2OHCl aq H Cl really morelike H3O NaOH aq Na OH acid base Good fundamental definition of acids and bases for aqueoussolution Of limited use in non-aqueous solvents or the gas phase wheredissociation doesn’t occur Also fails to capture analogous reactivity of substances that don’tdissociate into H or OH–, e.g., what do you do about NH3?

Brønsted-Lowry ModelAcids release hydrogen ions and bases take up hydrogen ionsHCl H2OacidstrongH3O Cl-baseweakacidweakerbasevery weak Conjugate acids and bases – when an acid gives up a hydrogenion, it forms the conjugate base Acid-base reactions always proceed to form the weaker acid/basepair The Brønsted-Lowry Model accommodates non-aqueous solventsand gas-phase reactions 4 2NH NH 2NH 3 4 NH H NH 3acidNH 3acidbaseNH 3 2 NH H NH 3baseacidNH 3acid

Solvent System ModelThis model for acid-base reactivity was developed for solvents thatcan dissociate into cations (acid) and anions (base) The classic example is water:acidbase Now we can say that sulfuric acid is an acid because it increasesthe concentration of H3O : On the other hand, ammonia is a base because it increases theconcentration of OH–:

Solvent System ModelMany solvents dissociate to some degree:14.0pKion -log Kion -log([H3O ][OH-])34.4

The Lewis ConceptA base is an electron-pair donor and an acid is an electron-pairacceptor This model is consistent with the Arrhenius and Brønsted-LowryModels:acidbaseH : NH 3 NH 4 Lewis LewisAcid Base It also works with molecules that neither give up nor accept ahydrogen ionBF3 : NH 3 H 3 N : BF3Lewis LewisAcid BaseLewisAdduct

The Lewis ConceptThe Lewis concept also allows us to describe coordination complexeswithin an acid-base framework.Ag 2 :NH3LewisAcidLewisBaseCoordinate covalent or dative bond –used to show that both electrons in thebond come from a Lewis base Lewis Acid-Base adducts in which the Lewis acid is a metal ion aretypically called coordination compounds

The Lewis Concept and Molecular OrbitalsThe Lewis concept of acids and bases is readily interpreted usingmolecular orbital theory.BF3 : NH 3 H 3 N : BF3N–B σ*LUMOHOMOdriving forcefor adductformationN–B σAn acid-base reactionoccurs when HOMO ofthe base and LUMO ofthe acid combine tocreate new HOMO andLUMO in the product

The Lewis Concept and Molecular OrbitalsThe MO perspective on Lewis acids/bases is versatile and can help usunderstand many types of reactions. Consider water:water as oxidizing agent3d4slarge ΔE favorselectron transferLUMO (mostly on H’s)HOMO (O 2py)H2OCaHere, water acts as an “extreme Lewis acid” to oxidize Ca to Ca2 (complete electron transfer)2 H2O Ca Ca2 2 OH- H2

The Lewis Concept and Molecular OrbitalsThe MO perspective on Lewis acids/bases is versatile and can help usunderstand many types of reactions. Consider water:solvation of an anionsimilar E favorsstrong adductLUMO4s3pHOMOH2OCl-Here, water acts as a Lewis acid to form an adduct with an anionn H2O Cl- [Cl(H2O)n]-

The Lewis Concept and Molecular OrbitalsThe MO perspective on Lewis acids/bases is versatile and can help usunderstand many types of reactions. Consider water:solvation of a cationLUMOsimilar E favorsstrong adductHOMO3s2pH2OMg2 Here, water acts as a Lewis base to form an adduct with a metal cation6 H2O Mg2 [Mg(H2O)6]2

The Lewis Concept and Molecular OrbitalsThe MO perspective on Lewis acids/bases is versatile and can help usunderstand many types of reactions. Consider water:water as reducing agentLUMOlarge ΔE favorselectron transferHOMOσu*H2OF2πg*Here, water acts as an “extreme Lewis base” to reduce F2 to 2 F- (complete transfer)2 H 2O 2 F 2 4 F - 4 H O 2

HSAB ModelThe Hard-Soft Acid-Base concept seeks to understand the reactivity ofLewis acids and bases according to the polarizability of their valenceelectrons (i.e., their deformability by other molecules/ions). hard Lewis acids are small acids with a high positive charge soft Lewis acids are larger and typically have a lower positivechargeHard AcidsBorderline AcidsSoft AcidsH , Li , Na , K BH3, Tl , Tl(CH3)3Be2 , Mg2 , Ca2 , Sr2 Cu , Ag , Au , Cd2 ,B(CH3)3Hg22 , Hg2 , CH3Hg BF3, BCl3, B(OR)32 2 2 Fe , Co , NiAl3 , AlCl3, AlH3[Co(CN)5]2–, Pd2 , Pt2 2 2 Cu , ZnCr3 , Mn2 , Fe3 , Co3 Br2, I23 , Ir3 , Ru3 , Os2 RhMn (n 4)Mn (n 0)H-bonding moleculesπ-acceptor molecules

HSAB ModelThe Hard-Soft Acid-Base concept seeks to understand the reactivity ofLewis acids and bases according to the polarizability of their valenceelectrons (i.e., their deformability by other molecules/ions). hard Lewis bases are small bases with highly electronegativedonor atoms soft Lewis bases are larger and typically have smallerelectronegativitiesHard BasesBorderline BasesF–, Cl–H2O, OH–, O2–, ROH,Br–RO–, R2O, RCOO–NO2–, N3–NO3–, ClO4–, CO32–,SO32–C6H5NH2, C5H5N, N2SO42–, PO43–NH3, RNH2, N2H4Soft BasesH–I–H2S, SH–, S2–, RSH,RS–, R2SSCN–, CN–, RNC, COS2O32–PR3, P(OR)3, AsR3,C2H4, C6H6, R-

HSAB TheoryThe Hard-Soft Acid-Base concept seeks to understand the reactivity ofLewis acids and bases according to the polarizability of their valenceelectrons (i.e., their deformability by other molecules/ions). hard Lewis acids are small acids with a high positive charge, softLewis acids are larger and typically have a lower positive charge hard Lewis bases are small bases with electronegative donoratoms, soft Lewis bases are larger and typically have smallerelectronegativitiesHard acids prefer to interact with hard bases, soft acids prefer tointeract with soft bases.ZnO 2LiC4 H 9 Zn C4 H 9 2 Li2Oborderline acidhard basehard acidsoft baseborderline acidsoft basehard acidhard base

HSAB Theory and Exchange ReactionsHSAB theory is consistent with the large differences in Keq for simpleexchange reactions:Hg2 is a soft acid. As the halide becomes softer, the reaction becomesincreasingly favorable.

HSAB Theory and SolubilitiesBecause the dissolution of salts in water typically requires thereplacement of a Lewis base with water, HSAB can predict trends insolubilities. Consider the silver halides: F– and Cl– are hard bases; however, so is water. It turns out thatwater is softer than F– but harder than Cl– Br– and I– are soft bases Because Ag is a soft acid, it is less prone to give up the halide aswe move down the series

HSAB Theory and SolubilitiesBecause the dissolution of salts in water typically requires thereplacement of a Lewis base with water, HSAB can predict trends insolubilities. Lithium halides show the opposite trend: F– and Cl– are hard bases; however, so is water. It turns out thatwater is softer than F– but harder than Cl– Br– and I– are soft bases In this case, Li is a hard acid and it prefers to interact with thehardest base available

HSAB Theory and Metal Thiocyanate ComplexesIn coordination complexes the metal is the Lewis acid. The Lewis baseis normally called a ligand. Thiocyanate (SCN–) is an interesting ligand because there are twoLewis base sites:“linkage isomerism”

Pearson’s Absolute Hardness ScaleThe absolute hardness is defined asI A 2 I is the ionization energy of the molecule in eV, approximatelyequal to -EHOMO A is the electron affinity of the molecule in eV, approximately equalto -ELUMO So the absolute hardness is just half the HOMO-LUMO difference(i.e., half the band gap!) Hard acids have large values for η, soft acids have smaller valuesfor η Softness of a donor atom is given by the inverse of hardness, i.e., 1

Pearson’s Absolute Hardness Scale

The Nature of the AdductHard acid/hard base adducts tend to have more ionic characterin their bonding. These are generally more favored energetically.Soft acid/soft base adducts are more covalent in nature.

Acid/Base StrengthIt is important to realize that hard/soft considerations havenothing to do with acid or base strength. An acid or a base maybe hard or soft and also be either weak or strong.In a competition reaction between two bases for the same acid,one must consider both the relative strength of the bases, andthe hard/soft nature of each base and the acid.ZnO 2LiC4 H 9 Zn C4 H 9 Li2O2borderline acidhard basehard acidsoft baseborderline acidsoft basehard acidhard baseZinc ion is a strong Lewis acid, and oxide ion is a strong Lewis base,suggesting the reaction is unfavorable as written.However, the reaction proceeds to the right (K 1), because hard/softconsiderations override acid-base strength considerations.

Drago’s EC ApproachA quantitative system for calculating the thermodynamics of acid-basereactions takes account of both covalent and ionic components of theacid-base interaction. For the reaction, EA and EB are the capacity for electrostatic (ionic) interactions ofthe acid and base CA and CB are the capacity for covalent interactions of the acid andbase I2 is the reference acid with CA EA 1.00 kcal/mol Reference bases are MeC(O)NMe2 (EB 1.32) and SEt2 (CB 7.40)

Using the EC ModelThe enthalpy of a given Lewis acid-base reaction can be determinedusing tabulated values (see Table 6.17 in your textbook).LewisacidLewisbaseLewisadduct H EA EB C AC Bkcal kcal kcal kcal H 1.000.525 1.000.681 mol mol mol mol kcal H 1.21molwithin 9% of experimental value

Summary There are several models to describe acids and bases. The Lewismodel is one of the most general since it does not depend on thetransfer of a hydrogen ion. A dative bond (coordinate covalent bond) is often used to denote abond between a neutral Lewis base and a Lewis acid: The Lewis model is readily interpreted using MO theory: the HOMOof the Lewis base interacts with the LUMO of the Lewis acid. HSAB provides a semi-quantitative method for understandingtrends in acid-base reactivity: hard acids like hard bases and softacids like soft bases. The Drago EC model provides a more quantitative method forunderstanding the thermodynamics of an acid-base reaction.

be hard or soft and also be either weak or strong. In a competition reaction between two bases for the same acid, one must consider both the relative strength of the bases, and the hard/soft nature of each base and the acid. ZnO 2LiC4 H9 Zn C 4 H9 Li2 O borderline acid hard base hard acid soft base borderline acid soft b

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