Capture 5. Acids and bases

Transcription

1 Capture 5 Acids and bases Chap. 5 1 Arrhenius s conception acids produce + ions in aqueous solutions bases produce O ions in aqueous solutions water required, so only allows for aqueous solutions only protic acids are allowed; required to produce hydrogen ions only hydroxide bases are allowed Chap

2 AcidArrhenius BaseArrhenius Chap. 5 3 Johannes Nicolaus Brønsted Thomas Martin Lowry acids are proton donors bases are proton acceptors aqueous solutions are permissible bases besides hydroxides are permissible only protic acids are allowed Brønsted Lowry Chap

3 BrønstedLowry acid must have a removable (acidic) proton BrønstedLowry base must have a pair of nonbonding electrons. Chap. 5 5 Autoionization of Water As we have seen, water is amphoteric. In pure water, a few molecules act as bases and a few act as acids. K w This special equilibrium constant is referred to as the ionproduct constant for water,k w. At 25 C,K w = = [ 3 O + ] [O ] Chap

4 α=120 sp 2 4 O + [ 3 O] + [ClO 4 ] 9 O O + 2 O 2 O 2 O 3 O + 2 O 2 O 2 O 3 O + igh Mobility of + and O ydrogen bonding Chap. 5 7 O + Configuration diffusion of 3 and O : O O + O O O O O + O O O O O Chap. 5 8 O O O O 4

5 Conjugate Acids and Bases From the Latin word conjugare, meaning to join together. The conjugate base of an acid is the species that is left after a proton is lost. The conjugate acid of a base is the species formed when a proton is gained. Chap. 5 9 Conjugate Acid F + 2 O 3 O + + F (1) Acid Base Conjugate Base Conjugate Acid 3 O + + F F + 2 O (2) Acid Base Conjugate Base Chap

6 Base Conjugate Acid N O N 4+ + O Acid Conjugate Base Base Conjugate Acid N O 3 O + + N 3 Acid Conjugate Base Chap The strengths of Brønsted acids p is defined as the negative base10 logarithm of the hydronium ion concentration. p = log [ 3 O + ] In pure water: K w = [ 3 O + ] [O ] = This equilibrium constant is called the aciddissociation constant, K a. The greater the value of K a, the stronger the acid Chap

7 Strong acids are completely dissociated in water. Their conjugate bases are quite weak. Weak acids only dissociate partially in water. In any acidbase reaction, the equilibrium favors the reaction that moves the proton to the stronger base. Cl(aq) + 2 O(l) 3 O + (aq) + Cl (aq) 2 O is a much stronger base than Cl, so the equilibrium lies so far to the rightkis not measured (K>>1). Chap Substances with negligible acidity do not dissociate in water. Their conjugate bases are exceedingly strong. Acetate is a stronger base than 2 O, so the equilibrium favors the left side (K<1). The stronger base wins the proton. C 2 3 O 2 (aq) + 2 O 3 O + (aq) + C 2 3 O 2 (aq) Acid Base Conjugate Acid Acetate Conjugate Base Chap

8 The strengths of Brønsted bases K a and K b are linked: Combined reaction =? Chap Polyprotic acids Acid Formula Ka pka phosphoric acid 3PO4 K1 = PO4 K2 = PO4 2 K3 = PO O 2 PO O + K 1 = [ 3 O + ][ 2 PO 4 ] [ 3 PO 4 ] 2 PO O PO O + K 2 = [ 3 O + ][PO 2 4 ] [ 2 PO 4 ] PO O PO O + K 3 = [ 3 O + ][PO 3 4 ] [PO 2 4 ] Chap

9 Figure. The distribution diagram for the various forms of the triprotic acid phosphoric acid in water, as a function of p. Chap Solvent levelling Acid stronger than 3 O + in water : 2 O 3 O + water levelling Cl, Br, I, NO 3, 2 SO 4, ClO 3, ClO 4 The strengths of such acids can be distinguished by using a less basic solvent AcO: I>Cl Base stronger than 2 Oin water : 2 O O water levelling Chap

10 The strongest acid that can exist in a solvent is the conjugate acid (lyonium species) of the solvent. 2 SO O 3 O + + SO 4 pk a ~ 4 Cl + 2 O 3 O + + Cl pk a ~ 7 The strongest base that can exist in a solvent is the conjugate base (lyoxide species) of the solvent. N O N 3 + O (ipr) 2 N + 2 O (ipr) 2 N + O pk W = 14 pk a ~ 3540 for neutral amines Chap An acid that is weak in water may appear strong in a solvent that is a more effective proton acceptor, and vice versa (pka =0 to pka solvent solvent levelling) Which solvents could be used to differentiate the acidities of Cl (pka =6) and Br (pka =9)? COO Which solvents could be used to differentiate the acidities of P 3 (pka =27) and Ge 4 (pka =25)? N 3, DMSO The acid base discrimination window for a variety of solvents. Chap

11 Characteristics of Brønsted acids There are three classes of acids to consider: 1) An aqua acid: E(O 2 ) 2) A hydroxoacid, in which the acidic proton is on a hydroxyl group without a neighbouring oxo group (=O). E(O) An example is Te(O) 6 Chap ) An oxoacid, in which the acidic proton is on a hydroxyl group with an oxo group attached to the same atom. O E(O) > E(O) > E(O 2 ) An example of these successive stages is provided by a dblock metal in an intermediate oxidation state, such as Ru(IV): Chap

12 Simple acid strength X The stronger the bond + + X The weaker the acid F << Cl < Br < I The more polar the X bond and/or the weaker the X bond, the more acidic the compound. Acidity increases from left to right across a row and from top to bottom down a group. Chap Periodic trends in aqua acid strength The correlation between acidity constant and the electrostatic parameter of aqua ions. electrostatic parameter ξ = zazb d aqua acid strength z/r [Fe(O 2 ) 6 ] 2+ < [Fe(O 2 ) 6 ] 3+ < [Al(O 2 ) 6 ] 3+ [g(o 2 )] ppm 78.5 ppm 68 ppm 116 ppm Chap

13 δ δ+ Simple oxoacids Z O Z O + + The O bond will be more polar and easier to break if: Z is very electronegative or Z is in a high oxidation state 1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z ClO 3 > BrO 3 Chap Oxoacids having the same central atom (Z) but different numbers of attached groups. Acid strength increases as the oxidation number of Z increases. (or number of oxygens). ClO 4 > ClO 3 > ClO 2 > ClO Chap

14 Substituted oxoacids One or more O groups of an oxoacid may be replaced by other groups to give a series of substituted oxoacids O 2 S(O) 2 < O 2 S(N 2 )O Oxidation Number: N 2 : x + 2= 1 O 2 : 2 x= 3 Chap Pauling s rules Predicting the acidity of oxoacids For a formula EOp(O)q, Ka = 10 5p8 which gives: pka1 = 85p If q > 1, successive Ka values for additional ionisations are 10 5 smaller (or pka 5 units larger) pka2 = pka1+5 pka3 = pka2 +5 Work out K1 and K2 for 2SO4 (or SO 2 (O)2) p=2 and q=2 pka1 = 85p=85(2)=2 (pka1 (experimental) = 2) pka2 = pka1+5=2+5=3 (pka2 (experimental) = 1.9) Chap

15 Structural anomalies pka=85(1)=3 pka=6 (experimental) CO O pka=85(1)=3 pka=1.8 (experimental) SO O Chap Chap

16 Anhydrous oxides Classifications of oxides: Acidic: e.g. + 2O + + SO3 SO4 Basic: e.g. CaO + 2O Ca O Amphoteric: An amphoteric oxide is an oxide that reacts with both acids and bases. in acid: Al2O3 + 63O O 2 [Al(O2)6] 3+ and in base: Al2O3 + 2O + 3 2O 2 [Al(O)4] Al = amphoteric(in all oxidation states) As = As V acidic; As III basic in the highest oxidation state and amphoteric oxides in lower oxidation states. Chap Figure. The oxidation numbers for which elements in the fist row of the d block have amphoteric oxides. Predominantly acidic oxides are shown shaded pink, predominantly basic oxides are shaded blue. Cr II Cr IV Cr VI basic Amphoteric Acidic oxide oxide oxide Chap

17 Polyoxo compound formation In acidic solutions, both form octahedral hexaaqua ions, [Al(O 2 ) 6 ] 3+ and [Fe(O 2 ) 6 ] 3+. In solutions of p > 4, both precipitate as gelatinous hydrous oxides: Polyoxoanion formation from oxoanions occurs by protonation of an O atom and its departure as 2 O: oxoanion 2+ 2 O Polyoxoanion Chap Al: +3 1Al: Fe: +3 1Fe: +2 Chap

18 Orthophosphate condensation They also include the phosphate polymers (such as ATP) used for energy transfer in living cells. condensation reaction, At p=7.4, the POP bond is unstable with respect to hydrolysis. In the presence of water, it will dissociate, releasing energy in the process chains Polyoxoanion Two dimensional paper chromatogram Chap rings Nonaqueous solvents 1) Solvent systems similar to water, in which selfionisation involves proton transfer. In the case of water: 2 2 O 3 O + + O An acid is a species which increases the concentration of 3 O +, while a base is a species which increases the concentration of O. Also, species which react with the solvent to give changes in 3 O + or O concentration can similarly be classified as an acids or bases e.g the OEt ion in NaOEt would be a base, because: OEt + 2 O EtO + O Chap

19 Liquid Ammonia An acid will increase the concentration of the cationic species The selfionisation reaction is: 2N 3 N 4+ + N 2 A base will increase the concentration of the anionic species Ammonium salts (e.g. N 4 Cl) are acids because they increase the concentration of N 4+. Alkali metal amides (e.g. NaN 2 ) are bases because they increase the concentration of N 2. Species which can protonate 2 O to form 3 O + (e.g. Cl) will protonate N 3 to give N 4+. Chap Liquid F An acid will increase the concentration of the cationic species The selfionisation reaction is: 3F 2 F + + F 2 A base will increase the concentration of the anionic species Some species which are acids in water will also be acids in liquid F: e.g. 2 SO 4 + F 2 F + + SO 4 But, some species which are only weakly acidic in water can act as bases in F: e.g. C 3 COO + 2F + C 3 COO 2 + F 2 Chap

20 Fluoride ion donors (e.g. NaF) become bases: F + F F 2 Fluoride ion acceptors (e.g. SbF 5 ) become acids: e.g. SbF 5 + 2F SbF F + Chap Gilbert Newton Lewis Acids are electron pair acceptors Bases are electron pair donors Chap

21 The LUMO and OMO are called frontier orbitals. If there is a net lowering of energy, the adduct is stable Chap Examples of Lewis acids and bases 1) A metal cation can bond to an electron pair supplied by the base to form a coordination compound [Co(O 2 ) 6 ] 2+ 2) A molecule with an incomplete octet can complete its octet by accepting an electron pair. Chap

22 3) A molecule or ion with a complete octet may be able to arrange its valence electrons and accept an additional electron pair. 4) A molecule or ion may be able to expand its valence shell (or simply be large enough) to accept another electron pair) to accept another electron pair). Chap Other examples of donoracceptor complexes Chap

23 Other examples of donoracceptor complexes Chap Reactions and properties of Lewis acids and bases Carbon monoxide is a much better Lewis acid than O 2 and forms a strong, almost irreversible, bond to the iron(ii) site of haemoglobin: The fundamental types of reaction (a)adduct (b)displacement reactions (c) Metathesis reactions b Fe II + CO b Fe II CO A + :B A:B AB + A A B + A AB + B AB + B AB + A B AB + A B Chap

24 Example: AB + A A B + A Chap Example: AB + A B AB + A B Chap

25 Pearson s hard and soft acids and bases The strength of the donoracceptor interaction in Lewis acidbase complexes can be measured quantitatively in terms of the stability constant K. A + B AB K = [AB] a [A] [B] igh K indicates a strong interaction Low K indicates a weak interaction Why does the interaction change for different complexes? Chap η= ( I A e ) 2 I, A e η LUMO ard OMO LUMO OMO soft (a) Low polarizability (b) high polarizability

26 ard acids and bases Anion(Soft) z & r Cation(ard) z & r These tend to be small and not easily polarised. e.g ard acids: +, Li +, Na +, Mg 2+, Ca 3+, Al 3+, BF 3 ard bases: F, O, O 2, 2 O, N 3, NO 3 Soft acids and bases These tend to be large and more easily polarised. e.g Soft acids: Cu +, Ag +, g +, g 2+, Cd 2+, B 3 Soft bases:, I, CN, SCN, CO, PR 3 Remember this: ard acids tend to bind to hard bases (high ionic character) Soft acids tend to bind to soft bases (high covalent character) Chap Acids (electrophiles) Type Species Examples ard high charge/radius cations +, Li +, Mg 2+, Ca 2+, Al 3+, Cr 3+, Ti 4+, I 5+ group II species Be(C 3 ) 2 Intermediate Soft group III species Al(C 3 ) 3, BF 3, B(OR) 3 bond donors RO, O, RN 2, RN + 3 moderate charge/radius cations Fe 2+, Cu 2+, Zn 2+, Pb 2+, Sn 2+, Co 2+, Ni 2+ carbocations (C 3 ) 3 C +, Ar + low charge/radius cations carbon electrophiles diatomic halogens radicals Cu +, Ag +, Au +, g +, g 2+, I +, Br +, RO + RL, ArL (L = nucleofuge) I 2, Br 2, ICN O, Cl, Br, I, RO Chap

27 Bases (nucleophiles) Type Species Examples ard small halide anions F, Cl oxygen nucleophiles 2 O, RO, O, RO, ROR, MeCO 2, SO 4 2, PO 4 3 Intermediate Soft amine nucleophiles RN 2, 2 NN 2 larger halide anions Br nitrogen nucleophiles C 6 5 N 2, C 5 5 N, N 3 oxygen nucleophiles NO 2, SO 2 3 sulfur nucleophiles RSR, RS, RS phosphorus nucleophiles R 3 P, (RO) 3 P, R 3 As carbon nucleophiles CN, CO, R, C 2 4, Ar others, I Chap AB + A A B + A A + B AB K = AB + A B AB + A B ard ard igh K Soft Soft igh K ZnO + 2 LiC 4 9 Zn(C 4 9 ) 2 + Li 2 O soft hard hard soft soft soft hard hard K>1 [AB] a [A] [B] ard Soft Low K BX3 + N(C 3 ) 3 X 3 BNMe 3 The stability of the adduct is:bbr 3 < BCl 3 > BF 3 empty 2p orbital π bonding in BF 3 filled orbitals Chap

28 MO diagram for BF 3 σ and π bond e a 2 B e * a 2 a 1 * 3F 8 BO= = B.O everybond: =1 3 3 a 1 e e n.b e e e a 2 a 1 a 2 a 2 e B.O: BF 3 > BCl 3 lewis acidity: BF 3 < BCl 3 a 1 Chap Figure. The trends in stability constants for complex formation with a variety of halide ion bases. ard ions are indicated by the blue lines, soft ions by the red line. Borderline hard or borderline soft ions are indicated by green lines. M n+ + X MX n M n+ :ard ions M n+ :soft ions M n+ :Borderline hard or soft Chap

29 Important factors of Lewis acids and bases 1) Electronic Effect hard hard & softsoft 2) Steric Effect Repulsions by bulky groups make reactions less favorable for adduct formation. N 71 Kj/mol Me 3 B: Me N 74 Kj/mol N 42 Kj/mol Me Chap )The Rearrangement The rearrangement of the substituents of the acid and base that may be necessary to permit formation of the complex. + sp 2 4)The ybrid Effect %s I χ acidity sp 3 Base Base??? Chap

30 5) Solvent Effect Competition with the solvent in reactions in solution. Donor Number or DN Acceptor Number or AN Dielectric Constant or ε (charge in reaction) Donor Number or DN (Gutmann) In chemistry a donor number or DN is a quantitative measure of Lewis basicity. The standard Lewis acids SbCl 5 /1,2 DCM DN=0 DN <0 = DN Base effect of solvent Chap Acceptor Number or AN In chemistry a acceptor number or AN is a quantitative measure of Lewis Acidity. The standard Lewis Base Et 3 PO(TEPO) AN=0 Et 3 PO + A Et 3 POA AN <0 = AN Acid effect of solvent DMSO: Great e donor, but poor acceptor Methanol: Good edonor, but great eacceptor (donor) Chap

31 F 3 BN 3 BF 3 + N 3 AcidBase Solvent: MeO or Et 2 O??? DN: MeO Et 2 O AN: MeO >Et 2 O F 3 BN 3 BF 3 + SN 3 Acid F 3 BN 3 SBF 3 + N 3 Base BF 3 + N 3 F 3 BN 3 Solvent: MeO or Et 2 O??? Chap Dielectric constant (ε): Measure of polarity, polarizability Values obtained by measuring capacitance across solvent Taft α/β: Measure of proton donating/proton accepting character Interesting contrast: Acetic acid & ethyl acetate Same dielectric constant, but very different miscibility with water (AcO infinitely miscible, EtOAc immiscible) Chap

32 Thermodynamic acidity parameters A(g) +B(g) A B(g) r (A B) Drago Wayland equation (for gas and nonpolar solvent) electrostatic factor r (A B)/kJ mol 1 = E A E B + C A C B covalent factor The parameters E and C must accommodate all factors except solvation. N 3 BF 3 : E=20.2 and C=3.31 for BF 3 and E=2.78 and C=7.08 for N 3. r = [( ) + ( )]= kjmol 1 (exothermic reaction) Chap I 2 is the reference acid (C A and E A = 1); N,Ndimethylacetamide and diethyl sulfide are the reference bases. I 2 has very little electrostatic attraction for bases (small E A value); however, it has a strong tendency to form covalent bonds (high C A values) I 2 + C 6 6 I 2 C 6 6 I 2 + Aceton I 2 Aceton IfEandCvalues have been tabulated, Drago s method can predict acidbase strength fairly accurately. If no data is available, Pearson s SAB method can be useful for rough predictions. Neither method covers every case. Additionally, neither method includes solvation effects. Chap

33 eterogeneous acid base reactions The surfaces of many catalytic materials and minerals have Bronsted and Lewis acid sites. Surface reactions carried out using the Bronsted acid sites of silica gels are used to prepare thin coatings of a wide variety of organic groups using surface modification reactions such as: Bronsted acid Lewis acid Chap