Dr. Wolf's CHM 201 & What Happened to pK b ?

Dr. Wolf's CHM 201 & About pK a and pK b A separate “basicity constant” K b is not necessary. Because of the conjugate relationships in the Brønsted-Lowry approach, we can examine acid-base reactions by relying exclusively on pK a values.

Dr. Wolf's CHM 201 & ExampleExample Which is the stronger base, ammonia (left) or pyridine (right)? Recall that the stronger the acid, the weaker the conjugate base. Therefore, the stronger base is the conjugate of the weaker acid. Look up the pK a values of the conjugate acids of ammonia and pyridine in Table 1.7. N HH H N

Dr. Wolf's CHM 201 & ExampleExample N H H H H + pK a = 9.3 N H + pK a = 5.2 weaker acid stronger acid Therefore, ammonia is a stronger base than pyridine

Dr. Wolf's CHM 201 & How Structure Affects Acid Strength

Dr. Wolf's CHM 201 & The Main Ways Structure Affects Acid Strength The strength of the bond to the atom from which the proton is lost. The electronegativity of the atom from which the proton is lost. Changes in electron delocalization on ionization.

Dr. Wolf's CHM 201 & Bond Strength Bond strength is controlling factor when comparing acidity of hydrogen halides. HFHClHBrHI pKapKapKapKa weakest acid strongest acid strongest H—X bond weakest H—X bond

Dr. Wolf's CHM 201 & Bond Strength Recall that bond strength decreases in a group in going down the periodic table. Generalization: Bond strength is most important factor when considering acidity of protons bonded to atoms in same group of periodic table (as in HF, HCl, HBr, and HI). Another example: H 2 S (pK a = 7.0) is a stronger acid than H 2 O (pK a = 15.7).

Dr. Wolf's CHM 201 & The Main Ways Structure Affects Acid Strength The strength of the bond to the atom from which the proton is lost. The electronegativity of the atom from which the proton is lost. Changes in electron delocalization on ionization.

Dr. Wolf's CHM 201 & ElectronegativityElectronegativity Electronegativity is controlling factor when comparing acidity of protons bonded to atoms in the same row of the periodic table.

Dr. Wolf's CHM 201 & ElectronegativityElectronegativity pKapKapKapKa weakest acid strongest acid least electronegative most electronegative CH 4 NH 3 H2OH2OH2OH2OHF

Dr. Wolf's CHM 201 & ElectronegativityElectronegativity The equilibrium becomes more favorable as A becomes better able to bear a negative charge. Another way of looking at it is that H becomes more positive as the atom to which it is attached becomes more electronegative. H A O R H....RH.. O H A –

Dr. Wolf's CHM 201 & Bond strength versus Electronegativity Bond strength is more important when comparing acids in which the proton that is lost is bonded to atoms in the same group of the periodic table. Electronegativity is more important when comparing acids in which the proton that is lost is bonded to atoms in the same row of the periodic table.

Dr. Wolf's CHM 201 & Acidity of Alcohols pKapKapKapKa Alcohols (RO—H) resemble water (HO—H) in their acidity. HO—H CH 3 O—H CH 3 CH 2 O—H (CH 3 ) 2 CHO—H (CH 3 ) 3 CO—H In many acids the acidic proton is bonded to oxygen.

Dr. Wolf's CHM 201 & Acidity of Alcohols Electronegative substituents can increase the acidity of alcohols by drawing electrons away from the —OH group. CH 3 CH 2 OH CF 3 CH 2 OH weakerstronger pKapKapKapKa

Dr. Wolf's CHM 201 & Inductive Effect The greater acidity of CF 3 CH 2 OH compared to CH 3 CH 2 OH is an example of an inductive effect. Inductive effects arise by polarization of the electron distribution in the bonds between atoms. C C O HHH F FF ++++

Dr. Wolf's CHM 201 & Electrostatic Potential Maps CH 3 CH 2 OH CF 3 CH 2 OH The greater positive character of the proton of the OH group of CF 3 CH 2 OH compared to CH 3 CH 2 OH is apparent in the more blue color in its electrostatic potential map.

Dr. Wolf's CHM 201 & Another example of the inductive effect O CH 3 C OHO CF3CCF3CCF3CCF3COH weakerstronger pKapKapKapKa

Dr. Wolf's CHM 201 & The Main Ways Structure Affects Acid Strength The strength of the bond to the atom from which the proton is lost. The electronegativity of the atom from which the proton is lost. Changes in electron delocalization on ionization.

Dr. Wolf's CHM 201 & Electron Delocalization Ionization becomes more favorable if electron delocalization increases in going from right to left in the equation. Resonance is a convenient way to show electron delocalization. H A O R H....RH.. O H A –

Dr. Wolf's CHM 201 & Nitric Acid O + – ONH O O H H H O H H + O + – ON O – + pK a =

Dr. Wolf's CHM 201 & Nitric Acid O + – ON O – Nitrate ion is stabilized by electron delocalization.

Dr. Wolf's CHM 201 & Nitric Acid –O + ON – O O + – ON O –O + – ON O – Negative charge is shared equally by all three oxygens.

Dr. Wolf's CHM 201 & Acetic Acid OOCH O H H pK a = CH 3 H O H H + + O OC – CH 3

Dr. Wolf's CHM 201 & Acetic Acid Acetate ion is stabilized by electron delocalization. O OC – CH 3

Dr. Wolf's CHM 201 & Acetic Acid O OC – Negative charge is shared equally by both oxygens. O OC – CH 3

Dr. Wolf's CHM 201 & Acid-Base Equilibria

Dr. Wolf's CHM 201 & GeneralizationGeneralization The equilibrium in an acid-base reaction is favorable if the stronger acid is on the left and the weaker acid is on the right. Stronger acid + Stronger base Weaker acid + Weaker base

Dr. Wolf's CHM 201 & Example of a strong acid HH.. O H Br– H Br OHH pK a = -5.8 stronger acid pK a = -1.7 weaker acid + + The equilibrium lies to the side of the weaker acid. (To the right)

Dr. Wolf's CHM 201 & Example of a weak acid pK a = 4.7 weaker acid pK a = -1.7 stronger acid H—OCCH 3 O + OHH O—H HH + + OCCH 3 O – The equilibrium lies to the side of the weaker acid. (To the left)

Dr. Wolf's CHM 201 & Important Points A strong acid is one that is stronger than H 3 O +. A weak acid is one that is weaker than H 3 O +. A strong base is one that is stronger than HO –. A weak base is one that is weaker than HO –. The strongest acid present in significant quantities when a strong acid is dissolved in water is H 3 O +. The strongest acid present in significant quantities when a weak acid is dissolved in water is the weak acid itself.

Dr. Wolf's CHM 201 & Predicting the Direction of Acid-Base Reactions Phenol pK a = 10 stronger acid Water pK a = 15.7 weaker acid H—OC 6 H 5 + – The equilibrium lies to the side of the weaker acid. (To the right) Phenol is converted to phenoxide ion by reaction with NaOH. H—O – OC 6 H 5 + H—O—H

Dr. Wolf's CHM 201 & Predicting the Direction of Acid-Base Reactions Phenol pK a = 10 weaker acid Carbonic acid pK a = 6.4 stronger acid The equilibrium lies to the side of the weaker acid. (To the left) Phenol is not converted to phenoxide ion by reaction with NaHCO 3. H—OC 6 H 5 + – OC 6 H 5 + – HOCOO HOCO—H O

Dr. Wolf's CHM 201 & Lewis Acids and Lewis Bases

Dr. Wolf's CHM 201 & DefinitionsDefinitions Arrhenius An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions. Brønsted-Lowry An acid is a proton donor. A base is a proton acceptor. Lewis An acid is an electron pair acceptor. A base is an electron pair donor.

Dr. Wolf's CHM 201 & Lewis Acid-Lewis Base Reactions The Lewis acid and the Lewis base can be either a neutral molecule or an ion. Lewis acid Lewis base +A + A—BB + – A + A—BB + + A + A—BB –– – A + A—BB +

Dr. Wolf's CHM 201 & Example: Two Neutral Molecules F3BF3BF3BF3B+ O CH 2 CH 3 – + F3BF3BF3BF3B O CH 2 CH 3 Lewis acid Lewis base Product is a stable substance. It is a liquid with a boiling point of 126°C. Of the two reactants, BF 3 is a gas and CH 3 CH 2 OCH 2 CH 3 with a boiling point of 34°C.

Dr. Wolf's CHM 201 & Example: Ion + Neutral molecule Reaction is classified as a substitution. But notice how much it resembles a Brønsted acid-base reaction. + Lewis base Lewis acid H—O – H 3 C—Br + H—OCH 3 – Br + H—O – H—Br + H—O—H – Br

Dr. Wolf's CHM 201 & Example: Ion + Neutral molecule Brønsted acid-base reactions are a subcategory of Lewis acid-Lewis base reactions. + Lewis base Lewis acid H—O – H 3 C—Br + H—OCH 3 – Br + H—O – H—Br + H—O—H – Br

Dr. Wolf's CHM 201 & End of Chapter 1