Presentation on theme: "Acids and Bases. Different Definitions of Acids and Bases Arrhenius definitions for aqueous solutions. acid: acid: a substance that produces H + (H 3."— Presentation transcript:
Acids and Bases
Different Definitions of Acids and Bases Arrhenius definitions for aqueous solutions. acid: acid: a substance that produces H + (H 3 O + ) ions aqueous solution base: base: a substance that produces OH - ions in aqueous solution Bronsted-Lowry definitions for aqueous and non-aqueous solutions. Conjugate acid – base pair: molecules or ions interconverted by transfer of a proton. acid: transfers the proton. base: receives the proton.
Lewis Acids and Bases Focuses on the electrons not the H +. An acid receives electrons from the base making a new bond. Acid electron receptor. Base electron donor. lone pairs pi bonding electrons sigma bonding electrons Energy Basicity Types of electrons:
Acid – Base Eqilibria The position of the equilibrium is obtained by comparing the pK a values of the two acids. Equivalently, compare the pK b values of the two bases.
Acid – Base Eqilibria Same equilibrium with electron pushing (curved arrows).
Lone Pair acting as Base. Note the change in formal charges. As reactant oxygen had complete ownership of lone pair. In product it is shared. Oxygen more positive by 1. Similarly, B has gained half of a bonding pair; more negative by 1.
An example: pi electrons as bases Bronsted Lowry Acid Bronsted Lowry Base The carbocations are conjugate acids of the alkenes. For the moment, just note that there are two possible carbocations formed.
Sigma bonding electrons as bases. Much more unusual!! Super acid A very, very electronegative F!! A very positive S!! The OH becomes very acidic because that would put a negative charge adjacent to the S.
Trends for Relative Acid Strengths Totally ionized in aqueous solution. Aqueous Solution Totally unionized in aqueous solution
Example Ethanol, EtOH, is a weaker acid than phenol, PhOH. It follows that ethoxide, EtO -, is a stronger base than phenolate, PhO -. For reaction PhOH + EtO - PhO - + EtOH where does equilibrium lie? pK a = 9.95 Stronger acid H 2 O + PhOH H 3 O + + PhO - K a = [H 3 O + ][PhO - ]/[PhOH] = H 2 O + EtOH H 3 O + + EtO - K a = [H 3 O + ][EtO - ]/[EtOH] = pK a = 15.9 Weaker acid Stronger base Weaker base. Query: What makes for strong (or weak) acids? K = / =
What affects acidity? 1. Electronegativity of the atom holding the negative charge. Increasing electronegativity of atom bearing negative charge. Increasing stability of anion. Increasing acidity. Increasing basicity of anion. 2. Size of the atom bearing the negative charge in the anion. Increasing size of atom holding negative charge. Increasing stability of anion. Increasing acidity. Increasing basicity of anion.
What affects acidity? Resonance stabilization, usually of the anion. Increasing resonance stabilization. Increased anion stability. Acidity Increasing basicity of the anion. No resonance structures!! Note that phenol itself enjoys resonance but charges are generated, costing energy, making the resonance less important. The more important resonance in the anion shifts the equilibrium to the right making phenol more acidic.
An example: competitive Bases & Resonance Two different bases or two sites in the same molecule may compete to be protonated (be the base). Acetic acid can be protonated at two sites. Which conjugate acid is favored? The more stable one! Which is that? Recall resonance provides additional stability by moving pi or non-bonding electrons. Pi bonding electrons converted to non-bonding. Non-bonding electrons converted to pi bonding. No valid resonance structures for this cation.
An example: competitive Bases & Resonance All atoms obey octet rule! The carbon is electron deficient – 6 electrons, not 8. Lesser importance Comments on the importance of the resonance structures.
What affects acidity? Inductive and Electrostatic Stabilization. Due to electronegativity of F small positive charges build up on C resulting in stabilization of the anion. Increasing anion stability. Acidity. Increasing anion basicity. Effect drops off with distance. EtOH pK a = 15.9
What affects acidity? Hybridization of the atom bearing the charge. H-A H + + A: -. sp 3 sp 2 sp More s character, more stability, more “electronegative”, H-A more acidic, A: - less basic. Increasing Acidity of HAIncreasing Basicity of A - Note. The NH 2 - is more basic than the RCC - ion. Know this order.
Example of hybridization Effect.
What affects acidity? Stabilization of ions by solvents (solvation). Solvation provides stabilization. Crowding inhibiting solvation Solvation, stability of anion, acidity pK a = (CH 3 ) 3 CO -, crowded Comparison of alcohol acidities.
Example Para nitrophenol is more acidic than phenol. Offer an explanation The lower lies further to the right. Why? Could be due to destabilization of the unionized form, A, or stabilization of the ionized form, B. A B
Examine the equilibrium for p-nitrophenol. How does the nitro group increase the acidity? Resonance structures A, B and C are comparable to those in the phenol itself and thus would not be expected to affect acidity. But note the + to – attraction here Structure D occurs only due to the nitro group. The stability it provides will slightly decrease acidity. Examine both sides of equilibrium. What does the nitro group do? First the unionized acid. Note carefully that in these resonance structures charge is created: + on the O and – in the ring or on an oxygen. This decreases the importance of the resonance.
Resonance structures A, B and C are comparable to those in the phenolate anion itself and thus would not be expected to affect acidity. But note the + to – attraction here Structure D occurs only due to the nitro group. It increases acidity. The greater amount of significant resonance in the anion accounts for the nitro increasing the acidity. Now look at the anion. What does the nitro group do? Remember we are interested to compare with the phenol phenolate equilibrium. In these resonance structures charge is not created. Thus these structures are important and increase acidity. They account for the acidity of all phenols.