1 / 22

Acids and Bases

Acids and Bases. Different Definitions of Acids and Bases. Arrhenius definitions for aqueous solutions . acid : a substance that produces H + (H 3 O + ) ions aqueous solution base : a substance that produces OH - ions in aqueous solution.

paiva
Download Presentation

Acids and Bases

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Acids and Bases

  2. Different Definitions of Acids and Bases • Arrhenius definitions for aqueous solutions. acid: a substance that produces H+ (H3O+) ions aqueous solution 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.

  3. 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. Types of electrons: Energy lone pairs pi bonding electrons sigma bonding electrons Basicity

  4. Acid – Base Eqilibria The position of the equilibrium is obtained by comparing the pKa values of the two acids. Equivalently, compare the pKb values of the two bases.

  5. Acid – Base Eqilibria Same equilibrium with electron pushing (curved arrows).

  6. 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.

  7. An example: pi electrons as bases Bronsted Lowry Base Bronsted Lowry Acid For the moment, just note that there are two possible carbocations formed. The carbocations are conjugate acids of the alkenes.

  8. Sigma bonding electrons as bases. Much more unusual!! 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. Super acid

  9. Totally unionized in aqueous solution Aqueous Solution Trends for Relative Acid Strengths Totally ionized in aqueous solution.

  10. Example pKa = 15.9 Weaker acid pKa = 9.95 Stronger acid H2O + PhOH H3O+ + PhO- H2O + EtOH H3O+ + EtO- Ka = [H3O+][EtO-]/[EtOH] = 10-15.9 Ka = [H3O+][PhO-]/[PhOH] = 10-9.95 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? Weaker base. Stronger base K = 10-9.95 /10-15.9 = 106.0 Query: What makes for strong (or weak) acids?

  11. Increasing basicity of anion. Increasing basicity of anion. 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. 2. Size of the atom bearing the negative charge in the anion. Increasing acidity. Increasing size of atom holding negative charge. Increasing stability of anion.

  12. What affects acidity? - 2 3. Resonance stabilization, usually of the anion. Increasing resonance stabilization. Increased anion stability. Increasing basicity of the anion. Acidity 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.

  13. 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. Pi bonding electrons converted to non-bonding. Which conjugate acid is favored? The more stable one! Which is that? Recall resonance provides additional stability by moving pi or non-bonding electrons. No valid resonance structures for this cation. Non-bonding electrons converted to pi bonding.

  14. An example: competitive Bases & Resonance Comments on the importance of the resonance structures. All atoms obey octet rule! The carbon is electron deficient – 6 electrons, not 8. Lesser importance All atoms obey octet rule!

  15. What affects acidity? - 3 4. Inductive and Electrostatic Stabilization. Increasing anion stability. Increasing anion basicity. Acidity. d+ d+ Due to electronegativity of F small positive charges build up on C resulting in stabilization of the anion. Effect drops off with distance. EtOH pKa = 15.9

  16. What affects acidity? - 4 Note. The NH2- is more basic than the RCC- ion. 5. Hybridization of the atom bearing the charge. H-A  H+ + A:-. sp3 sp2 sp More s character, more stability, more “electronegative”, H-A more acidic, A:- less basic. Increasing Acidity of HA Increasing Basicity of A- Know this order.

  17. Example of hybridization Effect.

  18. What affects acidity? - 5 6. Stabilization of ions by solvents (solvation). Solvation provides stabilization. Comparison of alcohol acidities. 17 18 pKa = 15.9 Crowding inhibiting solvation (CH3)3CO -, crowded Solvation, stability of anion, acidity

  19. 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. B A

  20. Examine the equilibrium for p-nitrophenol. How does the nitro group increase the 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. Structure D occurs only due to the nitro group. The stability it provides will slightly decrease 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

  21. 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. 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. 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

  22. Sample Problem

More Related