Chapter 4 Acids and Bases PDF

CHAPTER 4 Acids and Bases Key Concepts Brønsted-Lowry Acids and Bases Acid Dissociation Constants and pKa Acidity Trends Electronegativity Atom size Hybridization at carbon Resonance delocalization Inductive effects Assigned Reading All...

CHAPTER 4 Acids and Bases Key Concepts Brønsted-Lowry Acids and Bases Acid Dissociation Constants and pKa Acidity Trends Electronegativity Atom size Hybridization at carbon Resonance delocalization Inductive effects Assigned Reading All of Chapter 4 except 4.2B, 4.2C, 4.5, 4.7 Practice Problems from the Textbook Ch. 4 (8th ed) In chapter 4.1, 2, 5, 6 End of chapter 4.10, 11, 13, 15, 26, 29, 39 (38), 47 (45), 54 (48), 55 (49), 58 (52), 59 (53) 1  Chapter 4.1-4.4 Acids, Bases, and pKas Hydrangeas and pH Image from https://www.pinterest.com/pin/440649144794332162/?nic_v2=1a6lTpSGJ Reading: Chapter 4.1, 4.2A, 4.3, 4.4 Textbook problems: 4.1, 4.2, 4.5, 4.6, 4.10, 4.11, 4.15, 4.26, 4.29, Note: Textbook problems are for your practice. They will not be turned in.  Brønsted-Lowry Acids and Bases Definitions An acid is a proton ___________ donor A base is a proton ___________ acceptor & & ⑦ A - H= B - & A H - B acid base Conjugate conjugate base acid Remember in our discussion of resonance structures, we used curved arrows to depict the flow of electrons We can also use this convention for chemical reactions (including acid-base reactions) conjugate _________ A ________ base is formed when an acid loses a proton A ________ acid conjugate _________ is formed when a base accepts a proton It -N O i - H - - Il O - & it Conju Conju acid base bade and Strong acids are deprotonated to form weak conjugate bases Strong bases are protonated to form weak conjugate acids 3  iClicker Question Ch4#1 Which represent aG base and its conjugate G acid is (listed in that order)? & It' acceptor donor A) NH4+, NH3 B) CH3CH3, CH3,CH2- ⑧ C) CH S , CH SH 3 - 3 D) HCOOH, HCOO- 4 Acid Dissociation Constants and pKa Strength and Definitions A strong acid completely donates H+ to a base A strong base completely accepts H+ from an acid A weak acid/base does not completely donate/accept H+ We use pKa to define the strength of an acid Low pKa = strong acid High pKa = weak acid The strength of an acid can be expressed in terms of the following equilibrium: & Hit + He + H From the above equation, we can determine an equilibrium constant (Ka): [A-][H30] Kag [ [H20] Ka = Keq[H20]-[ pka -log(ka) = strae - more [] - - due on [A] - Lower the pKa = STRONGER THE ACID 5 Acid Dissociation Constants and pKa pKas for some common acids - Table 4.1 from textbook 6 Acid Dissociation Constants and pKa Predicting Equilibriums We can use pKa values to predict where the equilibrium lies in reactions between an acid and a base. Example: Will acetic acid (CH3COOH, pKa = 4.76) react with ammonia (NH3)? 9 24 pla = ammonium. *Hint: Compare the pKa values for acetic acid and the conjugate acid of ammonia equilibrium favors products -"A ① HiNHa O - Il - bake ad acid low pla > - high pla Keq = [b] imm [Con ; Guid I H30] 3 Keq (Holacod The : cong and ⑫(pKa) plug pkegs and-(pKa) = 4 -. 76 4 48. - (9 24). crus a Negate play positive - cid chen products > Reactants plaq- Keq = 10 4 48. 7  iClicker Question Ch4#2 What is the equilibrium constant (pKeq) of the following reaction? coul contacid a did base base - HF + H 2O F + H 3O + Acid Conjugate Base pKa Water (H2O) Hydroxide (-OH) 15.7 Hydronium ion (H3O+) Water (H2O) -1.74 Hydrogen fluoride (HF) Fluoride (F-) 3.2 plu-(pHa)ad-(pha) play = 3 2. - (1 74). 4 94 Farre reactants pheq =. 8  Sosted uts Mean : 80 3 %. Gede Students te = B 141 27 8. C 84 16. S 5 18 ↑ Su 10 6. Begrada request due Wednesday Oct # - written description including question name , , an for reason believing need regrade a - deliver to Prof Drown/Parkinson. - Recitation worksheet due today Quiz S due tomorrow - - 24 his to start - 30 minutes to complete once started - Informal Early poll Feedback starts today - See Brightspace announcement - Chapter 5 starting soon  Chapter 4.6 Acidity Trends Reading: Chapter 4.6 Textbook problems (8th ed): 4.39 (38), 4.47 (45), 4.54 (48), 4.55 (49), 4.58 (52), 4.59 (53) Note: Textbook problems are for your practice. They will not be turned in. 9  Functional Groups and Acidity Predicting Equilibriums Conjugate acid pKa Conjugate base ROH (alcohols): 15-19* RO- (alkoxide) RSH (thiols) 10-12* RS- (thiolate) R3NH+ (trialkylammonium ion) 9-11* R3N (trialkylamine) RCOOH (carboxylic acids) 4-5* RCOO- (carboxylate) *Precise value varies with the structure of R THIS KNOW e thints hols adv cinium " ⑪ - acidic protons It most I TV Th # so 110 10  Acidity Trends An Overview ***For neutral organic acids, it is most useful to evaluate the stability of the anionic conjugate base – the more stable the anion (A-), the greater the acidity of HA*** Stable anion > - strong and in conjugate base HA A mon O O more Stable ac d =2 OH O less less stable acidic OH O 11  Acidity Trends Charge Delocalization A general observation is that spreading charge across a larger space (larger atoms or multiple atoms via resonance) is improved stability Cat in Carrier Cat on Large Bed High Energy/Unstable Low Energy/Stable Localized Charge Delocalized Charge High Energy/Unstable Low Energy/Stable - O - - O - O - N O- 12  Acidity Trends An Overview 5 factors to consider when evaluating anion stability: Atom Directlyattached to H [ 1A. Electronegativity of the atom bearing the negative charge *More electronegative element = more stable anion 1B. Size of atom bearing the negative charge *Larger element = more stable anion Same Element Attached to H [ 2A. Hybridization of the carbon center bearing the negative charge *More s-character = more stable anion 2B. Extent of resonance delocalization of the negative charge *More delocalization = more stable anion 2C. Inductive effects and electrostatic stabilization *Presence of electron-withdrawing groups = more stable anion *This is generally a second-order consideration compared with the other factors 13  Acidity Trends Electronegativity 1A) Electronegativity of atom bearing the negative charge in conjugate base (atom directly attached to acidic proton) Acid H–CH3 0 H–NH2. Er H–OH H–F - Conjugate Base –CH 3 –NH 2 –OH –F Electronegativity 2.5 3.0 3.5 4.0 (Pauling scale) pKa 51 38 15.7 3.5 for alomunts Comparison in same now Remember that electronegativity increases going across the periodic table More electronegative atoms can better accommodate negative charge Example: Which of the following molecules is more acidic? E H H3C H :OH I more H3C N CH3 C die vs. ac C C * H2 H2 CH3 Th VN 0. ↳ 14  Acidity Trends Atom Size 1B) Size of atom bearing the negative charge Acid Et H–OH get H–SH Conjugate Base –OH –SH pKa 15.7 7 Compare elements in same column The valence shell for larger elements is more diffuse Orbitals that are more delocalized (occupy a larger volume of space) are at a lower energy (i.e. are more stable) NOTE: This trend does NOT follow the trend of electronegativity!!! Example: Rank the following molecules in order of acidity? HF HBr HCl HI 1 TV I Th FO Bo 20 - biggest smallest #3 #I # Y #2 15  Acidity Trends Hybridization 2A) Hybridization of the atom bearing the negative charge Acid ad H–CH CH H–CH=CH 2 g H–C CH 3 2 Conjugate Base –CH CH 2 3 –CH=CH 2 –C CH Hybridization sp3 sp2 sp (25% s) (33% s) (50% s) pKa 51 44 25 character > more - stable - > more Higher s acodic Example: Indicate the most acidic proton in the following molecule. sp2 mosta ↓H [ H C C C ↑ C CH3 p H Sp 16  Acidity Trends Resonance 2B) Extent of resonance delocalization of the negative charge H2 CH2 O E H C [ E Acid O CH3 H O C CH3 H O C CH3 spat H2 &CH2 & O Conjugate Base - C C C O CH3 · O CH3 O CH3 · m pKa 15.9 ~13 T 4.8 ↑ O : CH2 · I o Hy :0. CH3 Orbitals that are more delocalized (occupy a larger volume of space) are at a lower energy (i.e. more stable) Evaluate the relative stability of the 3 conjugate bases shown above: 17  Acidity Trends Inductive Effects and Electronic Stabilization 2C) Inductive effects and electronic stabilization O O O Acid E H C F E H C F E H O C CH3 O C O C H2 F F O O O C F Conjugate Base C O C C F O C O CH3 H2 F F pKa 4.8 2.7 -0.3 Electronegative elements that withdraw electron density stabilize nearby negative charge Evaluate the relative stability of the three conjugate bases shown above H · O · O destabilizing stabilizes. 18  Acidity Trends Identify the most acidic site acron O bagger mosi HS E NH scidia [ HN N E H O A Cyclic dipeptide O O OH H E OH H resonance stabilizatio · " E HO ⑧ OH most cardic H Cholic Acid 19  iClicker Question Ch4#3 Which group will lose a proton first (i.e. is most acidic)? It H O most acidic ⑧ - [ HO Hi OH NH[2 serine Ous N O is wor EN O Ho It OooH U NHz NHz To Holto NIz 20  iClicker Question Ch4#4 Which pair has the stronger acid listed first (on left)? Select all that apply. Stronger? morsetronegative Weaker? Fitso & F O Br O i A OH go OH B v OH # SH Spi T H C H CH2OH ↳ CO2H D Hort o * Sto 21