Chapter 3 Acids and Bases. The Curved-Arrow Notation Arrhenius Acids and Bases • Acid: a substance that, when dissolved in water, increases the concentration of H+ (protons) H2O • HCl • HCl + H2O H+ + ClH3O+ + Cl- • Base: a substance that, when put in water, increases the concentration of OH- ions or a substance that accepts H+ ions • NaOH(aq) Na+(aq) + OH-(aq) Bronsted-Lowry Acids and Bases • Acid: proton (H+) donor • Base: proton (H+) acceptor Lewis Acids and Bases • Lewis Acid – – – – Electron deficient/poor Electron acceptor Electrophile Tend to have less than an octet • Lewis Base – – – – Electron rich Electron donor Nucleophile Must have a lone pair of electrons • product called an adduct Fluorine is electron rich • Lewis acids tend to react so as to fulfill their valence-shell octet • Note the conservation of charge • Recall: FC = # valence e-’s – ( # LP e-’s + ½ # of bonding e’s) 6 Problems Complete the following Lewis acid-base reactions. Draw in any missing electrons, label the nucleophiles and electrophiles, identify the adduct, and calculate any formal charges needed for the adduct OH H C H + OH-1 Curved-Arrow Notation • A tool for tracking electrons in a chemical reaction • Electrons flow from the electron donor (Lewis base) to the electron acceptor (Lewis acid) 8 Problems Use the curved arrow notation to derive a structure for your product in each of the following reactions CH3NH2 + H+ Electron Pair Displacement Reactions • Not all acceptors are electron-deficient • When an atom is NOT electron deficient, an electron pair must depart from the atom before it receives another electron pair • This preserves the octet rule 10 Curved-Arrow Notation for Displacement • Displacement reactions require two arrows • Watch for conservation of total charge! • Donated electron pairs can also originate from a lone pair or a bond 11 The Wrong Way • Curved-arrows show the movement of electron pairs not nuclei • Electrons are responsible for chemistry! 12 Problems Provide a curved arrow notation for each of the following reactions H2O + HCl H3O+ + Cl- CH2CH2 + Br2 + Problems For each of the following reactions, give the product that results Two Reactions Represented by Curved Arrows • Most reactions in O-chem involve moving electrons – Every reaction involving electron pairs fits into one of these two categories: 1) Lewis base + Lewis acid 2) Electron-pair displacement reactions • Reactions may be a combination of the two types above 3.3 Review of the Curved-Arrow Notation 15 Problems • For the following reactions, indicate whether you have a Lewis acid-base reaction or an electron pair displacement reaction Curved-Arrow Notation for Resonance • Resonance structures differ only by movement of electrons (and usually electron pairs) • Curved-arrow notation is ideal to help derive resonance contributors • Note: the interconversion of resonance structure by movement of electron is NOT a reaction 3.3 Review of the Curved-Arrow Notation 17 Problems • Using the curved arrow notation, derive resonance structures for the following compounds: 1) Benzene 2) Aniline 3) Diazomethane BrØnsted Acid-Bases Reactions • A Bronsted acid-base reaction involves an electron-pair displacement on a proton • Bronsted Acid: A species that donates a H+ – Keeps the electrons that were bonding to H • Bronsted Bases: A Lewis base that donates its electron pair to a proton (in order to grab it) 19 Conjugate Acids and Bases • When a BrØnsted acid loses a proton, its conjugate base is formed • When a BrØnsted base gains a proton, its conjugate acid is formed 3.4 BrØnsted-Lowry Acids and Bases 20 Amphoteric Compounds • Compounds that can act as either an acid or a base are called amphoteric • Observe the behavior of a compound in a reaction to classify it as an acid or base • Water is amphoteric 22 Problems Identify the conjugate acids for the compounds on the left and the conjugate bases for the compounds on the right. Also, identify all amphoteric compounds • • • • H2O FHCO3SO42- • • • • H2O HCO3HPO42H2S Organic Reactions • The BrØnsted-Lowry acid-base concept is central to many reactions in organic chemistry • For example: • …looks similar to: 3.4 BrØnsted-Lowry Acids and Bases 24 Nucleophiles and Electrophiles • Nucleophile = Lewis base (“nucleus loving”) 3.4 BrØnsted-Lowry Acids and Bases 25 Nucleophiles and Electrophiles • Electrophile = Lewis acid (“electron loving”) • The atom that receives the electron pair 3.4 BrØnsted-Lowry Acids and Bases 26 Leaving Groups • The group or atom that receives electrons from the breaking bond is a leaving group 3.4 BrØnsted-Lowry Acids and Bases 27 Leaving Groups • Can also be applied to Lewis acid-base dissociation reactions 3.4 BrØnsted-Lowry Acids and Bases 28 Problems • Classify each of the following reactions as a Bronsted acid-base reaction or a Lewis acid-base association/dissociation. Identify each species in the following reactions as a Bronsted acid, Bronsted base, Lewis acid, Lewis base, nucleophile, electrophile, and/or leaving group. Draw in the appropriate curved arrow notation where appropriate. H 3 O+ HCl Strengths of BrØnsted Acids • A measure of the extent of proton release to a BrØnsted base • The standard base traditionally used is water • The equilibrium constant is: 3.4 BrØnsted-Lowry Acids and Bases 33 The Dissociation Constant • As [H2O] effectively remains constant: • Each acid has its own dissociation constant • A large Ka = many H+ transferred – Strong acid – Weak conjugate base 3.4 BrØnsted-Lowry Acids and Bases 34 The pKa Scale and pH • pKa = -log Ka • pKa values are more manageable than Ka values • Stronger acids have smaller pKa values • pH is a measure of [H+], a property of a solution (recall: pH = - log[H3O+]) • pKa is a measure of acid strength, a fixed property 3.4 BrØnsted-Lowry Acids and Bases 37 39 Strengths of BrØnsted Bases • Directly related to Ka/pKa of the conjugate acid • If a base is weak, its conjugate acid is strong • If a base is strong, its conjugate acid is weak 3.4 BrØnsted-Lowry Acids and Bases 40 Problems 1) Write out the dissociation constant expression for formic acid, HCO2H, in water 2) Identify the conjugate acid-base pairs in the equation for problem #1 3) Using the Ka for formic acid, calculate the pKa 4) What is the Ka for acetic acid if its pKa = 4.74? Is acetic acid’s conjugate base weaker or stronger than the conjugate base of HF? HF’s Ka = 7.2 x 10-4 Relationship of Structure to Acidity • Which of the following molecules is the weakest acid? Which is the strongest? • • • • HF HCl HBr HI • What about these: 42 The Element Effect • Evaluate the atom attached to the proton • The acidities of Bronsted acids (H-A) increase down a group • Acidities increase as the atomic # of A increases • Due to decrease in bond strength • The acidities of Bronsted acids (H-A) increase across a period from left to right • Due to increasing electronegativity of A 3.6 Relationship of Structure to Acidity 43 45 The Charge Effect • Who is more acidic, H2O or H3O+? • Positively charged compounds attract electrons better than neutral ones • pKa of H2O = 15.7 • pKa of H3O+ = -1.7 3.6 Relationship of Structure to Acidity 46 Problems • Which of the following is the stronger acid? 1) PH3 or SH2 2) H2O or SH2 3) NH3, NH4+, or NH2- 47 The Polar Effect • Which of the following is more acidic? 48 The Polar Effect • The presence of electronegative substituents has an acid strengthening effect = polar effect or inductive effect – Such substituents are said to be electron withdrawing • Acids with stable conjugate bases tend to be more acidic – Resonance = stabilization 49 • Consider the following series: • Which molecule is the most acidic and why? 50 Problems • Rank the following molecules, in each series according to order of increasing acidity and explain your reasoning 51 52 Homework Problems • 3.1 – 3.13, 3.19 – 3.21, 3.24 – 3.37, 3.39, 3.44, 3.49, 3.50 53