Chapter 3 - Acids And Bases

Chapter 3Acids and BasesCHEM 2301 – Dr. Houston Brown ‐ 20211‐1

3.1 Bronsted‐Lowry Acids and BasesBrønsted‐Lowry definition Acids donate a proton Bases accept a protonRecall from General Chemistry this classic exampleconjugateconjugatebaseacid1‐2CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.1 Conjugate Acids and BasesBrønsted‐Lowry definition A conjugate acid results when a base accepts a proton A conjugate base results when an acid gives up a protonLabel the acid, base, and the conjugates in the reaction below1‐3CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.2 Curved Arrows in ReactionsThe making and breaking of bonds involves electron movementWe use curved arrows to describe the flow of electron densityThey are the same as curved arrows used to draw resonance structures, BUT here, the curved arrows are actually describing the physical movement of electrons!!!Learning to draw mechanisms is one of the most valuable skills in this class1‐4CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.2 Curved Arrows in ReactionsConsider a specific acid/base exampleThe base “attacks” the acid, using a pair of electronsThe acid cannot lose its proton without the base taking it. All acid/base reactions occur in onestepThe mechanism shows two arrows indicating that two pairs of electrons move simultaneously(one shows a bond breaking, the other shows the bond being made1‐5CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.2 Curved Arrows in ReactionsA multistep reaction mechanism is shown belowWhich steps below are proton transfers?Before long, you will be drawing mechanisms like this one. For now, just worry aboutcorrectly using curved arrows to show acid‐base reactions (i.e. proton transfers).1‐6CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.3 Quantifying AcidityRecall from General Chemistry, how do “strong” acids/bases differ from “weak”acids/bases?The strength of an acid or base is helpful to predict how reactions will progress We will learn to do Quantitative strength analysis – using pKa values to compare the strengths of acids We will learn to do Qualitative strength analysis – comparing the general stability of structures.1‐7CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.3 Quantifying AcidityQuantitative strength analysis – using numerical data to compare how strong acids are.Ka is the acid dissociation constant of an acid dissolved in water. It is the measurementof an acid’s strength what water is the base.If the acid is strong, will Ka be bigger than 1, or smaller than 1?1‐8CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.3 Quantifying AcidityKa values range from 10‐50 to 1010 and so the size of these numbers (very smallor very big) are hard to work with.If you take the ‐log of the Ka, that will focus you on the exponent of the Ka value,which ranges from ‐10 to 50So, pKa values range from ‐10 to 50. Lower pka stronger acid1‐9CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.3 Quantifying AcidityThere are more acids and pKa values inTable 3.1 and the “inside cover of yourtextbook”. (Since you have an e‐book,search for “pka values” and you willfind this table.Each pKa unit represents an order ofmagnitude or a power of 10.For example, H2SO4 (pKa ‐9) is a 100times stronger acid than HCl (pKa ‐7)1‐10CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.3 Quantifying BasicityYou can also use pKa values to comparethe strengths of bases because The stronger an acid the weaker itsconjugate base.1‐11CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.3 Uing pKa values to predict equilibriaWith the relevant pKa values, you can predict which direction an acid/baseequilibrium will favor. Higher pKa weaker acidThis reaction demonstrates what is ALWAYS true in an acid‐base reaction:equilibrium favors the weaker acid and weaker base!!1‐12CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.3 Using pKas to analyze EquilibriaSubtracting the pKa values, (50 ‐ 15.7 34) also tells you that there will be 1034more products than reactants.It’s not much of an equilibrium, and more like an irreversible reaction** Your instructor hates this term 1‐13CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityTo determine the relative strength of two acids, without knowing their pKavalues, we compare the stability of their conjugate bases.The stronger the acid, the more stable it’s conjugate base!When an acid loses a proton, it forms the conjugate base, which has a lone pairof electrons that resulted from the loss of H To determine the stability of a conjugate base, we are actually looking at thestability of the lone pair1‐14CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityThe more effectively a conjugate base can stabilize its negative charge(i.e. lone pair), the stronger the acid.Four main factors affect the stability of a negative charge: The type of atom that carries the charge Resonance Induction The type of orbital where the charge residesThese factors can be remembered with the acronym, ARIO1‐15CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ The type of atom that carries the chargeIn order to compare the acidity of the two compounds belowWe need to draw and then analyze the stability of the negative charge on the conjugate basesO1‐16CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ The type of atom that carries the chargeHere, we can determine whether an oxygen or a carbon will better stabilize a negative chargeOLess stableMore stable(1) The larger the atom, the more stable a negative charge will be (size is the most important factor)(2) Since C and O are in the same period, they are similar sizes. In this case, the more electronegativeatom will better stabilize the negative charge1‐17CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ The type of atom that carries the chargeThe relative stability of the bases tells us the relative strength of the acidsOLess stableLess acidicMore stableMORE ACIDIC1‐18CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ Resonance stabilizes a negative charge (i.e. lone pair) by spreading it outacross multiple atomsCompare the acidity of the two compounds below by comparing the stabilitiesof their conjugate bases.1‐19CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ ResonanceCompare thestability ofthese conjugatebasesversusNow we know the relativestability of the acids (which canbe confirmed by looking uptheir pKa values)Less acidic1‐20CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and SonsMORE ACIDIC

3.4 Qualifying AcidityARIO ‐ Induction can also stabilize a formal negative charge by spreading it out.How is induction different from resonance?Electron withdrawing atoms/groups inductively withdraw electron density fromtheir surroundings, thus stabilizing a negative charge.less acidicmore acidic1‐21CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityMore electron withdrawing groups more stable conjugate baseThe closer the electron withdrawing groups to the negative charge more stablethe conjugate base1‐22CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ The type of orbital also can affect the stability of a formal negative chargeThe closer electrons are held to the nucleus, the the more stable they are.The shorter the atomic orbital,the closer to the nucleus.1‐23CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ The type of orbital also can affect the stability of a formal negative chargeConsider the relative stability of the H’s indicated below:To predict which H is more acidic, we first have to draw the two possible conjugate basesversusWhich carbanion is more stable?1‐24CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO ‐ The type of orbital also can affect the stability of a negative charge. Themore s‐character in the orbital, the more stable the negative charge.versusLone pair in a sp2orbital, not as closeto the nucleusLone pair in a sporbital, closer to thenucleusLESS STABLEMORE STABLEless acidicMORE ACIDIC1‐25CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityCompare the acidity of the compounds below by comparing the stabilities oftheir conjugate bases.1‐26CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityWhen assessing the acidity of protons, we generally use ARIO as the order ofimportance of these stabilizing effects.1.2.3.4.The type of atom that carries the chargeResonanceInductionThe type of orbital where the charge residesIt is typically helpful to use this order of priority when comparing thestability of conjugate bases, but it isn’t 100% reliable: there areexceptions1‐27CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityEthanol is more acidic than propylene. Therefore, the conjugate base of ethanol must be morestable.pKa 16pKa 43More stableLess stableThe type of atom (O vs. C) is consistent with this fact.But, propylene’s conjugate base is resonance stabilized, which would suggest it is more stableSo, in this case, our order of priority (ARIO) is accurate.1‐28CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityARIO is only a guideline of priority it sometimes failsIn this example, we know equilibrium lies to the right because we know the pkavaluesIf we had judged the conjugate base stability, we would’ve concluded that negative charge on Nis more stable than C, and predicted equilibrium to lie to the left, and we would’ve been wrongConclusion: for some acids, we simply need to know the pKa values becausethey are exceptions to the ARIO priority rule.1‐29CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.4 Qualifying AcidityPractice the Skill 3.23 – Predict which proton (red vs. blue) is more acidic ineach of these compounds.1‐30CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.5 Predicting Equilibrium PositionConsider any acid base reaction:There are two distinct ways to predict which side is favored at equilibrium1. the pKa values of H‐A and H‐B (the higher pKa will be favored)2. The relative stability of the bases, B‐ and A‐1‐31CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.5 Choosing a ReagentAnother important skill is to be able to choosean appropriate reagent for a acid/basereactionChoose an acid from Table 3.1 that couldeffectively protonate each of the followingmolecules1‐32CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons

3.6 Leveling EffectAnother important skill is to be able to choose an appropriate solvent for aacid/base reactionThe solvent should be able to surround the reactants and facilitate theircollisions without itself reactingBecause water can act as an acid or a base, it has a leveling effect on strongacids and bases Acids stronger than H3O can not be used in water. Bases stronger than OH‐ can not be used in water. WHY? – see next few slides1‐33CHEM 2301 – Dr. Houston Brown – 2021 ‐ Source: John Wiley and Sons