3.1 Bronsted-Lowry Acids And Bases - University Of Ottawa

3.1 Bronsted-Lowry Acids and Bases Brønsted-Lowry definition– Acids donate a proton– Bases accept a proton Recall from General Chemistry this classic exampleCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-1conjugateconjugatebaseacidKlein, Organic Chemistry 3e

3.1 Conjugate Acids and Bases Brønsted-Lowry definition– A conjugate acid results when a base accepts a proton– A conjugate base results when an acid gives up a proton Label the acid, base, and the conjugates in the reaction belowCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-2Klein, Organic Chemistry 3e

3.2 Curved Arrows in Reactions Consider a specific acid/base example The base “attacks” the acid, using a pair of electrons The acid cannot lose its proton without the base taking it. Allacid/base reactions occur in one step The mechanism shows two arrows indicating that two pairs ofelectrons move simultaneously (one shows a bond breaking, theother shows the bond being madeCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-3Klein, Organic Chemistry 3e

3.2 Curved Arrows in Reactions A multistep reaction mechanism is shown below Which steps below are proton transfers? Before long, you will be drawing mechanisms like this one. Fornow, just worry about correctly using curved arrows to show acidbase reactions (i.e. proton transfers). Practice with SkillBuilder 3.1Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-4Klein, Organic Chemistry 3e

3.3 Quantifying Acidity Recall from General Chemistry, how do “strong” acids/bases differfrom “weak” acids/bases? The strength of an acid or base is helpful to predict how reactionswill progress– We will learn to do Quantitative strength analysis – using pKavalues to compare the strengths of acids– We will learn to do Qualitative strength analysis – comparingthe general stability of structures.Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-5Klein, Organic Chemistry 3e

3.3 Quantifying Acidity Quantitative strength analysis – using numerical data to comparehow strong acids are. Ka is the acid dissociation constant of an acid dissolved in water. Itis the measurement of an acid’s strength what water is the base. If the acid is strong, will Ka be bigger than 1, or smaller than 1?Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-6Klein, Organic Chemistry 3e

3.3 Quantifying Acidity Ka values range from 10-50 to 1010 and so the size of thesenumbers (very small or very big) are hard to work with. If you take the -log of the Ka, that will focus you on the exponentof the Ka value, which ranges from -10 to 50 So, pKa values range from -10 to 50. Lower pka stronger acidCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-7Klein, Organic Chemistry 3e

3.3 Quantifying Acidity There are more acids and pKavalues in Table 3.1 and the insidecover of your textbook Each pKa unit represents anorder of magnitude or a powerof 10. For example, H2SO4 (pKa -9) is100 times stronger acid than HCl(pKa -7) Practice with SkillBuilder 3.2Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-8Klein, Organic Chemistry 3e

3.3 Quantifying BasicityYou can also use pKavalues to comparethe strengths ofbases because The stronger anacid the weaker itsconjugate base. Practice with SkillBuilder 3.3Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-9Klein, Organic Chemistry 3e

3.3 Using pKa values to predict equilibria With the relevant pKa values, you can predict which direction anacid/base equilibrium will favor. Higher pKa weaker acid This reaction demonstrates what is ALWAYS true in an acid-basereaction: equilibrium favors the weaker acid and weaker base!!Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-10Klein, Organic Chemistry 3e

3.3 Using pKas to analyze Equilibria Subtracting the pKa values, (50 - 15.7 34) also tells you that therewill be 1034 more products than reactants. It’s not really much of an equilibrium, and more like an irreversiblereaction Practice with SkillBuilder 3.4 and checkpoint 3.12Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-11Klein, Organic Chemistry 3e

3.4 Qualifying Acidity to determine the relative strength of two acids, without knowingtheir pKa values, we compare the stability of their conjugatebases The stronger the acid, the more stable it’s conjugate base! When an acid loses a proton, it forms the conjugate base, whichhas a lone pair of electrons that resulted from the lose of H To determine the stability of a conjugate base, we are actuallylooking at the stability of the lone pairCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-12Klein, Organic Chemistry 3e

3.4 Qualifying Acidity The more effectively a conjugate base can stabilize its negativecharge (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 chargeResonanceInductionThe type of orbital where the charge resides These factors can be remembered with the acronym, ARIOCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-13Klein, Organic Chemistry 3e

on will develop a methodical approach for comparing negative c3.4 QualifyingAcidityll consider four factors:(1) the atom bearingthe charge, (2) resonaals. ARIO - The type of atom that carries the chargebears the charge? The first factor involves comparing the atoms bege in each conjugate base. For example, consider the structures of bIn order to compare the acidity of the two compounds belowHButaneOHPropanolhe relativeacidityof thesetwo compounds,we mustfirstdeprotonaWe needto drawand thenanalyze the stabilityof thenegativeraw theconjugatechargeon the bases:conjugate bases O hese conjugate bases by looking at where the negative charge is locatKlein,Chemistryconj3eate base, the negative charge is on3-14a carbon atom.InOrganicthe secondCopyright 2017 John Wiley & Sons, Inc. All rights reserved.

3.4 Qualifying AcidityOHHButanePropanol ARIO - The type of atom that carries the chargeHere,aciditywe can ofdeterminewhetheran oxygena carbonhe relativethese lizebases:a negative chargeraw theconjugateLess stable O More stablethese conjugatebaseslookingat wherethea negativechargewillis locat(1) The largerthebyatom,the morestablenegative chargeate base, thenegativeis on a carbonbe (sizeis thechargemost importantfactor) atom. In the second conon an oxygen atom. To determine which of these is more stable, wements arein theC samein samethe samecolumnofsimilarthe periodic(2) Sinceand Oroware inortheperiod,they aresizes. tabIn this case, the more electronegative atom will betterstabilize the negative chargeIn the same rowIn the same columnCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-15Klein, Organic Chemistry 3e

3.4 Qualifying AcidityOHHButanePropanol ARIO - The type of atom that carries the chargerelativestabilityof thetwobasestells us the relativestrengthof thehe Therelativeacidityof thesecompounds,we mustfirst deprotonaacidsrawthe conjugate bases: OLess stable More stablethese conjugate bases by looking at where the negative charge is locatate base, the negative charge is on a carbon atom. In the second conon an oxygenatom. To determine which of theseMOREis moreLess acidicACIDICstable, wements are in the same row or in the same column of the periodic tabIn the same rowCopyright 2017 John Wiley & Sons, Inc. All rights reserved.In the same column3-16Klein, Organic Chemistry 3e

3.4 Qualifying Acidity ARIO - Resonance stabilizes a negative charge (i.e. lone pair) byspreading it out across multiple atoms Compare the acidity of the two compounds below by comparingthe stabilities of their conjugate bases.Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-17Klein, Organic Chemistry 3e

OO3.4 QualifyingAcidityO OOO OOO In both cases,negativechargeis onSuchoxygen.Therefore,factorwill1 dobs case, the charge is delocalizedoverthebothoxygenatoms.a negativechargehisthe- Resonancechargeisdelocalizedbothoxygenatoms.a negati case,But onthereis overa criticaldifferencebetweentheseSuchtwo oneoxygenatom:has no resonancethe secondle than a negative chargelocalizedstructures,on onewhileoxygenatom:conjugate base does:Compare thestability ofthese conjugatebasesOO Charge is localized(less stable)O versus OO Charge is delocalizedOO(more stable)O OOChargeis localizedis oxygendelocalizedIn this case,the chargeis delocalized Chargeover bothatoms. Such aFor this reason, compoundscontainingaC Obonddirectlynexttoanstable)(more stable) OH are gestable than a (lessnegativecharge localized on oneoxygen atom:y acidic, because their conjugate bases are resonance stabilized:Nowknow therelative containing a C O bond directly Onext toForthiswereason,compounds stabilityoftheacids(whichcanOOO–Odly acidic, because their conjugatebases are resonancestabilized:OHHbe confirmed by lookingupROtheir pKa values) O A carboxylic acid is localizedO ChargeR(less stable)RO–H Charge is delocalized (more stable)OOResonance-stabilizedFor this reason, compoundscontainingaMOREC OACIDICbond directlyconjugatebaseLessacidic Hmildlyacidic,because their conjugatebasesO are resonance stabilized:RROROPractice with Skillbuilder 3.6A carboxylic acidCopyright 2017 John Wiley & Sons, Inc. All rights reserved.–Resonance-stabilizedH OO3-18HconjugatebaseKlein,OrganicChemistry 3e

3.4 Qualifying Acidity ARIO - Induction can also stabilize a formal negative charge byspreading it out. How is induction different from resonance? Electron withdrawing atoms/groups inductively withdraw electrondensity from their surroundings, thus stabilizing a negativecharge.less acidicCopyright 2017 John Wiley & Sons, Inc. All rights reserved.more acidic3-19Klein, Organic Chemistry 3e

3.4 Qualifying Acidity More electron withdrawing groups more stable conjugate base The closer the electron withdrawing groups to the negative charge more stable the conjugate base Practice with SkillBuilder 3.7Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-20Klein, Organic Chemistry 3e

3.4 Qualifying Acidity ARIO - The type of orbital also can affect the stability of a formalnegative charge The closer electrons are held to the nucleus, the the more stablethey are. The shorter the atomicorbital, the closer to thenucleus.Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-21Klein, Organic Chemistry 3e

3.4 Qualifying Acidity ARIO - The type of orbital also can affect the stability of a formalnegative charge Consider the relative stability of the H’s indicated below: To predict which H is more acidic, we first have to draw the twopossible conjugate basesversus Which carbanion is more stable?Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-22Klein, Organic Chemistry 3e

3.4 Qualifying Acidity ARIO - The type of orbital also can affect the stability of a negativecharge. The more s-character in the orbital, the more stable thenegative charge.versusLone pair in a sp2orbital, not as closeto the nucleusLone pair in a sporbital, closer to thenucleusLESS STABLEMORE STABLEless acidicCopyright 2017 John Wiley & Sons, Inc. All rights reserved.MORE ACIDIC3-23Klein, Organic Chemistry 3e

3.4 Qualifying Acidity Compare the acidity of the compounds below by comparing thestabilities of their conjugate bases. Practice with SkillBuilder 3.8Copyright 2017 John Wiley & Sons, Inc. All rights reserved.3-24Klein, Organic Chemistry 3e

3.4 Qualifying Acidity When assessing the acidity of protons, we generally use ARIO asthe order of importance of these stabilizing effects.1.2.3.4.The type of atom that carries the chargeResonanceInductionThe type of orbital where the charge resides It is typically helpful to use this order of priority when comparingthe stability of conjugate bases, but it isn’t 100% reliable: thereare exceptionsCopyright 2017 John Wiley & Sons, Inc. All rights reserved.3-25Klein, Organic Chemistry 3e