CHE2060 Lecture 5: Acid-base chemistry 5.1 Acids & bases: overview & basics 5.2 Acid & base strength 5.3 Equilibrium acid-base reactions 5.4 The leveling effect of solvents 5.5 Estimation of acidity by conceptual knowledge 5.6 Classes of organic acids & bases 5.7 Functional groups: acid-base nature Daley & Daley Chapter 5: Acid-base theory

Estimating acidity …by using conceptual knowledge of the factors that increase or decrease acidity

How do we estimate acidity And what makes some protons (H atoms) more acidic than others? Estimate pKa by finding a similar group in a table or chart of pKa values. Look at the EN values of close neighbors (inductive effect). Look for nearby multiple bonds (π bonds). Is the H attached to a positively charged atom? What is the row position of the atom the H is bound to? What is the column position of the atom the H is bound to? What is the orbital hybridization of the atom the H is bound to? What is bound to the atom that the H atom is bound to? (pattern) M Karty, p.304

1. Estimating pKa Functional groups are the reactive parts of organic molecules, and pKa values are one great way to predict the outcome of reactions. Some functional group pKa tables can be found in pKa tables. What if you can’t find a pKa? 1. You can estimate pKa by finding a similar group in a table or chart of pKa values. Estimate the pKa values of these two molecules. Similar to ethanol: pKa 16 Actual isopropanol pKa is 16.5 M Similar to acetic acid: pKa 4.75 Actual benzoic acid pKa is 4.20 Karty, p.304

2. Inductive effect of high EN neighbors Electronegative atom(s) or group(s) close to the hydrogen of interest will increase acidity by pulling electrons through the molecule and towards the electronegative group, making the bond to H more polar and therefore more reactive. Anions are stabilized by EN groups near the negative charge. Cations are destabilized by EN groups near the positive charge. The electronegative π bonds pull electrons through the structure and towards themselves. This increases the polarity of the O – H bond and therefore increases acidity; pKa is significantly lowered. pKa 0.77 M pKa 10 pKa 4.76 The electronegative chlorine atoms pull electrons through the structure and towards themselves. This increases the polarity of the O – H bond and therefore increases acidity; pKa is significantly lowered. pKa 16 Karty, p.304

3. Nearby π bonds stabilize conjugate anions Acids are more powerful (lower pKa) when their conjugate bases are stable. Resonance stabilizes anions formed by reaction of acids. pKa 16 Ethanoic acid (aka acetic acid) forms the acetate anion when deprotonated. The acetate anion is stabilized by resonance. Note that the negative charge is delocalized over several atoms in the resonance hybrid. Delocalization of charge increases stability. pKa 4.76 M Ethanol is deprotonated to form its conjugate base, ethoxide. The negatively charged oxygen atom is fairly stable due to its high EN. Karty, p.304

3. Nearby π bonds stabilize conjugate anions Acids are more powerful (lower pKa) when their conjugate bases are stable. Resonance stabilizes anions formed by reaction of acids. EPM p.321 Karty M Karty, p.304

4. H on positively charged atoms Hydrogen atoms bound positively charged atoms are more acidic than those bound to uncharged atoms. H2O H3O+1 NH3 NH4+1 pKa: 15.7 - 1.7 36 9.4 Why? Generally, charged atoms are less stable, so higher energy. The products are both charged but the reactants are not. The reaction is not energetically favorable. M One reactant & one product are charged. The reaction is less energetically unfavorable. O Is more electronegative & better able to carry the charge. pKa: 15.7 - 1.7 pKa: 9.4 15.7 36 - 1.7 Karty, p.312-14

5. Effect of row position (EN) on pKa The further to the right an atom is, the more acidic are its hydrogen atoms. Because atoms become more electronegative from left to right. More electronegative atoms hold onto their electrons more strongly, polarizing the bond to hydrogen and weakening it. CH4 NH3 OH2 pKa 48 36 15.7 The stability of anions increases as the charged atom’s electronegativity increases. Because more electronegative atoms are able to hold free electron pairs. .. .. .. H3C:- H2N:- HO:- :F:- .. .. electronegativity anion stability M The stability of cations decreases as the charged atom’s electronegativity increases. Because less electronegative atoms are able to give up free electron pairs. H4N+ H3O+ H2F+ electronegativity cation stability Karty, p.312-14

6. Effect of column position (size) on pKa The further down a column an atom is, the more acidic are its hydrogen atoms. Because atoms become larger from top to bottom. Larger atoms (ions) have more room for free electron pairs: lower charge density. Which has a lower pKa value, CH4 or SiH4? Carbon is in row 2 of column IVA while Si is in row 3 of column IVA; Si is therefore larger than carbon. pKa = 48 M pKa = 35 So the conjugate base (-:SiH3) formed by deprotonation of SiH4 is more stable than the conjugate base (-:CH3) formed by deprotonation of CH4. Karty, p.312-14

7. Effect of orbital hybridization Orbital hybridization of the atom the H is bonded to affects that atoms ‘effective electronegativity’. Effective electronegativity: sp > sp2 > sp3 The stability of a charged molecule increases as the effective EN of negatively charged atoms increases. The stability of a charged molecule decreases as the effective EN of positively charged atoms increases Which is a stronger acid? (CH3)2C = OH+ or (CH3)2CH - OH2+? pKa = 20 Less stable cation, so stronger acid. M Check? Stronger acids have weaker conjugate bases. Strength 0f base increases with pKa. More stable cation, so weaker acid. pKa = 16.5 Karty, p.312-14

8. Effect of 1°, 2°, or 3° ions Molecular geometry can affect the stability of carbocations and carbanions. Carbocations are molecules in which carbon is positively charged. Carbanions are molecules in which carbon is negatively charged. Methyl: carbon bound only to hydrogen atoms Primary: carbon bound to one alkyl group Secondary: carbon bound to two alkyl groups Tertiary: carbon bound to three alkyl groups Delocalization increases ion stability. Least stable M Most stable Karty, p.312-14

Quick checklist? Try this quick set of questions to assess relative acid strength or pKa. 1. Do the molecules have different charges? + charges increase acidity - charges decrease acidity 2. Do the molecules carry charges on different atoms? size EN hybridization 1°, 2°, 3° pattern 3. Does either molecule have resonance? Resonance stabilizes charge of conjugate bases by delocalization. M 4. Do inductive effects occur in either molecule? Nearby EN atoms increase acidity Karty, p.312-14

Example Estimate the pKa for the hydrogen atoms shown in these molecules. The ring doesn’t include significantly electronegative atoms or bonds. So, pKa is similar most amines like dimethyl amine. pKa ~ 38 This is the protonated version of diethyl ether. Protonation and a positive charge increase acidity, reducing pKa. Compare hydronium to water. pKa < 45 This ring has electronegative π bonds. So, pKa will be lower (more acidic) than propanone. pKa < 20 M Karty, p.310 -11

Example Estimate the relative acidities of: Ethane Ethene Ethyne (aka acetylene) Least acidic The stability of the conjugate base (anion) increases from sp3 to sp. Note that the most acidic H is attached to the carbon with the most π bonds. M Most acidic Karty, p.310 -11

Example Which molecule is more acidic based on structure? More acidic: Replacement of H with more EN Cl destabilizes the positively charged molecule. Loss (donation) of H removes the positive charge and increases stability. Inductive effect M Karty, p.312-14

Example Which alcohol is more acidic based on structure? More acidic: Replacement of H with more EN Cl stabilizes the negatively charged conjugate base. A more stable base allows the H to leave. Inductive effect M EPM Karty, p.324 Karty, p.312-14

Example Rank the relative acidities of these molecules. Least acidic (4) The EN F is located far from the polar OH hydrogen. (3) The EN F is located closer to the polar OH hydrogen. Most acidic (1) Positive charge causes this molecule to be least stable. Losing a H will remove the charge and stabilize the molecule. (2) Two EN groups (F and =O) withdraw electrons and increase the polarity of the OH hydrogen. M Karty, p.312-14