CHE2060 Lecture 5: Acid-base chemistry. CHE2060 Lecture 5: Acid-base chemistry

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1 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 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 1

2 Acids & bases Overview & definitions Basic acid-base reaction Electrophiles & neutrophiles Bases vs. Nu An orbital view Acids & bases: a shift in perspective 1884 Arrhenius Acids are sources of H+ ions; bases source of OH- ions 1923 Brønsted-Lowry Acids are H+ donors; bases source of H+ acceptors 1916 G.N. Lewis Acids accept electron pairs (:); bases donate : M&B p.6-7 2

3 Acids & bases: a shift in perspective 1884 Arrhenius Acids are sources of H+ ions; bases source of OH- ions 1923 Brønsted-Lowry Acids are H+ donors; bases source of H+ acceptors 1916 G.N. Lewis Acids accept electron pairs (:); bases donate : Note that this reaction moves a pair of electrons in a single-step reaction. A free pair from the hydroxide oxygen forms a dative bond with the hydrogen ion to form water. M&B p.6-7 Acids & bases: a shift in perspective 1884 Arrhenius Acids are sources of H+ ions; bases source of OH- ions 1923 Brønsted-Lowry Acids are H+ donors; bases source of H+ acceptors 1916 G.N. Lewis Acids accept electron pairs (:); bases donate : Note that this reaction moves a pair of electrons in a single-step reaction. A free pair from the hydroxide oxygen forms a dative bond with the hydrogen ion to form water. M&B p.6-7 3

4 4/10/17 Acids & bases Acid-base reactions are central to biochemistry because most biological reactions are catalyzed by acids or bases. So we must understand the basics of acid-base reactions and be able to answer these questions: Where s the acid? Where s the base? How can the two react? What will happen? Brønsted-Lowry acids & bases: acids donate protons (H+1) & bases accept them Conjugate pairs: before & after loss of proton N s free pair forms a dative bond; π become free pair, breaking bond. M&B p.6-7 Examples: draw the products Karty 4

5 Examples: donor? acceptor? products? Step 1. Line-bond structures, showing all free : M&B p.6-7 Examples: donor? acceptor? products? Step 1. Line-bond structures, showing all free : Step 2. Identify LA & LB M&B p.6-7 5

6 Examples: donor? acceptor? products? Step 1. Line-bond structures, showing all free : Step 2. Identify LA & LB Step 3. Free : from the LB move to create a dative bond with the LA M&B p.6-7 Examples: donor? acceptor? products? Step 1. Line-bond structures, showing all free : Step 2. Identify LA & LB Step 3. Free : from the LB move to create a dative bond with the LA Step 4. Create the products M&B p.6-7 6

7 Examples: donor? acceptor? products? Step 1. Line-bond structures, showing all free : Step 2. Identify LA & LB Step 3. Free : from the LB move to create a dative bond with the LA Step 4. Create the products. Step 5. Identify the conjugate acid & conjugate base Conjugate acid-base pair: two molecules that differ only by one H! Note that these reactions move one H, and or one pair of electrons. M&B p.6-7 Examples: donor? acceptor? products? Which product is the conjugate acid & conjugate base? conj base conj acid M&B p.6-7 7

8 4/10/17 Lewis acids & bases Critical for organic chemistry and acids don t always have H! Lewis acid: accepts an e- pair from a Lewis base.. Doesn t have to be H+1 Lewis base: donates an e- pair from to Lewis acid TCA cycle example: Here Fe is the Lewis acid in the enzyme s cofactor. Metal cations often act cofactor acids/bases in enzymes. M&B p.10 Electrophiles & nucelophiles Electrophiles & nucleophiles are what organic chemists call Lewis acids & Lewis bases. Electrophile: + charged & therefore e- loving; looking for an e-rich nucleophile E+ = acids (H+1); trialkylsulfonium (R3S+1); carbonyls (δ+c=o) Nucleophile: e-rich (sometimes -) & looking for an e- seeking electrophile Nu: = amines (:NH2); water (H2O); hydroxide ion (OH-1); alkoxide ions (RO-1); thiolate ions (RS-1) Nu:- Nu: E+ E M&B p.11 8

9 Who s who? A common theme of organic reactions is nucleophilic attack: a nucleophile uses its electron pairs to attack an electrophile, a chemical with low electron density. It s critical to be able to identify reacting chemicals as Nu: or E+. 1. Identify each player. 2. Predict the outcome of the attack... E+ δ- :O:.. Cl: δ+.. Nu: :OH.. δ- δ+ E+ Nu:.. :O:.. OH E+ Nu: OH2.. Klein p.178 Bases vs. Nu: Most organic rxns involve either bases or Nu: & students often confuse them. The difference is purely functional because an ion, molecule or group may function as both. Base: the group removes a proton and takes it away Nu: attacks the other reactant & is added to it Hydroxide can act as either a base or a Nu:. Which is it doing in these examples? OH acting as a base: removes proton & becomes H2O OH acting as a Nu; it s added to the other reactant Klein p.179 9

10 4/10/17 Examples: bases vs. Nu: For each example below, determine whether hydroxide or water is acting as a base or a Nu:, and predict the outcome. base Nu:.. :O:.. Cl:.. :OH.. Nu: :O:.... H2O + :O.. S O.. H :O: :O: H3O+1 + base base Klein p An orbital perspective Let s look at electrophiles, nucleophiles & how they react at the level of obitals: sp3 base unbonded : Nu: empty p acid not enough e- NH3 sp3 overlaps with BF3 p NH3 donates its : to form the bond BF3 becomes sp3 hybridized The new molecule has no overall charge, but does have formal charges Dative bonds occupy empty orbitals by fusing them with full orbitals. D&D p

11 Acidity of alkanes vs. alkenes vs. alkynes The relative acidity of alkanes, alkenes and alkynes is a result of the shapes & geometry of their hybrid orbitals. alkane sp3 alkene sp2 alkyne sp acidity increases Why? The shorter, more spherical shape of the sp alkyene orbitals places electrons closer to the nucleus. The more tightly electrons are held to the nucleus, the less they are pulled toward hydrogen atoms they are bonded to. Alkynes are therefore better acids than alkenes and alkanes.... becuase their hydrogen are less tightly bound. D&D, p.213 In biochemistry Biochemical examples of acid-base (aka nucleophile-electrophile) reactions. M&B p.11 11

12 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 Acid & base strength Ionization & ph Strength: Ka & pka Common organic acids & bases Predicting direction of equilibrium 12

13 Ionization of water & ph The strength of a Brønsted-Lowry acid is determined by the extent to which it ionizes in water: autoionization. hydronium ion Because both the acid and base are water, the degree of dissociation or auto-ionization is very slight. For water, the concentration of the dissociated products are constant, Kw. Kw = [H3O +1 ][OH -1 ] = Kw = 1.00 x While Kw is constant, the concentration of hydronium ion can change greatly. It is this [H3O +1 ] that chemists measure as ph! ph = -log[h3o +1 ] often expressed as ph = -log[h +1 ] M&B p.6-8 Acid strength is measured as Ka or pka An acid s strength is measured as its dissociation constant, Ka. So, imagine an acid, Ka: hydronium ion The larger the Ka value stronger the acid s strength. Strong acids have pka values between at zero or below. Most organic acids are quite weak and have Ka values between 10-2 and To make it easier to compare this very wide range of values, Ka is often converted to pka, a logarithmic scale: pka = -log(ka) Since this scale is inverse logarithmic, but lower the pka the stronger the acid. D&D, p

14 Common pka values weakest acid Strongest acid D&D, p.217 pka s of common organic acids M&B p

15 Examples: using pka to gauge acid strength Which of each pair is the stronger acid? And what are the molecules named? p-methylphenol p-chlorophenol How much stronger is the p-chlorophenol? The difference in pka values = = The difference in acid strength = = 6.8. So p-chlorophenol is 6.8 times as strong an acid. Karty p.300 Examples: pka for most acidic proton? For each molecule, identify the most acidic proton (hydrogen). methyl 48 carboxylic acid 4.75 phenol 10 ethanol 16 benzoic acid 4.2 thiol 10.3 hydronium -1.7 methyl ammonium amine 38 Karty p

16 Strong bases are weak acids. You can predict the strength of a base if you know the Ka or pka values of the base s conjugate acid. So bases are stronger when their conjugate acids have: lower Ka values; or higher pka values. So which is the stronger base, hydroxide (OH -1 ) or ammonia (NH3)? The conjugate acid of hydroxide (OH -1 ) is water: pka 15.7 The conjugate acid of ammonia is ammonium: pka 9.4 So, hydroxide is the stronger base. How much stronger? Difference in pka: = 6.3 Difference in strength: = 2.0E6 = 2 million times stronger! Karty p.301 Example: predict the strength of a base Which is the stronger base, chloride ion or phenoxide ion (C6H5O -1 )? The conjugate acid of chloride ion is hydrochloric acid: pka ~ - 7 The conjugate acid of phenoxide ion is phenol: pka 10 So, phenoxide ion is the stronger base. How much stronger? Difference in pka: 10 (-7) = 17 Difference in strength: = 1 E17 times stronger! Karty p

17 pka s of common organic bases Stength of bases is measured by the acidity of their conjugate acid. pka + pkb = 14 -> pkb = 14 - pka So stronger bases have higher pka values. M&B p.8-9 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 17

18 Equilibrium reactions Using pka to predict whether reactants or products predominate Equilibrium of conjugate acid-base pairs When a reaction is reversible, the side with the weaker conjugate acid (higher pka) is favored. Are reactants or products are favored here? pka = 15.1 pka = 36 pka ~ 4.76 pka = pka ~ 26 pka = 35 pka = 4.76 pka ~ D&D, p

19 Why is the weaker conjugate acid favored? It s as if the two acids, on the reactant & product sides, of the reaction are in competition with one another. In essence, the stronger acid wins and acts like an acid, accepting an electron pair, and driving the reaction to producing the weaker acid and the conjugate base of the stronger acid. B: - + H A B H + :A- base acid conjugate conjugate acid base Karty p.302 A more detailed look the driving acid Let s look at a specific example with numbers. The strong acid pushes equilibrium to the right acetic acid methyl acetate methyl amine ion ammonium pka 4.76 pka stronger equilibrium We can see how strongly the reaction favors products by calculating Δ pka. Δ pka = = 5.88 So the equilibrium constant for this reaction can be calculated: Keq = = 7.6E10 5 In other words, products are favored by a factor of 760,000. Karty p

20 Example: quantitate the push Predict which side is favored and by how much. 2-methyl- 2-propoxide propanone 2-methyl- 2-propanol pka 20 pka 19 equilibrium propanone anion We can see how strongly the reaction favors reactants by calculating Δ pka. Δ pka = = 1 So the equilibrium constant for this reaction can be calculated: Keq = 10 1 = 1o In other words, reactants are favored by a factor of 10. Barely pushed. Karty p.302 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 20

21 The leveling effect Solvents may hijack intended reactions Remember solvents? Solvents are compounds that are: Present in large amounts (larger than solvents); Used to create a soluble environment for solute reactants; and Sometimes plays a role in solute reactions. Everyday example: water is the solvent for extraction of caffeine and other coffee or tea compounds from beans or leaves. Ideally, solvents either have no impact on reactions or actually enhance them. However, if solvents are not selected carefully they can interfere with or hijack our intended reactions. Karty, p

22 So, what s leveling? Leveling is a reaction of the solvent with one of the reactants. This reaction uses up some or most of that reactant and restricts the intended reaction. Acids & bases are often impacted by leveling because they react with a wide variety of solvents, like water. pka ~ - 7 pka ~ So HCl is a powerful acid until it is diluted in water. Once HCl reacts with water, the only acid remaining is hydronium which is 1.99E5 times weaker than HCl. Water levels (or reduces or restricts) the strength of HCl. Karty, p.304 Base strength can also be leveled by solvents. Deprotonated dimethyl amine is a base. Note its two free electron pairs. pka 15.7 pka 38 Deprotonated dimethyl amine is a powerful base. Its conjugate acid, dimethyl amine, has a pka of 38! For bases, the equilibrium is pushed towards the acid with the higher pka. The strength of deprotonated dimethyl amine pushes equilibrium towards products by a factor of 2.oE22! However, if water is used as a solvent when deprotonated dimethyl amine is used as a base, water will act as an acid, react with and neutralize the powerful base. Reaction with the water solvent produces hydroxide as the remaining base. And hydroxide is a weaker base than deprotonated dimethylamine. Karty, p

23 Conclusions about leveling If water is used as the solvent, no acid stronger than hydronium, and no base weaker than hydroxide, can exist to appreciable extents. For other solvents: The strongest acid that can exist in solution, to appreciable concentrations, is the protonated solvent. The strongest base that can exist in solution, to appreciable concentration, is the deprotonated solvent. Take-home message? Solvents must be chosen carefully! Karty, p.304 Example: wise choice of solvent If you want to use deprotonated dimethyl amine as a base, diethyl ether is a better solvent than water. Use pka values to show that diethyl ether is a better solvent! pka 45 pka 38 For bases, the equilibrium is pushed towards the acid with the higher pka, so when deprotonated dimethyl amine is placed in diethyl ether solvent the two barely react. Karty, p

24 Example: you choose the solvent! Would each of these solvents be appropriate for the reactant? Why? Reactant: pka 25 for its conjugated acid Solvents: pka: Deprotonated ethylyne (acetylene) is a base. For bases, the equilibrium is pushed towards the acid with the higher pka. To preserve the baseness of the reactant we want to push equilibrium back towards it; to the left. So use solvents that are stronger acids than the conjugate acid of the reactant. DMSO (dimethylsulfoxide) and diethyl ether have higher pka values then acetylene. Karty, p.304 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 24

25 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? 1. Estimate pka by finding a similar group in a table or chart of pka values. 2. Look at the EN values of close neighbors (inductive effect). 3. Look for nearby multiple bonds (π bonds). 4. Is the H attached to a positively charged atom? 5. What is the row position of the atom the H is bound to? 6. What is the column position of the atom the H is bound to? 7. What is the orbital hybridization of the atom the H is bound to? 8. What is bound to the atom that the H atom is bound to? (pattern) Karty, p

26 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 Similar to acetic acid: pka 4.75 Actual benzoic acid pka is 4.20 Karty, p 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. pka 0.77 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 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

27 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 Ethanol is deprotonated to form its conjugate base, ethoxide. The negatively charged oxygen atom is fairly stable due to its high EN. Karty, p 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 Karty, p

28 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: 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. pka: pka: One reactant & one product are charged. The reaction is less energetically unfavorable. O Is more electronegative & better able to carry the charge. Karty, p 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 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 The stability of cations decreases as the charged atom s electronegativity increases. Because less electronegative atoms are able to give up free electron pairs. electronegativity H4N+ H3O+ H2F+ cation stability Karty, p

29 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 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 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+? Less stable cation, so stronger acid. More stable cation, so weaker acid. pka = 20 pka = 16.5 Check? Stronger acids have weaker conjugate bases. Strength 0f base increases with pka. Karty, p

30 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 Most stable Karty, p 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? 3. Does either molecule have resonance? Resonance stabilizes charge of conjugate bases by delocalization. size EN hybridization 1, 2, 3 pattern 4. Do inductive effects occur in either molecule? Nearby EN atoms increase acidity Karty, p

31 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 ring has electronegative π bonds. So, pka will be lower (more acidic) than propanone. pka < 20 This is the protonated version of diethyl ether. Protonation and a positive charge increase acidity, reducing pka. Compare hydronium to water. pka < 45 Karty, p 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. Most acidic Karty, p

32 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 Karty, p Example Which alcohol is more acidic based on structure? EPM Karty, p.324 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 Karty, p

33 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. Karty, p 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 33

34 Classes of organic acids & bases Organic acids & bases Organic acids & bases have carbon skeletons. We ll look at these types: Neutral organic proton acids Neutral organic bases Positively charged carbon acids Negatively charged carbon bases D&D, p

35 Neutral organic proton acids: carboxylic acid Three main types: Carboxylic acids Phenols Alcohols OH -1 functional group OH bond polarity makes these groups acidic & reactive. Degree of acidity depends on the stability of the conjugate base. Carboxylic acids are the most acidic of the three although they are weak acids. Acetic acids (2 carbons) has a pka of 4.8. Anion has 3 unhybridized p orbitals w/ total of 4 e-. They form a 3-atom π molecular orbital system; ½ π connects C to each O. The conjugate base (anion) is stabilized by resonance. The (-1) is delocalized over 2 oxygen atoms in the hybrid. Therefore stronger acid than alcohol! D&D, p Neutral organic proton acids: phenol Phenols are much less acidic than carboxylic acids: pka of 10. The phenolate anion is stabilized by resonance & therefore has some acidity. But some of phenolate anion s resonance structures disrupt aromatic resonance & creates (-) carbon. Because phenolate s carbon can be charged: 1. the phenolate ion is less stable than the carboxylate ion; and 2. phenol is less acidic than carboxylic acid. D&D, p

36 Neutral organic proton acids: alcohols Alcohols are alkyl molecules with OH -1 functional groups: R OH. Alcohols are much less acidic than phenols. Alcohols are just a bit more acidic than water. The pka range of alcohols are ~ Is there any resonance stabilization for this alcohol? Nope. D&D, p Neutral organic bases: amines Neutral organic bases contain one or more pairs of nonbonding electrons. When acting as Lewis bases these e- pairs are donated. The more available the :, the more powerful the base. Any molecule with a pair (:) can act as a base. We ll look at just two of the many neutral organic bases: Amines; and Ethers. Amines are derivatives of ammonia, NH3, and act as weak bases in water. methyl amine pka 10.6 relatively strong base methyl ammonium ion Amines are strong bases because N holds its : less strongly than O, S, halogens. D&D, p

37 (+) acids & (-) bases Positively charged acids are e- deficient; carbons don t have an octet Carbocation is a carbon with a full (+) charge Highly reactive and seldom seen since the act as intermediates Carbocations are Lewis acids (hard acids) React with the first Lewis base they see Prefer reacting with hard bases Carbocations sp2 with empty p orbital carrying the + charge. Negatively charged bases are carbons bonded only to three other atoms * carrying an unbonded electron pair (:). Carbanion is a carbon with a full (-) charge Looks very much like an amine Carbon holds the : loosely since C is not very electronegative Thus carbanions are strong bases D&D, p Example Rank the acidity of these related carboxylic acids: pka: Acidity increases as the number of EN subsitutents increases. More substituents help to carry the : (or the negative charge) of the acid s conjugate anion. D&D, p

38 Examples Rand the acidities of these two sets of molecules. Most stable cation pka: 5.2 vs Resonance stabilizes the (+) & holds : closer to the nucleus making them less available for donation. (11.1 is the better base) sp2 sp3 pka: - 10 sp hybridization keeps : very close; less available Least stable cation More stable cation because the + is pka: delocalized over the CH3 group The CH3 alkyl group helps to delocalize (or share) the (+) charge; better base However, adding more alkyl groups can decrease basicity by sterically crowding the N & destabilizing the ion. D&D, p 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 38

39 Functional groups Acidity & reactivity Functional groups M&B p.4 39

40 4/10/17 Functional group polarity M&B p.5-6 Graphical representation Note that this scale shows both molecules & some functional groups. D&D, p

41 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 Key concepts from Lecture 5 According to the Brønsted-Lowry definition of acids and bases, an acid is a proton donor and a base is a proton acceptor. Acid strength is related to the stability of the acid s conjugate base. The stronger the acid, the more stable is its conjugate base. A measure of acid strength is the value of the acid dissociation constant, Ka. Higher Ka = stronger acid According to the Lewis definition of acids and bases, an acid is an electron-pair acceptor and a base is an electron-pair donor. The character of the acidic or basic functional group, as well as the inductive effect of any adjacent functional group, affects acid or base strength. 41