C H C H 3. aspirin CHEMISTRY Topic #4: Organic Chemistry Fall 2018 Dr. Susan Findlay See Exercises in Topic 12

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1 = = 3 EMISTY 2000 aspirin Topic #4: rganic hemistry Fall 2018 Dr. Susan Findlay See Exercises in Topic 12

2 rganic Acids (arboxylic Acids) When you hear the term organic acid, it s generally referring to a carboxylic acid. arboxylic acids are readily deprotonated by bases such as Na e.g acid base conjugate base conjugate acid The reason that this reaction is favoured in the forward direction is that the products are more stable than the reactants. In particular, the conjugate base (acetate; 3 2- ) is much more stable than the original base (hydroxide, - ). This makes hydroxide a stronger base than acetate. It also makes acetic acid (pk a =4.7) a stronger acid than water (pk a =14). 2

3 rganic Acids (arboxylic Acids) The strength of an acid is dependent on the stability of its conjugate base: The conjugate base of l (a strong acid; pk a =-7) is l - (a stable anion and very weak base) The conjugate base of 2 (a weak acid; pk a =14) is - (less stable than l - and a relatively strong base) The strength of an acid can also be said to be inversely related to the strength of its conjugate base (and vice versa). Why is 3 2- more stable than -? 3

4 rganic Acids (arboxylic Acids) Is the reaction below product-favoured or reactant-favoured? Since this reaction is equivalent to the acid dissociation equation (as long as the solution is sufficiently dilute that the X 2 1) 3 2 (aq) 3-2 (aq) + + (aq) its equilibrium constant is the K a for 3 2 : ecall that we can relate the K a and pk a for an acid via the following equation: An acid is stronger if it has a large K a and a small pk a. 4

5 rganic Acids (arboxylic Acids) We can increase the strength of an acid by adding electronwithdrawing groups, further stabilizing its conjugate base. e.g. To increase the acidity of acetic acid, replace one or more hydrogen atoms of the methyl group with halogens: F F F F F F pk a = 4.74 pk a = 2.66 pk a = 1.24 pk a = 0.23 This stabilization through σ bonds is called an inductive effect. Inductive effects are strongest when close to the acidic hydrogen. (F is not significantly more acidic than ) We saw inductive effects in EM 1000 when we looked at the strength of the oxoacids (e.g. l 2 vs. l 3 vs. l 4 ) 5

6 rganic Acids (thers) arboxylic acids are among the most acidic organic molecules; however, a number of other functional groups contain acidic hydrogen atoms: Alcohols are typically about as acidic as water. Most have pk a values of ~ (Inductive effects can lower these values to ~12) Phenols are typically more acidic than alcohols. Most have pk a values of ~8-10. (The pk a can go as low as 1 with the right groups attached to the aromatic ring) Thiols (-S) are also typically more acidic than alcohols. Most have pk a values of ~ (Also subject to inductive effects) Amines are not acidic in water, but can be deprotonated by some *very* strong bases in unreactive solvents like alkanes or ethers. Their pk a values are typically ~35-40! Sulfur and nitrogen are both less electronegative than oxygen. Why are thiols more acidic than alcohols, but amines less acidic? 6

7 rganic Acids (thers) Phenol is just an alcohol with a benzene ring. Why is phenol more acidic than most non-aromatic alcohols? 7

8 alculating p of an Acidic Solution We know that a strong acid (pk a < 0) dissociates fully in water because it is a stronger acid than 3 + (pk a = 0) so reacts fully with 2 to generate 3 + and its conjugate base. This effect is known as solvent leveling: No acid stronger than the conjugate acid of the solvent can exist in any solution. No base stronger than the conjugate base of the solvent can exist in any solution. (ydroxide isn t the strongest base not by a long shot! It s just the strongest base that can exist in water.) Weak acids aren t subject to solvent leveling, so we must consider what percentage of acid molecules have dissociated. If we have a relatively concentrated solution of a relatively weak acid, it is likely that the percent dissociation of the acid is small and that the actual concentration of acid is relatively close to the nominal concentration (i.e. concentration listed on the bottle or in the question). As a solution becomes more dilute, this assumption becomes less fair. ALWAYS EK YU ASSUMPTINS AFTE ALULATING!!! 8

9 alculating p of an Acidic Solution onsider a 0.32 M solution of phenol (pk a =9.95) at

10 alculating p of an Acidic Solution onsider a M solution of 3 2 (pk a =4.74) at

11 Distribution urves We can also work backwards, using the K a equation to work out the percent dissociation of an acid at a given p. The p tells us the activity (and therefore the concentration) of + (aq) so, as long as we know the K a value for the acid, we re set. a + a A K a [ A ] K a = so a A = = a a + a [ A] A alculate the percent dissociation of acetic acid (pk a =4.74) at p A 11

12 Distribution urves If we repeat this calculation at a large number of different p values, we generate what is known as a distribution curve: acetate ( 3 2- ) acetic acid ( 3 2 ) Note that the two curves cross at the pk a of the acid! The pk a of an acid is the p at which it is exactly 50% dissociated. 12

13 Distribution urves It is easy to see why an acid will be 50% dissociated at its pk a if we take the negative logarithm of the K a equation on page 11: pk a log K log log a + log a + A ( ) = a aa This is the enderson-asselbach equation. We can see that if a A- /a A = 1 then log(a A- /a A ) = 0 and p = pk a. As the acid s percent dissociation rises, so does the p and vice versa. a A ( K ) = ( ) a log a + log aa pk A a = A a = a p log a p A A a 13

14 Distribution urves for Polyprotic Acids If an acid has multiple acidic protons whose pk a values are different by several units, the distribution curve will look like several monoprotic distribution curves superimposed: If the pk a values are too close, the distribution curve will become more complex. 14

15 rganic Bases (Amines) When you hear the term organic base, it s generally referring to an amine. Amines are readily protonated by acids: e.g. N N base acid conjugate acid conjugate base The basicity of amines is due to the lone pair on the nitrogen atom which makes all amines ( =, alkyl or combination) Lewis bases. 15

16 rganic Bases (Amines) We have seen one other functional group containing nitrogen. Why is an amine considered a good base, but an amide is not? The nitrogen atom of an amide is so weakly basic that the oxygen of the carbonyl group will be protonated over it! 16

17 Quantifying Basicity We can measure a base s strength using either of two measures: We can refer to the K a (or pk a ) of the conjugate acid. Since the strength of a base is inversely related to the strength of its conjugate acid, we can deduce the strength of a base from the strength of its conjugate acid. N + pk a = N + pk a = 10.6 Smaller pk a = Stronger onjugate Acid = Weaker Base 17

18 Quantifying Basicity Alternatively, we can refer to the K b (or pk b ) of the base itself. This is the equilibrium constant for reaction of the base with water: N N K b is inversely proportional to K a for the conjugate acid: or where K w = under standard conditions. N pk b = 4.7 K K a = K w b K K = K a b 3 w N pk b = 3.4 Larger K b = Smaller pk b = Stronger Base 18

19 alculating p of a Basic Solution We can calculate the p of a solution of weak base by: Using the K b expression and nominal solution concentration to calculate a - then Using the K w expression (K w = a + a - ) to calculate a + then Using a + to calculate p. ecall that, at 25, K w = The pk a of aniline ( 6 5 N 2 ) is 28 and the pk a of 6 5 N 3+ is 4. What are the K b and pk b values for aniline? 19

20 alculating p of a Basic Solution alculate the p of a 0.71 M aqueous solution of aniline at

21 Amino Acids: Acid and Base Some textbooks present the structure of an amino acid as follows N in order to show that the name comes from having an amine group and a carboxylic acid group... This is not how an amino acid actually exists in biological systems (or other p ~7 solution). Instead, it exists as the zwitterion: N

22 Amino Acids: Acid and Base Why is this? The pk a for the 2 proton is typically ~2 (slightly lower than the standard 3-5 range for carboxylic acids). The pk a for the conjugate acid of the amine group (i.e. for the N + 3 proton) is typically ~9-10. Use this data to sketch a distribution curve for the amino acid, labeling with the major species within each p range. 22

23 Beyond Water: Implications of Acid-Base hemistry It s easy to forget that not all chemistry is done in aqueous solution, but the pk a scale doesn t just go from 0 to 14! ften times, the solvent for a reaction is chosen based (at least in part) on acid-base considerations onsider the following reaction:. - + I I What would happen if we tried to perform it in aqueous solution? 23

24 Beyond Water: Implications of Acid-Base hemistry Solvent leveling can be useful, though. Sodium methoxide (Na 3 ) can be prepared via a couple of different methods, both of which use methanol as the solvent: We saw in EM 1000 that adding Na (s) to 2 gives Na and 2(g) Similarly, adding Na (s) to 3 gives Na 3 and 2(g) : Adding a very strong base (e.g. a source of - such as Na) to 3 will also produce Na 3 along with the conjugate acid of the base: 24