Chapters 13/14: Carboxylic Acids and Carboxylic Acid Derivatives

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1 CHM 201 (Elements of Organic Chemistry) Dr. Virgil Lee Cal Poly Pomona Chapters 13/14: Carboxylic Acids and Carboxylic Acid Derivatives resonance stabilized OH group donates electron density to carbonyl RCO 2 H and its derivatives are more electron rich than aldehydes/ketones o aldehydes/ketones are better E + because they are more δ + Carboxylic Acid Derivatives General Reactions of Carboxylic Acid Derivatives (B&P 14.2) Overall, a substitution reaction (but not S N 1/S N 2) Mechanism called addition-elimination Can be catalyzed by either acids or bases 13/14-1

2 Carboxylic Acids (R-CO 2 H) Nomenclature Identify the longest parent carbon chain containing the CO 2 H group designating the C of the C=O group as #1. Replace the alkane (or alkene or alkyne) -e with -oic followed by the word acid to give an alkanoic acid (or alkenoic acid or alkynoic acid ). Since acids are always terminal and at the #1 position, no number designation is necessary for the position of the CO 2 H unit. Carboxylic acid groups have naming priority over alkenes, alkynes, and alcohols. Acid Chlorides (R-COCl) Nomenclature Acid chlorides have two-word names based on the corresponding carboxylic acid derivative. The name is obtained by replacing the -ic acid with -yl chloride This gives an alkanoyl chloride. Since acid chlorides are always terminal and at the #1 position, no number designation is necessary for the position of the COCl unit. Acid chlorides groups have naming priority over alkenes, alkynes, and alcohols. 13/14-2

3 Esters (R-CO 2 R ) Nomenclature Esters have two-word names based on the corresponding carboxylic acid derivative. The first word of the name corresponds to the alkyl substituent (R ) bonded to the oxygen (e.g., methyl when R =CH 3 ). The second word of the name is obtained by replacing the -ic acid with -ate. This gives an alkyl alkanoate. Since esters are always terminal and at the #1 position, no number designation is necessary for the position of the CO 2 R unit. Ester groups have naming priority over alkenes, alkynes, and alcohols. Amides (R-CO 2 NH 2 ) Nomenclature Amide have one-word names based on the corresponding carboxylic acid derivative. The name is obtained by replacing the -oic acid with amide to give an alkanamide. Since amides are always terminal and at the #1 position, no number designation is necessary for the position of the CO 2 NH 2 unit. Amide groups have naming priority over alkenes, alkynes, and alcohols. Draw cis-2-hexenoic acid Draw isopropyl butanoate 13/14-3

4 Physical Properties of Carboxylic Acids (B&P ) Acidity The acid is 10,000,000,000,000 (10 13!) times more acidic that the alcohol!!! Why are the pk a values so different? Look at conjugate bases! Something that stabilizes a compound s conjugate base makes it a stronger acid. 13/14-4

5 Which is the stronger acid? Note: Inductive effects decrease with distance. Common Acid pk a Values (Table 2.2) Strong Acids Weak Acids (pk a < 0) (pk a ~ 5) (pk a ~ 10) (pk a ~ 16) H 3 O ROH 2 ROH H 2 O HX H 2 SO 4 HNO 3 13/14-5

6 K a, pk a, and ph (B&P 2.3) Equilibrium if HA is a STRONG acid Equilibrium if HA is a WEAK acid K a Acid dissociation constant pk a pka = -log(k a ) Used because K a is often VERY large or VERY small (easier to work with than K a If K a is a LARGE number (>1), then the acid is: stronger / weaker If an acid is stronger, the pk a is: higher/lower ph = -log [H 3 O ] FYI: How does the ph of a solution affect chemical structures? (B&P 18.3) 13/14-6

7 Relative Reactivities of Carbonyl-containing Compounds (B&P 14.6) All C=O groups are electrophiles (E), but some are more reactive than others. Their reactivity depends on what groups are attached to the carbonyl) 13/14-7

8 Carboxylic Acid Interconversion: Hydrolysis of Carboxylic Acid Derivatives (B&P 14.3) Mechanism of base-promoted ester hydrolysis: 13/14-8

9 An addition/elimination reaction in which a LG is substituted for a Nu, followed by deprotonation.! OR (alkoxide) is a bad LG for S N 1/S N 2 reactions! OR (alkoxide) is an okay LG for addition/elimination reactions Base-promoted ester hydrolysis is the historical method of making soap Acid-Catalyzed Ester Hydrolysis Mechanism of acid-catalyzed ester hydrolysis: 13/14-9

10 Need an acid or a base for ester hydrolysis to occur (will not occur in neutral conditions) For Ester Hydrolysis In base, there is a strong Nu: In acid, there is a strong E: Fisher Esterification Reaction Transesterification Reaction 13/14-10

11 Reactions of Grignards/Hydrides with Esters A) Hydride Nucleophiles (B&P 13.5, 14.8) Mechanism of ester hydride reduction While aldehydes and ketones can be reduced by milder reducing reagents like NaBH 4, carboxylic acids derivatives like esters require stronger reducing reagents like LiAlH 4. This is due to carboxylic acid derivatives being more electron rich than aldehydes and ketones. B) Grignard Nucleophiles (B&P 14.7) 13/14-11