Topic 10a Carboxylic Acids
Topic 10a Carboxylic Acids What Are Carboxylic Acids? Functional group of a carboxylic acid is a carboxyl group (carbonyl + hydroxyl = COOH) Aliphatic carboxylic acid – RCOOH Aromatic carboxylic acid – ArCOOH What Are The Physical Properties Of Carboxylic Acids? Significantly higher boiling points than other types of organic compounds Result from their polarity and from the fact that they form very strong intermolecular H bonds Also interact with water molecules by H bonding through both their C=O and OH groups more soluble in water Hydrophilic carboxyl group increases water solubility while hydrophobic hydrocarbon chain decreases water solubility What Are The Acid-Base Properties Of Carboxylic Acids? Carboxylic acids are weak acids Stronger acids than alcohols because resonance stabilizes carboxylate anion by delocalizing its negative charge React with NaOH, KOH, and other strong bases to form water-soluble salts How Are Carboxyl Groups Reduced? Most resistant to reduction, not affected by catalytic reduction (H2/M) Lithium aluminum hydride (LiAlH4) reduces carboxylic acid to a primary alcohol o Initial product is an aluminum alkoxide, which is then treated with water to give the 1° alcohol and LiOH + Al(OH)3 Catalytic hydrogenation does not reduce carboxyl groups but does reduce alkenes to alkanes o Therefore, we can use H2/M to reduce the functional group selectively in the presence of a carboxyl group It is possible to reduce an aldehyde / ketone carbonyl group selectively in the presence of carboxylic group via NaBH4 What Is Fischer Esterification? Treatment of a carboxylic acid with an alcohol in the presence of an acid catalyst gives an ester Acid-catalyzed esterification is reversible In the Fischer esterification, the –OR portion of an alcohol replaces the – OH portion of a carboxylic acid
After addition of a nucleophile to a carbonyl, one possible mechanism is for the tetrahedral carbonyl intermediate to collapse back to a C=O while ejecting a leaving group Fischer esterification: o Proton transfer from the acid catalyst to the carbonyl O increases the electrophilicity of the carbonyl C o Carbonyl C is then attacked by the nucleophilic O atom of the alcohol to form an oxonium ion o Proton transfer from the oxonium ion to a second molecule of alcohol gives a tetrahedral carbonyl addition intermediate o Proton transfer to one of the –OH groups of the intermediate gives a new oxonium ion o Loss of water from the oxonium ion gives ester and regenerates aid catalyst
What Are Acid Chlorides? Functional group of an acid halide is carbonyl group bonded to a halogen atom Most common way to prepare an acid chloride is to treat carboxylic acid with thionyl chloride o Carboxyl group adds to S of thionyl chloride to generate a tetrahedral S intermediate o Loss of Cl from tetrahedral S intermediate regenerates the sulfonyl group o Chloride ion attacks the carbonyl C, forming tetrahedral carbonyl addition intermediate o Lone pair of electrons to collapse back toward bond to regenerate carbonyl C while expelling leaving group sulfochloridous acid (breaks down to yield sulfur dioxide and HCl) What Is Decarboxylation? Loss of CO2 from a carboxyl group Carboxylic acids that have a carbonyl group β to carboxyl group undergoes decarboxylation quite readily on mild heating (β-ketoacids) o Redistribution of 6 e- in a cyclic 6-membered transition state gives carbon dioxide an an enol o Tautomerism of the enol gives the more stable keto form of the product Once all carboxyl groups with β-carbonyls are identified, replace carboxyl groups with a H Presence of aldehyde / ketone carbonyl group on the carbon β to the carboxyl group is sufficient to facilitate decarboxylation
After addition of a nucleophile to a carbonyl, one possible mechanism is for the tetrahedral carbonyl intermediate to collapse back to a C=O while ejecting a leaving group Fischer esterification: o Proton transfer from the acid catalyst to the carbonyl O increases the electrophilicity of the carbonyl C o Carbonyl C is then attacked by the nucleophilic O atom of the alcohol to form an oxonium ion o Proton transfer from the oxonium ion to a second molecule of alcohol gives a tetrahedral carbonyl addition intermediate o Proton transfer to one of the –OH groups of the intermediate gives a new oxonium ion o Loss of water from the oxonium ion gives ester and regenerates aid catalyst
What Are Acid Chlorides? Functional group of an acid halide is carbonyl group bonded to a halogen atom Most common way to prepare an acid chloride is to treat carboxylic acid with thionyl chloride o Carboxyl group adds to S of thionyl chloride to generate a tetrahedral S intermediate o Loss of Cl from tetrahedral S intermediate regenerates the sulfonyl group o Chloride ion attacks the carbonyl C, forming tetrahedral carbonyl addition intermediate o Lone pair of electrons to collapse back toward bond to regenerate carbonyl C while expelling leaving group sulfochloridous acid (breaks down to yield sulfur dioxide and HCl) What Is Decarboxylation? Loss of CO2 from a carboxyl group Carboxylic acids that have a carbonyl group β to carboxyl group undergoes decarboxylation quite readily on mild heating (β-ketoacids) o Redistribution of 6 e- in a cyclic 6-membered transition state gives carbon dioxide an an enol o Tautomerism of the enol gives the more stable keto form of the product Once all carboxyl groups with β-carbonyls are identified, replace carboxyl groups with a H Presence of aldehyde / ketone carbonyl group on the carbon β to the carboxyl group is sufficient to facilitate decarboxylation
