LA.4.1Carboxylic Acids
Key Points
Carboxylic acids are compounds with the formula of that illustrated in Figure 1 and the general formula R-COOH where R is part of a larger organic molecule e.g.CH , C H , C H . To name 3 2 5 6 5 carboxylic acids you must look at the alkyl chain, take its prefix and add “oic acid” to it.
CH3CH2COOH is PROPANOIC ACID. Salts of Carboxylic Acids
Carboxylic acids are more acidic than alcohols as their conjugate base, the carboxylate ion, is resonance stabilised (see Figure 2). This anion then can form a salt ionically bonding with a metal such as sodium (see Figure 3).
Figure 1 The general formula of a carboxylic Figure 2 The stabilised Figure 3 The salt, carboxylate ion. sodium ethanoate. acid. Physical Properties of Carboxylic Acids The boiling point and solubility of carboxylic acids is related to their ability to form hydrogen bonds. Carboxylic acids are polar and can act as both hydrogen bond donors through the hydroxyl group and hydrogen bond acceptors through the carbonyl.
Boiling Point Solubility
Carboxylic acids have a higher boiling In the presence of water carboxylic acids won’t point than the corresponding alcohol due dimerise but will form hydrogen bonds with water their ability to dimerise through hydrogen instead. Smaller carboxylic acids (up to 5 bonds which is illustrated in Figure 4. carbons) are soluble in water but the solubility This dimerization causes the size of the decreases rapidly with size. This is due to the molecule to double and the van der hydrophobic nature of alkly chains. Waals’ interactions to increase. For EHANOIC ACID boiling to occur either the dimer must is considerably more soluble than break first or the whole dimer must be HEPTANOIC ACID vaporised requires more energy than the due the difference in chain length monomer.
Reactions of Carboxylic Acids
Reactions of Carboxylic Acids
1. Carboxylic acids react with alcohols to give ESTERS. e.g. CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O
2. Carboxylic acids react with metals and bases to give a METAL CARBOXYLATE SALT.
- + e.g. CH3COOH + Na CH3COO Na + H2 - + CH3COOH + NaOH CH3COO Na + H2O
3. Carboxylic acids react with amines to form AMIDES. e.g. 1) 2CH3COOH + (NH4)2CO3 2CH3COONH4 + H2O + CO2
2) CH3COONH4 Heat CH3CONH2 + H2O 4. Carboxylic acids can be reduced to give ALCOHOLS. e.g. CH3COOH + 4[H] CH3CH2OH + H2 (H= LiAlH4, a strong reducing agent.)
5. Carboxylic acids can be used to synthesis ACYL CHLORIDES. e.g. CH3COOH + PCl5 CH3COCl + POCl3 + H2O CH3COOH + SOCl2 CH3COCl + SO2 + H2O
6. Carboxylic acids can be used to synthesis ACID ANHDRIDES. e.g. 2 CH3COOH (CH3CO)2 + H2O
Esterification: The Mechanism
Designed by Leticia Bonita Prince Newcastle University 4th Year MChem Student