Download Carboxylic Acids Theory Sheet

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.CH3, C2H5, C6H5. To name
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
acid.
Figure 2 The stabilised
carboxylate ion.
Figure 3 The salt,
sodium ethanoate.
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
point than the corresponding alcohol due
their ability to dimerise through hydrogen
bonds which is illustrated in Figure 4.
This dimerization causes the size of the
molecule to double and the van der
Waals’ interactions to increase. For
boiling to occur either the dimer must
break first or the whole dimer must be
vaporised requires more energy than the
monomer.
In the presence of water carboxylic acids won’t
dimerise but will form hydrogen bonds with water
instead. Smaller carboxylic acids (up to 5
carbons) are soluble in water but the solubility
decreases rapidly with size. This is due to the
hydrophobic nature of alkly chains.
EHANOIC ACID
is considerably more soluble than
HEPTANOIC ACID
due the difference in chain length
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
CH3COOH + NaOH
CH3COO-Na+ + H2
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
CH3COOH + SOCl2
CH3COCl + POCl3 + H2O
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