What are carboxylic acids?
Carboxylic acids are compounds which contain a -COOH group.
Methanoic Acid, Ethanoic Acid, Propanoic Acid, Butanoic Acid, Pentanoic Acid.
Naming Carboxylic Acids:
The name counts the total number of carbon atoms in the longest chain - including the one in the -COOH group. If you have side groups attached to the chain, notice that you always count from the carbon atom in the -COOH group as being number 1.
Salts of Carboxylic Acids
Why are carboxylic acids acidic? - Because of the hydrogen in their OH group. When the acids form salts this H+ is lost and replaced by a metal compound.
- In solution in water, a hydrogen ion is transferred from the -COOH group to a water molecule. For example, with ethanoic acid, you get an ethanoate ion formed together with a hydroxonium ion, H3O+.
- This reaction is reversible and, in the case of ethanoic acid, no more than about 1% of the acid has reacted to form ions at any one time.
These are therefore weak acids.
This equation is often simplified to:
pH of carboxylic acids
The pH depends on both the concentration of the acid and how easily it loses hydrogen ions from the -COOH group.
Ethanoic acid is typical of the acids where the -COOH group is attached to a simple alkyl group. Typical lab solutions have pH's in the 2 - 3 range, depending on their concentrations.
Methanoic acid is rather stronger than the other simple acids, and solutions have pH's about 0.5 pH units less than ethanoic acid of the same concentration.
Infrared Spectometry for Carboxylic Acids
Reactions of Carboxylic Acids
2. The reaction ofcarboxylicacids with carbonates.
Carboxylic acids are neutralised by carbonates, for example
ethanoic acid + sodiumcarbonate sodium ethanoate + carbon dioxide + water.
2CH3CO2H(aq) + Na2CO3(s) 2CH3CO2Na(aq) + CO2(g) + H2O(l)
butanoic acid + zinccarbonate zinc butanoate +carbon dioxide + water.
2C3H7CO2H(aq) + ZnCO3(s) (C3H7CO2)2Zn(aq) + CO2(g) + H2O(l)
The reaction of carboxylicacids with metals.
magnesium + ethanoic acid - magnesium ethanoate + hydrogen
Mg(s) + 2CH3CO2H(aq) (CH3CO2)2Mg(aq) + H2(g)
zinc + propanoic acid - zincpropanoate + hydrogen.
Zn(s) + 2C2H5CO2H(aq) (C2H5CO2)2Zn(aq) + H2(g)