Carboxylic Acids - Evaluative Prep.

Carboxylic acids are compounds which contain a -COOH group.

FOR EXAMPLE:

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.

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:

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.

Carboxylicacids react in the same wayas dilute mineral acids.
1. The reaction of carboxylicacids with alkalis.

Carboxylic acids are neutralised by alkalis, for example.

methanoic acid+ sodium hydroxide sodium methanoate + water.
HCO₂H_(aq) + NaOH_(aq) HCO₂Na_(aq) + H₂O_(l)

2. The reaction ofcarboxylicacids with carbonates.

Carboxylic acids are neutralised by carbonates, for example

ethanoic acid + sodiumcarbonate sodium ethanoate + carbon dioxide + water.
2CH₃CO₂H_(aq) + Na₂CO_3(s) 2CH₃CO₂Na_(aq) + CO_2(g) + H₂O_(l)

butanoic acid + zinccarbonate zinc butanoate +carbon dioxide + water.
2C₃H₇CO₂H_(aq) + ZnCO_3(s) (C₃H₇CO₂)₂Zn_(aq) + CO_2(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)

4. The reaction of Carboxylic acids with alcohols.
Carboxylic acids will react with alcoholsto form esters