Carboxylic acids and esters
CARBOXYLIC ACIDS are generally weak acids and have the formula Cn H2n+1COOH Acids are named based on the number of carbon atoms in the entire formula with the suffix ‘oic’ added to the root name.
Carboxylic acids are commonly found in a number of substances such as citrus fruits, which contain citric acid. Other common acids are shown below.
Some acids are also known by their common names, for example:
Methanoic acid, also called formic acid; and ethanoic acid, also called acetic acid.
WHAT ARE THE CHARACTERISTIC REACTIONS OF CARBOXYLIC ACIDS?
Since carboxylic acids are weak acids, they are able to carry out typical reactions with substances such as metals, bases and carbonates. In these reactions, the -COOH group is the functional group that reacts.
1. Organic acids react with metals to form salts and liberate hydrogen.
2Na (s) + 2CH3COOH (aq) 2CH3COONa (aq) + H2 (g) sodium ethanoate
Only the hydrogen of the -COOH is replaced in these acid-type reactions. 2. Acids react with oxides and hydroxides of metals. MgO (s) + 2CH3COOH (aq) (CH3COO)2Mg (aq) + H2O Magnesium ethanoate
3. Organic acids react with carbonates and hydrogencarbonates to form salts and carbon dioxide. CH3COOH (aq) + NaHCO3 (aq) CH3COONa (aq) + CO2 (g) + H2O
4. Alcohols and carboxylic acids combine to produce ESTERS, which have the general formula RCOOR, where R is an alkyl group such as CH3 and C2H5. This is called an esterification reaction.
Esters have the functional group - COO- with two alkyl groups attached.
HOW ARE ESTERS NAMED?
Esters are named from the carboxylic acid and alcohol from which they are made. The alcohol gives the first part of the name and the carboxylic acid gives the second. The suffix - ‘oate’ is added to the acid part to get the name of the ester. In the example, ethanoic acid and methanol are reacted to form the ester methyl ethanoate.
Ester formation is a type of condensation reaction in which a large molecule is formed from the joining of two reactive groups with the elimination of a small molecule such as H2O.
In the formation of ethyl ethanoate, ethanol (alcohol) and ethanoic acid (carboxylic acid) are reacted to form the ester and water. This usually takes place in the presence of acid as the catalyst.
Esters can be hydrolysed (broken down) using acid or alkali to yield the acid (or salt of the acid) and alcohol that formed the ester.
In this hydrolysis reaction, acid is used as a catalyst and the reaction is reversible. The alcohol and acid that made up the ester is reformed.
HOW ARE ESTERS IDENTIFIED?
Esters are very important compounds known by a characteristic fruity or sweet smell. They are responsible for the smell of fruits, perfumes and flavourings in fruits and cakes. Esters also form solvents for organic compounds such as glues, paints and varnishes.