In A-Level Chemistry - Organic chemisrtry, we look at different functional groups in hydrocarbons. Let's look at the homologous series of carboxylic and their properties here.
Carboxylic acids containthe group -COOH, known as the carboxylgroup, as it consists of the carbonyl group, C=O, and the hydroxyl group, OH.
Carboxylic acids are found in a number of fruits and vegetables:
· Citric acid is a tribasic acid. It is found in citrus fruits, particularly lemons and limes.
· Malic acid, HOOCCH2CH(OH)COOH, is present in apples and grapes and gives them their sharp taste. Its systematic name is 2-hydroxy-butanedioic acid.
· Ethanedioic acid, HOOCCOOH is found in rhubarb leaves
· Vegetable and fish oils are esters containing unsaturated carboxylic acids such as C17H29COOH, which is an omega-3 acid.
The names of carboxylic acids are derived from the number of carbon atoms in the chain, including the carbon atom of the –COOH group. This carbon atom is regarded as the first carbon atom. Some organic acids have two COOH groups, e.g. HOOCCOOH which is Ethanedioic acid.
Physical properties
·Melting Points:
The acids in the homologous series CnH2n+1COOH are liquids up to n=9. However, they do not fit the usual pattern of an increase with the number of electrons in the molecule. The reason for this is that with methanoic and ethanoic acids there is considerable hydrogen bonding. Pair of acid molecules (dimers) are formed. Thisimmediately doubles the size of the molecule and so increases the van der Waals dispersion forces between one of these dimers and its neighbours - resulting in a high boiling point. This occurs to a lesser extent with acids that have more carbon atoms, as the hydrogen bonding is inhibited by the zigzag chains of carbon atoms.
· Boiling Points:
When the solid acid is melted, many hydrogen bonds are broken so the boiling temperatures of the acids have the pattern that is expected from the increasing number of electrons.
· Smell:
All carboxylic acids have strong smells. Vinegar is a dilute solution of ethanoic acid. The smell of rancid butter comes from butanoic acid.
· Solubility:
Hydrogen bonding with water molecules enables those carboxylic acids with only a few carbon atoms to dissolve in water. The solubility in water decreases as the hydrocarbon chain lengthens because the chain is hydrophobic. Conversely, solubility in lipids increases as the chain lengthens.
Preparing carboxylic acid
Oxidation of a primary alcohol – when heated under reflux with acidified potassium dichromate, the primary alcohol is oxidised to a carboxylic acid
RCH2OH+ [O] ⇨ RCOOH + H2O
Oxidation of an aldehyde – Aldehydes are oxidised to carboxylic acids by the same reagent and under the same conditions as primary alcohol
RCHO + [O] ⇨ RCOOH
Hydrolysis of an ester
Esters care hydrolysed when heated under reflux with either aqueous acid or aqueous alkali:
- When heated with acid, the acid is a catalyst and the reaction is reversible, therefore the yield is low. CH3COOC2H5 + H2O <---H+---> CH3COOH + C2H5OH
- The hydrolysis with alkali isn’t reversible, so there’s a good yield of the salt of the acid and the alcohol.
CH3COOC2H5+ NaOH ⇨ CH3COONa + C2H5OH
- To get the acid from the salt, the solution is cooled and excess dilute strong acid, such as HCl is added
CH3COONa+ HCl ⇨ CH3COOH + NaCl
Reduction
Carboxylic acids are reduced by lithium tetrahydridoaluminate (III) dissolved in dry ether (ethoxyethane). The H- ion in the AlH4- is a powerful nucleophile. It adds on to the δ+ carbon atom in the –COOH group. A series of reactions takes place and the final product has to be hydrolysed by dilute acid. The carboxylic acid is reduced to a primary alcohol. The overall equation is:
CH3COOH+ 4[H] ⇨ CH3CH2OH + H2O
Salt forming
Carboxylic acids form salts with bases such as NaOH. These are simple neutralisation reactions and are just the same as any other reaction in which hydrogen ions from an acid react with hydroxide ions. For example:
CH3COOH+ NaOH ⇨ CH3COONa + H2O
Esterification
Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid (as a catalyst) in a reversible reaction to form esters:
Acid +alcohol <----> ester + water
for example: CH3COOH + C2H5OH <-----H2SO4(l)-----> CH3COOC2H5 + H2O
The catalyst acts by protonating the acid, which then loses a water molecule. The lone pair of electrons on the oxygen of the alcohol bonds to the positive carbon atom in the CH3C+O ion and then an H+ is removed, thus reforming the catalyst.
Drafted by Eunice (Chemistry)
References
https://www.lecturio.com/magazine/carboxylic-acids-and-derivatives/
https://www.khanacademy.org/test-prep/mcat/chemical-processes/carboxylic-acids/a/carboxylic-acid-reactions-overview