Carboxylic Acids Flashcards
How can we get carboxylic acids from nitriles?
By hydrolysis of a nitrile:
1) Acid hydrolysis - reflux nitrile with sulphuric acid
2) Alkaline hydrolysis - reflux nitrile with sodium hydroxide
What is meant by decarboxylation,how can it be done and what is its use?
Decarboxylation - removal of the ‘COO’ group from a molecule.
Heat carboxylic acid with soda lime (CaO and NaOH)
This reaction can be used to shorten a carbon chain - useful in chemical analysis and organic synthesis.
Why do carboxylic acids have higher mpt and bpts than hydrocarbons, carbonyl compounds and halogenoalkanes?
Carboxylic acids can form both dipole-dipole and hydrogen bonding as well as Van der Waals intermolecular forces.
Hydrocarbons (VDW), carbonyl compounds and halogenoalkanes (dipole-dipole)
What do carboxylic acids form in the liquid state?
Chains and some dimers on account of their ability to form intramolecular hydrogen bonds.
When do carboxylic acids dimerise?
In gaseous phase and in non-polar solvents.
Why are dioic acids all solids at room temperature?
2 carboxyl groups allows for significantly greater H bonding than in mono-carboxylic acid.
Carboxylic acids show good solubility in non-polar solvents. Why?
The dimers formed are essentially non-polar.
Give examples of non-polar solvents.
Hexane, benzene, tetrachloromethane
Despite having both C=O and O-H groups, carboxylic acids fail to give many of the reactions characteristic of carbonyl compounds and alcohols. Why?
- Lone pair on the oxygen of the O-H group conjugates with the π system of the C=O group [Mesomeric Effect]
- This conjugation spreads negative charge and increases stability.
- This interaction reduces the electropositive character of the carboxyl carbon atom; modifies chemical properties significantly.
Why do carboxylic acids resist nucleophilic addition reactions characteristic of carbonyl compounds?
An addition reaction across the C=O would result in a loss of electron delocalisation and its inherent stability.
Reduction to alcohols is possible but requires powerful reducing agent LiAlH4 in dry ether solvent followed by alkaline hydrolysis.
How is acidic character being promoted in carboxylic acids?
- Conjugation of the lone pair on the oxygen of the O-H group with the C=O group tends to reduce electron density of the O-H bond.
- This weakens the bond, allowing for the loss of a proton.
- Carboxylate ion formed by the bond breaking process is itself stabilised by the delocalisation of negative charge.
As a result, carboxylic acids are significantly stronger acids than phenol.
What happens to fairly powerful nucleophiles, e.g. ammonia, as a result of carboxylic acid’s acidic character?
They do not attack the electrophilic carbon atom of the carboxyl group but tend to act as bases, accepting the proton to form salts.
How are carboxylic acids much stronger acids than phenol?
1) Carboxylic acids are able to liberate carbon dioxide from a carbonate/hydrogencarbonate.
2) They are also able to displace phenol from a solution of sodium phenate.
