Carboxylic Acids, Esters and Polymers

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Carboxylic Acids ­ Answers
1. Carboxylic acids contain the group , known as the carboxyl group, as it consists of the carbonyl group,
C=O, and the hydroxyl group, OH.
2. Carboxylic acids are found in a number of fruits and vegetables:
Citric acid is a tribasic acid with skeletal formula: 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.
3. 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.
4. The physical properties of the carboxylic acids:
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. This immediately
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.
5. Carboxylic acids can be prepared by:
i. Oxidation of a primary alcohol ­ when heated under reflux with acidified potassium dichromate, the primary
alcohol is oxidised to a carboxylic acid
ii. Oxidation of an aldehyde ­ Aldehydes are oxidised to carboxylic acids by the same reagent and under the
same conditions as primary alcohol
iii. Hydrolysis of an ester ­ Esters care hydrolysed when heated under reflux with either aqueous acid or aqueous
When heated with acid, the acid is a catalyst and the reaction is reversible, therefore the yield is low.
The hydrolysis with alkali isn't reversible, so there's a good yield of the salt of the acid and the alcohol.
To get the acid from the salt, the solution is cooled and excess dilute strong acid, such as HCl is added
iv. Hydrolysis of a Nitrile ­ Nitriles are compounds that contain the C N group
When heated under reflux with dilute acid, it is hydrolysed to a carboxylic acid and an ammonium salt:
CH3CN + H+ + 2H2O CH3COOH + NH4+
If heated under reflux with aqueous alkali the salt of the carboxylic acid formed. Subsequent addition of a
strong acid, such as dilute sulphuric acid, protonates the carboxylate ion and the carboxylic acid is formed.
CH3CN + OH- + H2O CH3COO- + NH3 then

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Iodoform Reaction of a Methyl Ketone or Secondary Alcohol ­ On addition of a solution of iodine in aqueous
sodium hydroxide to a methyl ketone (e.g. butanone, CH3CH2COCH3), the salt of the acid with one fewer carbon
atoms is produced (propanoic acid, CH3CH2COOH). The free carboxylic acid is formed by adding excess strong
acid to the mixture.
CH3CH2COCH3 + 3I2 + 4OH- CH3CH2COO- + CHI3 + 3I- + 3H2O then
6.…read more

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Solubility: They are soluble in several organic solvents, but react with water (often violently) so can't be
dissolved in it
16. Acid chlorides are prepared from carboxylic acids. The ­OH group is replaced by a chlorine atom. The reagents
that can be used are phosphorus pentachloride, phosphorus trichloride or thionyl chloride.…read more

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Esters are hydrolysed when heated under reflux with either aqueous acid or aqueous alkali:
Here, the acid is a catalyst and the reaction is reversible, therefore the yield of acid and alcohol is low.
This reaction is not reversible, so there is a good yield of the salt of the carboxylic acid and the alcohol.…read more

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A condensation polymer is formed when monomers join together with the elimination of a simple molecule, such
as water of HCl. Polyesters are condensation polymers. The monomers that condense must have two groups,
one at each end of the molecule.
38. A polyester is formed ia dicarboxylic acid, such as benzene-1,4-dicarboxylic acid reacts with a diol, such as
ethane-1,2-diol. One of the ­COOH groups in the dicarboxylic acid reacts with one of the ­OH groups in the diol.…read more


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