Evaluation preparation Starter 1. Draw methyl benzoate 2. What are the 2 different conditions for the hydrolysis of an ester? Starter 1. Draw methyl benzoate 2. What are the 2 different conditions for the hydrolysis of an ester? Acid or alkaline. Learning objectives • Recall how to carry out hydrolysis on an ester • Describe the different stages of the process • Evaluate the practical HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL HCOOH + METHANOIC ACID C2H5OH ETHANOL ETHYL METHANOATE CH3COOH ETHANOIC ACID METHYL ETHANOATE + CH3OH METHANOL HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL The products of hydrolysis depend on the conditions used... acidic CH3COOCH3 + H2O CH3COOH + CH3OH alkaline CH3COOCH3 + NaOH CH3COO¯ Na+ + CH3OH If the hydrolysis takes place under alkaline conditions, the organic product is a water soluble ionic salt The carboxylic acid can be made by treating the salt with HCl CH3COO¯ Na+ + HCl CH3COOH + NaCl • T:\Science\KS5\Chemistry\Hydrolysis of an Ester cut and stick.doc Thinking about the experiment. 1. The solution is acidified after the reflux, why? 2. Explain why ethanoic acid is water soluble but benzoic acid is not soluble in cold water. 3. Suggest what might be the most significant procedural error and suggest a modification for it. 4. Give two reasons why your percentage yield is less than 100%. 5. A suggested mechanism for the hydrolysis of the ester involves the hydroxide ion as a nucleophile. Complete the mechanism below, putting dipoles and curly arrows where necessary. 1. Alkaline hydrolysis of an ester produces the carboxylate. The solution was acidified to convert the carboxylate into the carboxylic acid. 2. The polarity of the bonds in the COOH group in ethanoic acid produces interactions with polar water. As well as dipole-dipole interactions there will be hydrogen bonding between the acid and water. In benzoic acid the large benzene ring is non- polar. The delocalised π-electrons are attracted to the carbon in the COOH group, which reduces the polarity of the bonds. This means that the intermolecular interactions and the solubility of benzoic acid are also reduced. 3. The volume of methyl benzoate is measured in a 10 cm3 measuring cylinder and as the yield is based on this measurement it would more appropriate to measure it in a graduated pipette. 4. The yield will be less than 100% because: The methyl benzoate may not be completely hydrolysed when it is refluxed. Some of the product is lost during each step of the purification process. Learning objectives • Recall how to carry out hydrolysis on an ester • Describe the different stages of the process • Evaluate the practical Starter • Draw the mechanism to show the nitration of methyl benzoate Starter • Draw the mechanism to show the nitration of methyl benzoate Practical • nitration of methyl benzoate ss.doc Clamp a round bottomed flask into an ice bath 15 Into a boiling tube, pour 3cm3 of conc nitric acid and 3 cm3 of conc sulfuric acid, place the boiling tube into a beaker of ice and allow to cool until the solution reaches 5oC 16 Pour 5cm3 of conc sulfuric acid into the round bottomed flask and cool it until it reaches 5oC. Slowly add 3cm3 of methyl benzoate to the sulfuric acid, keeping the temperature at 5oC. 17 Pour the nitrating mixture into the dropping funnel and place the funnel into the central ground glass joint, allowing the temperature to be monitored through the other glass joint May have to remove the bung to reduce the pressure and allow all the nitrating mixture to drip out 18 When all the nitrating mixture has been added, leave the mixture for about 10 mins at room temperature. 19 Cool a test tube of methanol 20 Pour the solution into a small beaker half filled with ice. Allow the product to precipitate out 21 Filter the solid under reduced pressure and wash with a MINIMUM of cooled methanol. 22 Heat a test tube of methanol for recrystallisation. 23 Recrystallise the nitrated ester with the MINIMUM methanol. Filter under reduced pressure and allow to dry 24 Find the mass of your product then determine the melting point. The melting point should be between 76-80oC 25 Answers to the questions • Mass of methyl benzoate used: 3.27 g • Expected yield of methyl 3-nitrobenzoate: 4.35 g • percentage yield = actual yield / expected yield × 100% • Melting point methyl 3-nitrobenzoate: 76–77 °C 1. HNO3 + H2SO4 NO2+ + HSO4 – + H2O or HNO3 + H2SO4 H2NO3+ + HSO4 – 2. It is necessary to control the temperature at which nitration occurs to ensure that only mono-nitration takes place. Trinitro compounds are unstable. 3. Electrophilic substitution The proton substituted by the nitro group reacts with the hydrogen sulfate ion and produces sulfuric acid. As the sulfuric acid has been regenerated at the end of the reaction, it has acted as a catalyst. H+ + HSO4- H2SO4 5. The ester group must deactivate the ring for substitution to occur at carbon-3. The carbonyl group is polarised and the carbon carries a δ+ charge. The delocalised π-electrons in the benzene ring are attracted to the carbon atom which decreases the electron charge density of the ring. 4.