April 04, 2014
Esters are one of the most common derivatives of carboxylic acids and are widely distributed in both nature and industry. A Fischer Esterification is the formation of an ester and water from alcohol and an acid. It is the simplest means of synthesizing an ester and requires the reaction of a carboxylic acid and an alcohol. The general reaction of Fischer esterification is, CH3CO2H + ROH ↔ CH3CO2R + H2O (reaction1)
CH3COOH + ROH ↔ CH3COOR + H2O (reaction2)
An alcohol (ROH in reaction 1) is a functional group containing a hydrogen, oxygen bond. Carboxylic acids (CH3CO2H in reaction 1) are characterized by a carbon, oxygen double bond, with one alcohol group and an alkyl or aromatic side chain. Esters (CH3CO2R in reaction 1) are identified by a carbon, oxygen double bond and an oxygen alkyl or aromatic group. Carboxylic esters often have pleasant odors and are used in foods and beverages to create flavors mimicking fruit. Depending on the ester synthesized smells such as banana, pineapple, or orange can be obtained. Reaction is a double displacement reaction. Carboxylic esters can be made by a variety of methods from the corresponding carboxylic acids. The simplest method to synthesize an ester is to react a carboxylic acid with an alcohol. This reaction is called Fischer esterification.it is a nucleophillic acyl substitution reaction carried out under acidic conditions. Carboxylic acids alone are not reactive enough to be attacked by neutral alcohols, but they can be made much more reactive in the presence of a strong acid, such as sulfuric acid. The mineral acid protonates the carbonyl group oxygen atom and gives the carboxylic acid a positive charge. Now positively charged, the carboxylic acid is much more reactive toward a nucleophillic attack by the alcohol.
The chemical industry uses esters for a variety of purposes. Ethyl acetate, for example, is a common solvent found in nail-polish remover, and dialkyl phthalates are used as plasticizers to keep polymers from becoming brittle. They also have important medical uses. Ethyl nitrite is a diuretic and an antipyretic. Amyl nitrite is used in the treatment of asthma and epileptic convulsions as an antispasmodic.
The reaction of a carboxylic acid with an alcohol is equilibrium, and therefore governed by the equilibrium constant. The equilibrium constant for esterification with primary alcohols is usually very close to unity; the equilibrium constant for the reaction shown below for the synthesis of ethyl acetate is 3.38.The equilibrium should be shifted to the right, namely to completion, by applying Le Chateliar’s principle.
Keq = [Ester]/ [Acid] [ROH] = 3.38
Mechanism of the Fischer Esterification
Addition of a proton (e.g.: p-TsOH, H2SO4) or a Lewis acid leads to a more reactive electrophile. Nucleophilic attack of the alcoholgives a tetrahedral intermediate in which there are two equivalent hydroxyl groups. One of these hydroxyl groups is eliminated after a proton shift (tautomerism) to give water and the ester.
Unknown alcohol # 41, acetic acid, concentrated sulfuric acid, NM spectroscopy Saturated aqueous sodium carbonate (Na2CO3 (aq)), Granular sodium sulfate Hickman still head, Boiling chips, 25 ml round bottom flask, Assembled reflux apparatus, Stir glass rod, IR spectroscopy, Pasteur pipette Procedure:
1. An empty 25 ml round-bottom flask was weighed.
2. Approximately 1.5ml of the alcohol was placed in the flask, and the flask was reweighed. 3. ~ 3ml of glacial acetic acid was added to occur the esterification reaction. 4. Three boiling chips were placed in the flask then 2 drops of concentrated sulfuric acid was added, which act as catalyst. 5. A reflux apparatus was assembled.
6. The assembly was places in the heating mantle and the reflux...
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