Are you a chemistry student? Visit A-Level Chemistry to download comprehensive revision materials - for UK or international students!

Ester hydrolysis

Hydrolysis

It means cleavage of chemical bonds of compound through the addition of water. During hydrolysis reaction with pure water is slow and usually not used, but ester is catalyzed by dilute acid like dilute hydrochloric acid and dilute sulphuric acid [1].

For example:                                    H+ (aq)

          CH3COOCH2CH3 + H2O      ⇋   CH3COOH + CH3CH2OH

               Ethyl ethanote                         ethanoic acid   ethanol

Another example hydrolyzing methyl proponate

                                                             H+(aq)

         CH3CH2COOCH3 + H2O         ⇋     CH3CH2COOH + CH3OH

          Methyl propanoate                        propanoic acid     methanol

The reactions are reversible, use excess amount of water to make the hydrolysis as complete as possible, the water comes from dilute acid so ester mix with excess of dilute acid.

Hydrolysis through dilute alkali

Sodium hydroxide is used to hydrolyzing ester, ester are heated with dilute alkali like sodium hydroxide. The reaction is one way and the products are easily separated.

          CH3COOCH2CH3 + NaOH →    CH3COONa + CH3CH2OH

           Ethyl ethanoate                  sodium ethanoate       ethanol

Hydrolyzing methyl propanote in the same way

        CH3CH2COOCH3 + NaOH →     CH3CH2COONa + CH3OH

 Methyl propanote                           sodium propanote     methanol

Ester

Ester was first reported by German chemist Leopold Gmelin in first half of 19th century, class of organic compound, structurally an alkoxy (OR) group is attached to the carbonyl group. They react with water and produced alcohols and organic or inorganic acids. In reactions alkoxy group is replaced by another group like hydrolysis which means splitting with water. It is catalyzed by either through base or an acid like hydrochloric acid or sulfuric acid. It is the reverse process of esterification (preparation of an ester). R may be alkyl, aryl or may be H [2]. It can be prepared through reactions of acid anhydride with alcohol and reaction of carboxylic acid salts with alkyl halides. Esters can be converted into another type of ester through transesterified process, in which through reaction with an alcohol, a carboxylic acid or third ester in the presence of catalyst. Saponification is the process in which ester hydrolysis is carried out in the presence of alkalies, it is used in the preparation of soaps from fat or oils, also helpful for quantitative estimation of ester.

Ester hydrolysis 1

   Ester

Nomenclature of Ester

The ester first word is derived from alkyl group and second word is from carboxylate group of the carboxylic acid.

Preparation of Ester

Ester are most commonly prepared through the reactions of carboxylic acid, acid chlorides and acid anhydrides with alcohol [3].

Ester hydrolysis 2

Synthesis of Ester

Acid – Catalyzed Esterification

The reaction of carboxylic acid and alcohol forms ester. The catalytic amount of a strong inorganic acid like sulfuric acid (H2SO4), Hydrochloric acid (HCl) and H3PO4  while organic acid include benzenesulpohonic or ρ- toluenesulphonic acid are used because they are soluble in organic solvent and they are also used as catalyst without adding additional amount of water

In the process of acid hydrolysis ester is heated with large amount of water which contain a strong acid catalyst. This reaction is reversible and does not go for completion like esterification.

For example: butyl acetate reacts with water to produce acetic acid and 1- butanol.

   CH3COOCH2CH3CH3 + H2O ⇋ CH3COOH + CH3CH2CH2CH2OH

    Butyl acetate                 water       Acetic acid        1- Butanol (butyl alcohol)

Esters derived from an Acid chloride and an Alcohol

Acid chloride and alcohol reacts with each other in the presence of a base like pyridine or Na2CO3 to make esters, the weak base is neutralize the liberated HCl.

                                                  Pyridine

Butanoyl chloride + Ethanol         →    Ethyl butanoate + hydrochloric acid

Ester derived from an Acid Anhydride and an Alcohol or a Phenol

Acid anhydride react with phenol or alcohol via nucleophilic acyl substitution to produce ester in the presence of a catalyst like strong acid (H2SO4) or a weak base (pyridine) or through heating.

Ester hydrolysis 3

Hydrolyzing complicated esters for soap making

Hydrolyzing big ester are commonly found in animals and vegetables fats or oils. Large esters are heated with concentrated sodium hydroxide produce a salt of carboxylic acid and octadecanoic acid (stearic acid). The salt is very important by-product which is helpful in cleaning. An alcohol is also produced called as propane-1,2,3-triol (glycerol).

Ester hydrolysis 4

The alkaline hydrolysis of an ester called as saponification.

Uses and Application

  • Presence of ester in fruits and flower is responsible for flavor and fragrance like: isopentyl acetate in bananas, methyl salicylate in wintergreen and ethyl butyrate in pineapple [5].
  • Volatile ester is used in the preparation of synthetic flavor, perfumes and cosmetics.
  • Ester in volatile in nature also used as solvent in lacquers, paints and varnishes, for example ethyl acetate and butyl acetate.
  • In animals and plants waxes are secreted, they are ester formed from long chain carboxylic acid and long chain alcohol.
  • Low volatile liquid esters are used as softening agents in resins and plastics.
  • As a polymer, Polymethyl methacrylate is an ester, it is a glass substitute sold as Lucite and Plexiglas.
  • Phosphate esters are important on industrial point of view, they are used as solvent, plasticizers, flame retardants, gasoline, oil additives and as insecticides.
  • Sulfuric and sulfurous acid ester are used in the preparation of dyes and pharmaceuticals.

Mechanism of the acid catalyzed hydrolysis of esters

First step

Activate the ester because an acid/base reaction, weak nucleophile and a poor electrophile is present. So the protonation of the ester carbonyl make it more electrophilic [4].

Second step

The oxygen (O) of water function as nucleophile attack on the electrophilic C in the C=O and electron moving towards oxonium ion, making tetrahedral intermediate.

Third step

Deprotonate the oxygen which is came from the water molecules and neutralize the charge.

Fourth step

There is a need to make the –OCH3 leave, but first protonation and then convert it into good leaving group.

Fifth step

With the help of electron of adjacent oxygen “push out” the leaving group called neutral methanol molecule.

Sixth step

Now deprotonation of the oxonium ion make carbonyl C=O in the carboxylic acid product and reproduce the acid catalyst.

Ester hydrolysis 5

Mechanism of base hydrolysis of Esters

First step

The hydroxide nucleophiles attack on the electrophilic C of the ester compound C=O, they break the double bond and make the tetrahedral intermediate.

Second step

The intermediate collapses and again form the C=O results in the loss of alkoxide leaving group, leading to the carboxylic acid.

Third step

Equilibrium is formed where alkoxide, RO behave as a base deprotonating the carboxylic acid, RCO2H.

Ester hydrolysis 6

Applications of Ester Hydrolysis

  • Sodium acetic acid is a kind of salt. When water is added to sodium acetic acid derivation at that point the compound separate into sodium particles and acetic acid particle. Acetic acid the secondary particle combine with hydrogen and make acidic corrosive.
  • Alkyl halides used in refrigerants as CFCs (chlorofluorocarbons). When water is added in the alkyl halide substance it is changed in to another form called liquor, in this form it is secure for earth [6].
  • Hydrolysis of sucrose which is disaccharide and called as table sugar breakdown into fructose and glucose which is used for different purpose.

References

  1. https://www.chemguide.co.uk/organicprops/esters/hydrolysis.html
  2. https://chem.libretexts.org/Courses/Eastern_Mennonite_University/EMU%3A_Chemistry_for_the_Life_Sciences_(Cessna)/15%3A_Organic_Acids_and_Bases_and_Some_of_Their_Derivatives/15.09_Hydrolysis_of_Esters
  3. https://profiles.uonbi.ac.ke/andakala/files/upc_213_esters.pdf
  4. http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch20/ch20-3-3-1.html.
  5. https://www.britannica.com/science/ester-chemical-compound
  6. https://byjus.com/chemistry/ester-hydrolysis/
No ratings yet.

Please rate these notes