ester Hydrolysis

In a reflux apparatus, the sodium hydroxide solutions were used to perform the hydrolysis of the ester in an alkaline medium. Following the hydrolysis, the mixture's free alcohol component was eliminated using a distillation device. The chromate ions caused the alcohols to oxidize, forming the corresponding aldehyde groups. Tollens reagents or Bayers reagents are used in a straightforward qualitative test to determine whether the presence of aldehyde groups is present. To obtain pure crystals of carboxylic acid, the chemical was isolated and then purified using sulfuric or hydrochloric acid. Finally, there was a determination of the melting point (136-140) and testing the sample in the infrared spectrometer. The monogram is compared with the pure monogram of 99.99% purity. The peaks of the sample and pure compounds should be similar in the absorbance and the peaks of the molecules in the wavenumber between 4000 and 400cm-1.















Introduction

Hydrolysis is a chemical reaction where water is used as a reactant to break down a functional group into constituent parts, therefore separating a portion of the molecule. For instance: hydrolysis of ester in the presence of a base catalyst or an acid to form carboxylic acid and alcohol. Ester belongs to a class of organic compounds that react with water to produces alcohols, inorganic or organic acids (Colquhoun et al. 2013). Carboxylic acid esters with the formula RCOOR1 are prepared by reactions of alcohols and carboxylic acid in the presence of a sulphuric acid or hydrochloric acid through a process known as esterification (Malik et al. 2014). The hydroxyl groups of carboxylic acid replace the alkyl group (R1O) of the alcohol. The hydrolysis of esters using alkaline medium is essential in industries when manufacturing soaps through the saponification process.

The reverse of the esterification is an example of hydrolysis. The hydrolysis of ester in the presence of alkali for instance sodium hydroxide is called saponification, mainly utilized in the preparation of soaps from oils and fats, and also a quantitative estimation of esters (Malik et al. 2014). And so, hydrolysis is an essential reaction of esters. Ester compound undergoes hydrolysis, distillation to remove free alcohols, isolations and purification of acid to obtain a carboxylic acid.



Figure 1: (Malik et al. 2014).

Method

Equipment and reagent

Anti-bumping reagent

Silicone grease or petroleum jelly

2(retort clamps and stands)

Electrically heated Isomantle (150ml)

2(laboratory jacks)

Round bottomed flask (150ml)

Condenser and 2(rubber tubes)

1Conical flask (50ml) and 1(250ml)

Weighing boat

10ml measuring cylinder

Still head

Distillation thermometer

Receiver adaptor

Glass rods

Ice bath

Bunsen burner and mat

2(sidearm test tubes Buchner funnel, filter paper and flasks)

Large rubber pressure hose

3(250ml conical flasks)

Hotplate

Filter paper melting paper capillary tubes

Melting point machine

Ester a and ester b

Ethanol

20% sodium hydroxide solution

2M hydrochloric acid

Sodium dichromate

Dilute Sulphuric acid

Brady’s reagent

Tollen’s reagent

Dilute ammonium hydroxide solution



Procedure

Weigh 4grams of ester and place in a 150ml round-bottomed flask.

Flux with 20% sodium hydroxide for 30 minutes

Remove the alcohol component from by mixture cooling down the mixture and then distilling the mixture

Distill off 6ml of the aqueous alcohol slowly

Cary out qualitative test using 3 ml aliquots of the distillate using Tollen’s or Brady’s reagent

Add 25 ml of distilled water to the round bottom flask

Add dilute hydrochloric acid to precipitate carboxylic acid and stop when you see no more precipitation

Collect the solid in Buchner and wash the solid cake with small amount of water

Place two 150 ml of beakers, one containing 75ml of water and one containing 75ml of alcohol on the hot plate as the sample filter on the Buchner.

Transfer crude product to a 250 ml conical flask place on a hot plate.

Add warm and not boiling alcohol to enhance recrystallization

After the solid has completely dissolved in minimum amount of warm ethanol, one can start to spot in warm water until the solutions go slightly cloudy

Remove the conical flask on a hot plate and give time to cools down.

The product of crystallization should appear on cooling.

After crystallization, collect the purified product and air dry the product for a few minutes

Dry the product in the oven at 800C for 30 minutes.

Record the melting point and yield, and infrared spectrum of the purified carboxylic acid and compare with that of original starting ester.

Results and Discussion



Figure 2 above: ester B (Parker 2012)





Figure 3 above: Ester B and pure compound (Parker 2012)



Figure 4 above: Carboxylic acid from ester B (Parker 2012)





Figure 5 above: carboxylic acid from ester B and pure compound (Parker 2012)

During the hydrolysis of esters, the esters are heated under reflux with a dilute alkali such as sodium hydroxide solution. There are two advantages of hydrolysis using alkali rather than using dilute acid (Malik et al. 2014). The products are readily separable and the reactions are reversible (one-way)

For instance ester hydrolysis using sodium hydroxide solution

RCOOR1 +NaOH RCOONa + R1OH (Colquhoun et al. 2013) 

Therefore, one gets the sodium salts and not the carboxylic acid. Provided one uses the excess sodium hydroxide solutions; the mixture is relatively smooth to separate (Malik et al. 2014). Alcohol can be distilled off if the main aim is to get acid rather than salts. For distillation, there is an addition of the strong acid such has diluted sulphuric or hydrochloric acid to the solution left after the first distillations. Thus, the acid floods the mixture with hydrogen ions. The ions are picked by the alkanoate ions present in the salts to forms alkaloid acid or carboxylic acid. Since they comprise weak acids, once the alkanoate and hydrogen ions combine, they tend to stay in that nature (Malik et al. 2014). Then the carboxylic acid can distill of easily. The hydrolysis of the ester using water or dilute acid is a slow process. The hydrolysis reactions are catalyzed by the dilute acid, and therefore it is heated in reflux (Malik et al. 2014). The reactions are reversible and one has to use excess water to make the reaction complete.

The wavelength of infrared radiations is between wavenumber 500 and 4000cm-1 (Ferraro and Basile 2012). An IR active bond is a bond that changes dipole during vibration while IR inactive is an asymmetric bond that does no change dipole in the process of vibration. Positions of great importance in an IR spectrum are the area of absorbance (wavenumber), an intensity of absorbance (dipole of the bond) and the shape of the absorbance (sharp or broad peaks). Alcohols have a stiff bond because of higher vibrational frequencies. Thus, the carboxylic acid will have 3000 cm-1 whereby the 0-H bond is involved in the hydrogen bonding (Ferraro and Basile 2012). And so, each bond will experience a different vibrational frequency. Carbonyl peaks of ester happen at 1735-1750cm-1 while acid shows carbonyl groups peaks at 1700-1730 and 3000 cm-1 broad peak (Parker 2012). The large dipole of carbonyl bonds makes all the carbonyls groups shows sharp and robust peaks. However, most carbonyls stretching frequencies happens around 1700-1730cm-1, and thus it can be distinguished from alkenes (low 1600cm-1) as a result of higher frequency and more intense absorbance (Ferraro and Basile 2012). For instance, esters have a higher stretching frequency of around 1742cm-1 (Ferraro and Basile 2012). The higher frequency of the carbonyl groups depends on the substituents on the carbonyl carbon; inductive or resonance effect (Colquhoun et al. 2013).

The inductive effect occurs where an electronegative atom (X) pulls electron density from the carbon, which subsequently draws electrons from the oxygen creating a stiffer bond (Cainelli and Cardillo 2012). The Lone pair of electrons on atom X can resonate to produce a C=X bond, and therefore a C-O single bond (weaker C-O bond) (Colquhoun et al. 2013). Hence, the more the resonance donation of an electron to the carbonyl carbon, the lower the frequency of the C=O stretch (Colquhoun et al. 2013).

Benzoic acid shows a broad and robust band for the O-H stretch. The stretch appears at the regions between 3000-2500cm-1, being centered at 3000cm-1(Parker 2012). Peak shows in the same areas as the C-H stretching of the aromatic and alkyl groups. The broad O-H band is superimposed on the sharp C-H stretching bands. The reason for the O-H band of the benzoic acid is that carboxylic acid usually exists as hydrogen-bonded dimers. The C-O stretch of the benzoic appears in the regions 1320-1210 cm-1 and the 0-H bend is in areas 1400-1395 and 950-910cm-1(Ferraro and Basile 2012).

Ethyl benzoate being alpha and beta-unsaturated esters, it C=O appears from 1730-1715 cm-1(Parker 2012). The C-O stretches show as two or more bands in the region between 1300 and 1000 cm-1. Above the 3000 cm-1, the ethyl benzoate shows a minimal peak resulting from the C-H stretch of aromatic. Also, there is a peak at around 3000 cm-1 due to the stretch of C-H of an alkyl group (Ferraro and Basile 2012).

Reactions on the left-hand tube

Partial Oxidation of the alcohol from the experiment

The oxidation reactions of the alcohols depend on the types of the alcohols. And so, the alcohol in this experiment was oxidized to form an aldehyde of the same carbon chain. Since there was the presence of vacuum to pull the aldehyde to the second tube, the alcohol did not oxidize to form carboxylic acid.

General Equation for partial oxidation reaction:

ROH + [O] RCHO+ H2O (Cainelli and Cardillo 2012) 

Observation: the color changes from orange to green because of dichromate ion (Cr2O72-) reduction to Cr3+ ion (Cainelli and Cardillo 2012).

Brady’s reagent from the aldehyde obtained from left-hand side tube

2, 4-dinitrophenyl hydrazine solution react with an aldehyde to give a red or orange precipitate of aldehyde 2, 4-dinitrophenylhydrazone derivative.



Figure 6: (Colquhoun et al. 2013) 

Tollens reagent (silver mirror test)

Ag2O (s) + 4 NH3 + 2 NaNO3 + H2O → 2 [Ag (NH3)2] NO3 + 2 NaOH (Colquhoun et al. 2013) 

Tollens’ reagent is an alkaline solution of ammoniacal silver nitrate which is used to test aldehydes. Silvers ions come out as a brown precipitate of silver (1) oxide in the presence of hydroxide ions. In aqueous ammonia, precipitate dissolves to form diamminesilver (1) ion, ({Ag (NH3)2} +, however, ketones do not show a positive test for Tollens’ reagent (Colquhoun et al. 2013).

The diamminesilver ion reduces to silver metal and aldehyde is oxidized to a carboxylic acid.

RCHO + 2[Ag (NH3)2] ++ 2OH- ---> RCOOH + 2Ag + H2O (Colquhoun et al. 2013) 

Question1

Loss of a proton from an acid with a general formula HA forms the anion A-. Thus, the more stable the A-, the stronger the HA acid. For instance, carboxylic acids (RCO2H) are stronger than alcohols (ROH) because the Alkoxide ions (RO-) are less stable than carboxylate ion (RCO2-) (Colquhoun et al. 2013). The carboxylate is stable because of the stabilization by the resonance. Resonance happens when the molecules with delocalized electrons are described by two or more structure. So, resonance energy is the extra stability gained by the electron delocalization.

Question 2

Oxygen is more electronegative than carbon and thus strong tendency to pull electrons in carbon-oxygen bonds. One of the lone pairs of electrons is quickly drawn to the oxygen; makes the double bond of carbon-oxygen highly polar (Eberson 2012). Therefore, nucleophile can attack a slight positive carbon in the carbonyl group. Ethanal is polar molecules due to the presence of the carbon-oxygen bond, and consequently, they have a Vander Waals dipole-dipole attraction and dispersion forces, and attractions between permanent dipoles in neighboring molecules(Eberson 2012). However, the alcohol has high boiling boing point because of the hydrogen bonding. The aldehyde is highly polar molecules, and therefore they do not have hydrogen bond. Ethanol has an OH group present. Thus, oxygen can pull electron density from the hydrogen making it partially positively charged (Eberson 2012). Positively charged hydrogen forms weak interactions with other molecules containing oxygen. However, more heat is needed to break the hydrogen bonds and boil. Ethanal has oxygen, but it is not attached to hydrogen. Hence there is no polarization of the hydrogen atom. The only bonding interaction present is weak Vander Waals forces and dipole-dipole interactions between the C=O groups (Colquhoun et al. 2013). Therefore, there is less intermolecular bonding to overcome in ethanal to separate all the molecules.

Question 3

The reaction of the Semicarbazide and aldehyde forms Semicarbazone through the condensation reaction with terminal -NH2 groups of the carbazide behaving like primary amines. A condensation reaction is a reaction involving two reactants, where a small molecule like water is formed as a by-product. However, the reactions tend to create two products. The most stable products are formed at the positions marked * because nitrogen atom at that position have lone pair of electrons that are not in resonance with C=O in Semicarbazide (Colquhoun et al. 2013). As a result, the reactions of the Semicarbazide and aldehyde are similar to the addition of ammonia derivatives to aldehydes or ketone.

H2*N-NH-(C=O)-NH2 + RHO → RCH=*N-NH-(C=O)-NH2 (De la Mare and Bolton 2013)

It is worth noting that although Semicarbazide has two amino groups (-NH2), only one group is a reactive amine. The other amide group is deactivated because of the adjacent carbonyl groups

Mechanism for imine formation

Nucleophile attack



Figure 7: (Colquhoun et al. 2013)



Transfer of proton



Figure 8: (Colquhoun et al. 2013)



OH protonation



Figure 9: (Colquhoun et al. 2013)



Removal of water



Figure 10: (Colquhoun et al. 2013)

Conclusion

Apparently, Hydrolysis of esters in alkaline medium gives the carboxylate ions and free alcohols due to the resonance effects. Alcohols obtained in the hydrolysis of esters shows a positive test with the Tollens and Bayers reagent. If the alkaline medium contains sodium hydroxide, then products after hydrolysis will be salt of the carboxylate. If pure carboxylic acid is required, then further purification with acid is essential. Therefore, the carboxylate ions will combine will free hydrogen ions in the mixture to form pure carboxylic acid. The test of carboxylic should be through melting point and test in the IR spectrometer. The process of the hydrolysis of an ester is important in making soaps through a process known as saponification.





























References

Cainelli, G. and Cardillo, G. (2012). Chromium oxidations in organic chemistry (Vol. 19). Springer Science & Business Media.

Colquhoun, H.M., Thompson, D.J. and Twigg, M.V. (2013). Carbonylation: direct synthesis of carbonyl compounds. Springer Science & Business Media.

De la Mare, P.B.D. and Bolton, R. (2013). Electrophilic additions to unsaturated systems (Vol. 9). Elsevier.

Eberson, L. (2012). Electron transfer reactions in organic chemistry (Vol. 25). Springer Science & Business Media.

Ferraro, J.R. and Basile, L.J. eds. (2012). Fourier transform infrared spectra: applications to chemical systems. Academic press.

Malik, S.R., Awan, B.A., Shafiq, U. and Mukhtar, A. (2014). Investigation of agitation effect on the conversion of saponification reaction in a batch reactor at STP conditions.

Parker, F. (2012). Applications of infrared spectroscopy in biochemistry, biology, and medicine. Springer Science & Business Media.