Experiment Product of Grignard Reaction

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The purpose of this experiment was to synthesize 3-methylheptan-3-ol using the Grignard reagent under refluxitil. The Grignard reaction utilized bromobutane in diethyl ether to react with 2-butanone.

Introduction

Grignard reagents are organomagnesium compounds with the formula RMgX (Carey, 186). X is a halogen that is normally Br or Cl. The R is an alkyl or an aryl group. The synthesis of these reagents involves the use of alkyl or aryl halides in diethyl ether. Making Grignard reagents requires addition of the halide into small bits of Mg (Carey, 187). These magnesium bits are contained in a flask already containing diethyl ether. The flask is fitted with reflux condenser so that the contents are heated under reflux. This solvent provides electrons to the Mg so that the octet configuration is attained by the Mg metal (Wade, 170). This enhances the stability of the reagent in the solvent (Sasian, 34). The reaction that generates the Grignard reagent proceeds by single electron transfer.

This Grignard reagent reacts with ketons and aldehydes to form alcohols. The alcohol produced can either be primary, secondary or tertiary (Carey, 190). Its reaction with formaldehyde generates primary alcohol and with ketons it generally generates tertiary alcohols (Sasian, 23). This is what the experiment is trying to verify. The Grignard reagents are nucleophilic in nature and can easily react with the electrohile to generate the alcohol (Wade, 174). The electrophile in this case is the 2-butanone due to partial positive charge on the carbon attached to the oxygen. This variation is generated by the high electronegativity of oxygen compared to carbon.

The reactants in this experiment were bromobutane and magnesium. The solvent used was diethyl ether. The table below shows their properties.

Chemical Symbol Molecular weight Density Melting point B.P BromobutaneC4H9Br 137.02 g/mol 1.26 g/ml -1120C Diethyl ether CH3CH2OCH2CH3 74.12 g/mol 0.71 g/ml -116.30C 34.60C Magnesium Mg 24.31 u 1.74 g/ml 6500C 1,0910C 

The Grignard reagent synthesized was spectroscopically analized to determine the success of synthesis. The IR and NMR spectra for carbon and proton were done. Synthesis involved heating under reflux. This is to ensure control of temperature under which the reaction takes place. This is emphasized since ether boils at 35oC and t is highly flammable. Heating the reacting contents under reflux therefore ensures that the reaction proceeds without direct heat sources or hot plates which would otherwise cause ignition.

The spectroscopy employed in analysis of Grignard reagent is used to detect specific peaks that correspond to the ones known to appear in spectra of these reagents. The only demerits associated with the analysis if any can be due to impurities. Impurities can be caused by detection of ether during the spectroscopic analysis. Some picks associated with ether can be detected in the proton, carbon NMR or even the IR detectors. These characterization techniques are otherwise essential in determining the success of the experiment.

Procedure

The procedure outlined in the laboratory manual was followed with some minor changes being made. The changes that were done involved the use of 2-butanone (Lab Manual). As a result, single necked flask fitted with Claisen adaptor was used instead of three necked flask. This in turn changed the method of adding bromobutane and diethyl ether. 2.4 g of Mg were weighed and a few pieces crushed. They were then added into the single neck flask. As small crystal of iodine was then added to the flask and the glassware dried with a Bunsen burner after the drying tube had been removed. The tube was then replaced but the system was not open to air again. The rest of the procedure was followed according to the manual.

Data

1.5g bromobutane * (1mol / 137.02g) = 0.01095 mol 2.4 g magnesium * (1mol / 24.3g) = 0.09877 mol limiting reagent = bromobutane0.09877 mol * (161.32 g / mol) molecular weight n-butyl magnesium bromide = 15.93 g 15.93 g n-butyl magnesium bromide * (1mol / 161.32 g) = 0.09877mol 0.5 g 2-butanone * (1mol / 72.11 g) = 0.00693 mol 2-butanone = limiting reagent. Theoretical Yield = 0.00693mol * (130.228 g / 1mol) 3-methylheptan-3-ol = 0.83g Actual Yield: 0.352g Percent Yield: (0.352g / 0.83 g)*100 = 42.17 %

Compound Expected yield Actual yield % yield

3-methylheptan-3-ol 0.83 g 0.35 2 g 42.17 %

18802351475740Fig. 2: IR data for 3-methylheptan-3-ol

4000020000Fig. 2: IR data for 3-methylheptan-3-ol

Discussion

The IR spectrum obtained indicated that the reaction reached completion. This implied that 3-methylheptan-3-ol was formed as a result of the reaction reaching end point. The product formed therefore indicated that Grignard reagent was formed during the reflux of bromobutane in diethyl ether with Magnesium. Further Grignard reaction with 2-butanone resulted into the formation of the product; 3-methylheptan-3-ol.

The peaks in the IR obtained from the experiment correspond to the ones known for the 3-methylheptan-3-ol. There is lack of a peak at frequency between 1710 and 1770 which indicates lack of carbonyl peak in the IR (Mol-Instincts). This shows that the reaction reached completion since if it were not to completion then the peak for the 2-butanone would have been detected. The peak at around 2900 cm-1 corresponds to the tertiary alcohol peak. This further illustrates the formation of the intended product (Mol-Instincts). The car bon and proton NMR spectra also qualify the formation of the product. The peak observed at 70 ppm is observed for the spectra in literature. The carbon peaks indicate presence of eight carbons and the ranges of these peaks for the products spectra is found to be similar to those in literature (Mol-Instincts).

The results obtained in this experiment showed that 3.352 g of the products was formed. This translated to 42.17% yield. The expected yield was 0.83 g. This deviation of about 64 % can be directed towards errors that could have been experienced during the experiment. Such errors include inaccuracy in measuring the accurate volumes and weights of the reactants. In addition, some of the Mg might have not reacted completely with the bromobutane. This in return could have contributed to the low yield of the Grignard reagent. The lower the low amount of Grignard, the lower the quantity of the alcohol produced since the alcohol production depended on the amount of Grignard reagent present.

Conclusion

The experiment was successful since the intended product of Grignard reaction was achieved. There could have been errors that might have led to the low yield of the product.

References

Carey, Francis A. Organic Chemistry. Boston: McGraw-Hill Higher Education, 2008. Print.

Sassian, Meeri. Reactions of Partially Solvated Grignard Reagents. Tartu, Estonia: Tartu University Press, 2003. Print. 60pg

Wade, L G. Organic Chemistry. Upper Saddle River, N.J: Prentice Hall, 1999. Print.

Mol-Instincts, A Fundamental Chemical Database based on Quantum Mechanics & QSPR, Copyright by ChemEssen, Inc. (http://www.mol-in.com). MIID: 0001-ixnd, http://search.molinstincts.com/spectra/spectra.ce?0001-ixnd (retrieved February 17, 2016).

 

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