Organic Chem Lab 2: The Separation of a Mixture of Benzoic Acid, 2-Naphthol and 1,4 Dimethylbenzene by Extraction PDF

Preview

Summary

Organic Chem Lab 2: The Separation of a Mixture of Benzoic Acid, 2-Naphthol and 1,4 Dimethylbenzene by Extraction...

Description

The Separation of a Mixture of Benzoic Acid, 2-Naphthol and 1,4 Dimethylbenzene by Extraction

Abstract: The objective of this experiment is to separate a mixture of benzoic acid, 2-naphthol and 1,4 dimethylbenzene by extraction. Introduction: Extraction is a useful means of separating organic compounds if one compound in the mixture can be chemically converted to an ionic form. The ionic form is soluble in an aqueous layer and can be extracted into it. Other, non-ionic organic compounds in the mixture will remain dissolved in the organic solvent layer. Separation of the two layers results in the separation of the two compounds. The extent to which an acid-base reaction proceeds to completion depends upon the relative acidity of the reactants and products. Reactions occur so that stronger acids and bases are converted into weaker conjugate base and conjugate acids, respectively.  The pK a value of the acids gives a measure of the acidity of each compound. Stronger acids have smaller pK a values and their conjugate bases are weaker. The position of an acid-base equilibrium can then be predicted from knowledge of the pK a values of the acids involved. In this experiment, the three components of the mixture will react differently to sodium bicarbonate and sodium hydroxide because each of the bases’ conjugate acids has a different pKa. The pKa’s of benzoic acid and 2-naphthol are 4.17 and 9.5, respectively, while naphthalene

is a neutral compound. Since benzoic acid is much more acidic than 2-naphthol, the weak base, sodium bicarbonate, will be able to effectively remove benzoic acid’s acidic hydrogen. It will take the much stronger base, sodium hydroxide, to remove the hydroxide hydrogen from 2-naphthol. Both of the sodium salts formed from the base extractions will be soluble in water, which allows for separation of the compounds. First, the water soluble benzoate anion will be separated from 2-napthol and 1,4-dimethylbenzene remained in the organic layer. Then the water soluble 2-naphtolate anion can be separated from 1,4-dimethylbenzene. To examine the purity of the samples of each compound recovered from extraction, thin layer chromatography was used and the melting point range of each sample was observed. Table1. Physical and Chemical Properties of Chemicals Used Throughout the Experiment

Compound

Molecular Formula

Molecular Weight (g/mol)

Density (g/mL)

Amount Used Melting Point (g) (°C)

Benzoic Acid

C7H6O2

122.7

1.316

1.023

122.4

2-Naphthol

C10H8O

144.17

1.217

1.059

123.0

1,4-Dimethyl -benzene

C8H10O2

138.17

1.053

1.043

56.0

Results: BENZOIC ACID Weight, original sample Weight, recovered sample Percent recovery Melting point (observed range)

1.023g 0.512g 50.0% 121.7-125.3 °C

2-NAPHTHOL Weight, original sample Weight, recovered sample Percent recovery Melting point (observed range)

1.059g 0.293g 27.7% 119.3-122.7 °C

1,4-DIMETHYLBENZENE Weight, original sample Weight, recovered sample Percent recovery Melting point (observed range)

1.043g 0.0433g 41.5% 42.7-61.0 °C

Thin Layer Chromatography

Table 2. Experimental data from TLC plate Compound

Spot 1 Length(cm)

Spot 2 Length(cm)

Spot 3 Length(cm)

Benzoic Acid

3.2

N/A

N/A

2-Napthol

3.1

3.9

N/A

1,4-Dimethylbenzene

N/A

3.9

4.7

Table 3. Calculated Rf values from experimental data Compound

Spot 1 Rf

Spot 2 Rf

Spot 3 Rf

Benzoic Acid

0.667

N/A

N/A

2-Napthol

0.645

0.813

N/A

1,4-Dimethylbenzene

N/A

0.813

0.979

Discussion: The objective of this experiment was to separate a mixture of benzoic acid, 2-Naphthol and 1,4-Dimethylbenzene of known proportions by extraction. The three components of the mixture reacted differently to sodium bicarbonate and sodium hydroxide because each of the bases’ conjugate acids has a different pKa. When the solution of benzoic acid, 2-napthol and 1,4-dimethylbenzene dissolved in an organic solvent was reacted with aqueous sodium bicarbonate, benzoic acid was converted to the water soluble benzoate anion while 2-napthol and 1,4-dimethylbenzene remained in the organic layer since they do not react with sodium bicarbonate. With benzoic acid removed from the organic layer, this allowed for further separation of 2-napthol and 1,4-dimethylbenzene. With the addition of aqueous sodium hydroxide, a strong base 2-napthol was converted to the water soluble 2-naphtolate anion. After both the sodium bicarbonate and sodium hydroxide extractions, only 1,4-dimethylbenzene remained in the organic layer. 1,4-dimethylbenzene did not react with either sodium bicarbonate and sodium hydroxide because of its neutrality. To recover benzoic acid and 2-napthol they were each converted into precipitates. Neutralization of the aqueous solution of sodium benzoate with HCl caused benzoic acid to precipitate. Similarly, neutralization of the aqueous solution of sodium 2-naphtholate with HCl caused 2-naphthol to precipitate. The purity of the recovered samples were indicated by their melting point ranges. The purer the sample, the narrower the melting point range. The recovered benzoic acid and 2-napthol had very narrow melting point ranges, whereas the recovered 1,4-dimethylbenzene had

a wide melting point range of 42.7-61.0 °C indicating the lack of purity of the recovered compound. This is consistent with the findings of the TLC where we can see 2-napthol present in the recovered 1,4-dimethylbenzene. It is also consistent with the weight of the recovered samples. There was only a 22.7% recovery of 2-napthol whereas there was a 41.5% recovery of 1,4-dimethylbenzene, even with human error leading to loss of the recovered sample. This suggests that 2-naphthol was not extracted all the way by aqueous sodium hydroxide, allowing for it to be present in the recovered 1,4-dimethylbenzene sample. The 50.0% recovery of benzoic acid also suggests that benzoic acid was not completely extracted by sodium bicarbonate leading to its presence in the recovered 2-naphthol sample, which was also reflected in the TLC performed. At the end of the experiment three different compounds were successfully separated and recovered, however, the yeild and TLC findings reflect the impurity of these extracted compounds....