Question

In a isolation of organic compounds phenanthrene, methyl 4-aminobenzoate and 1,4-dibromobenzene was dissolved in diethyl ether. HCl was used to separate the aqueous and organic layer. The aqueous layer contains methyl 4-aminobenzote +H⁺ +Cl^-. 6M NaOH was added to the aqueous layer, methyl 4-aminobenzote + water and NaCl was yield after the NaOH addition. This was followed by the addition of dichloromethane. After this addition you have two layers: one layer contains methyl 4-aminobenzoate + water +dichloromethane, the other layer contains water and NaCl.

(1)Can you explain and draw the reaction that takes place when HCl is added? Why was it necessary to add HCl?

(2)Can you explain and draw the reaction that happens when NaOH is added? why was it necessary to add NaOH?

(3) in a flask you had two layers one contained methyl 4-aminobenzoate, water and dichloromethane and the other layer contained water and NaCl. How can you identify the aqueous layer from the organic layer. Which layer is on top and which layer is on the bottom and why?

(4) when doing a crystallization a student is given a flask with phenanthrene, 1,4-dibromobenzene and diethyl ether. The solvent of choice for the crystallization is ethyal alcohol. Can you draw the reaction that takes place when ethyl alcohol is added to the flask containing: phenanthrene, 1,4-dibromobenzene and diethyl ether? Why was ethyl alcohol the solvent of choice? Do you think there is a better solvent of choice?

Answer 1

#1

The following reaction takes place when HCl is added to it

Methyl-4-aminobenzoate is slightly soluble in water. when we add HCl to it, the amino group gets protonated in acidic conditions and forms hydrochloride salt which is water soluble.

In this way we can extract the salt form which is water soluble into aqueous layer.

Therefore in order to extract methyl-4-aminobenzoate from organic layer to aqueous layer, we add HCl

#2

When NaOH id added, it neutralizes the acidic conditions created by HCl , as a result amino group get deprotonated again forming original compound , methyl-4-aminobenzoate , most of which is water insoluble.

This compound can then be separated using organic solvent like dichloromethane

The reaction is given below

#3

Two layers that we have are

Organic layer containing dichloromethane + methyl-4-aminobenzoate

Aqeuos layer containing water + NaCl

density of dichloromethane is about 1.33 g/ml as compared to water which is around 1 g/ml

As a result more dense layer would be heavier and would remain on the bottom and aqueous layer would float on it

So the layer on top is Aqueous layer and layer on bottom is organic layer

#4

The given mixture contains phenanthrene, 1,4-dibromobenzene and diethyl ether. Out of these , diethyl ether is liquid at room temperature and itself acts like a solvent.

Both phenanthrene and 1,4-dibromobenzene are solid at room temperature.

phenanthrene is a non polar substance whereas 1,4-dibromobenzene is slightly polar.

Due to non polar nature of phenanthrene, it is more soluble in diethyl ether as compared to ethyl alcohol.

On the other hand , 1,4-dibromobenzene is soluble in ethyl alcohol.

Therefore when ethyl alcohol is added to mixture, 1,4-dibromobenzene get dissolved in it and forms an aqueous layer which occupies bottom layer

and diethyl ether dissolves phenanthrene and floats on aqueous layer.

when 2 layers are separated, get aqueous layer containing 1,4-dibromobenzene. Ethyl alcohol is allowed to vaporize and we get crystals of 1,4-dibromobenzene.

The organic layer containing diethyl ether and phenanthrene can be subjected to fractional distillation, where diethyl ether can be distilled from the mixture and solid that is left behind is phenanthrene crystals

Therefore ethyl alcohol is the solvent of choice.

The solvent needed here should be such that it has low affinity for non polar substances at the same time dissolving slightly polar substance .