Chromatographie, Separation of the products formed in the nitration of Phénol

Tabarnoust                                                                                        28.04.2016

Ali-François                                                                                 Raum F3.045-24

[pic 1]

                Introduction

   In this experiment, we start with solid phenol and the objectif is to make nitrophenol.

For that, we are going to use nitric acid, water and dichloromethane to have the final result.

Reactions with nitric acid are complicated because nitric acid is an oxidising agent, and phenol is very easily oxidised to give complex tarry products.

                Reaction equation

[pic 2]

                Reaction Mechanism

   The reaction start with a substitution electrophile where an atom of Hydrogen H which is connected to a carbon of the Phenol is substitued by a group Nitro NO2 to create a Nitrophenol.

The electrophile used is the nitrium  NO2+ .

The reaction is slower than if we used Sulfuric Acid H2SO4 for catalyse.

We used nitric acid diluted and at the end, we will have a majority of 2-Nitrophenol (ortho) and 4-Nitrophenol (Para).

3-Nitropheno will be create in a very small amount, because the mesomere effects shows that Ortho and Para have more effects than Meta.

                Experimental Procedure

   A solution of Nitric acid diluted (Water+Nitric acid) of 5mL is added to solid phenol (1,5g) in a bottom flask.

The solution is stirred to accelerate the reaction at room temperature.

We extract the product which dichloromethane to separate the aquous and the organic phase.

At the end, we use rotary evaporation to separate the product and the dichloromethane.

We make a chromatography column with Silica Gel and the eluent used is composed by 20% of Hexano and 80% of dichloromethane. We choose this mixture after making preparatory test with thin layer column.

We put the half of the product in the column.

After taking approximately 40 fractions of the column, we make thin layer chromatography and finally, we take the seventeen first fractions.

These fractions are the fractions which have products.

When me make the thin layer chromatography, we put for reference the original product diluted, to be less concentrate (1 or 2 drops in 5mL of dichloromethane)

And then, we put the product on the thin layer and we observe that there are 3 portions. One very high (the 2 first fractions), one in the middle (3-7 fractions) and one low (8-17 fractions).

The fractions are collected in a bottom flask and we remov the solvent by rotoary evaporation.

Then, a high vaccum durung 20 minutes is making before to make a sample for the  1H NMR Spectroscopy with Chlorophorm.