Preparation of Organic Compounds

Objectives

Our objectives are to prepare:

  • A sample of acetanilide from aniline.
  • A sample of dibenzal acetone.
  • A sample of p-nitro acetanilide from acetanilide.
  • A sample of 2-naphthol aniline dye.

The Theory

What is Organic Chemistry?

Organic chemistry is a branch of chemistry that deals with the study of the structure, properties, reactions and preparation of organic compounds. As carbon and hydrogen are the essential constituents of all organic compounds, they are known as hydrocarbons. Organic compounds not only contain carbon and hydrogen, but also certain other elements such as: oxygen, sulphur, nitrogen, and halogens. These are regarded as the derivatives of hydrocarbon.

Which was the first organic compound to be synthesised in the lab and by whom?

Urea was the first organic compound to be prepared in the laboratory, which was by synthesised chance. It was prepared by Friedrich Wohler, a German chemist in the year 1828. Wohler synthesised urea from an inorganic compound, ammonium cyanate.

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What is organic synthesis?

Organic synthesis is a method of preparation of organic compounds. It is concerned with the preparation of organic compounds through organic reactions.

Synthesis of some simple organic compounds.

1. Acetanilide

Acetanilide is prepared from aniline by acetylating it with acetic anhydride in the presence of glacial acetic acid. Aniline or phenylamine is a primary amine with molecular formula C6H5NH2. It is basic in nature. Acetic anhydride is an anhydride of acetic acid and acts as a source of acyl group. Aniline reacts with acetic anhydride by nucleophilic substitution reaction. The reaction between aniline and acetic anhydride is called acetylation. In this reaction, aniline acts as the nuclepohile and acyl (CH3CO-) group from acetic anhydride acts as the electrophile. Here, the hydrogen atom of –NH2 group is replaced by the acyl group. The chemical reaction is shown below.

2. Dibenzal acetone

It is prepared from benzaldehyde and acetone by Claisen-Schmidt Reaction.

Claisen Schmidt Reaction is a condensation reaction between an aromatic aldehyde and an aliphatic ketone in the presence of a strong base like NaOH leading to unsaturated ketone. In this reaction, two molecules of water are eliminated.

In the preparation of Dibenzal acetone, two moles of benzaldehyde condenses with one mole of acetone. The chemical reaction is represented as follows;

3.  p-Nitro acetanilide

The direct preparation of p-nitroacetanilide from aniline using a nitrating mixture is very difficult. This is because, in the presence of the nitrating mixture, the amino (-NH2) group of aniline is oxidised to the nitro (-NO2) group and forms nitro benzene.

The amino group of aniline is first protected by acylation with acetic anhydride to produce acetanilide which is then nitrated to form p-nitroacetanilide as the major product and o-nitroacetanilide as the minor product. To separate p-nitroacetanilide from this mixture, the reaction mixture is recrystallised in ethanol, so that the more readily soluble o-nitroacetanilide dissolves in ethanol and pure p-nitroacetanilide separates out.

4.  2-Naphthol aniline dye

2-Naphyhol aniline dye is an azo compound and it is a scarlet red dye. It is mainly used for dyeing textiles. It contains the characteristic group. Azo compounds have an extended conjugated system and are often coloured and are used as dyes. These compounds are prepared by the reaction known as coupling reaction. 2-Naphthol aniline dye is prepared from aniline.

Aniline is first converted to diazonium salt by treating it with nitrous acid at >5oC. Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid. The procession of conversion of aromatic primary amines into diazonium salt is known as diazotisation. This benzene diazonium compound undergoes a coupling reaction with 2-Naphthol to form 2-Naphyhol aniline dye.

Learning Outcomes

  1. Students understand the concepts of organic compounds, classification of organic compounds, functional groups, substitution, addition, elimination and rearrangement reactions.
  2. Students acquire the skills to prepare:
    • Acetanilide
    • Dibenzal acetone
    • p-Nitro acetanilide
    • 2-Naphthol aniline dye 
  3. Students understand the chemicals required for the preparation of the above compounds.
  4. Students understand the theory behind the formation of acetanilide, dibenzal acetone, p-Nitro acetanilide and 2-Naphthol aniline dye.
  5. Once they understand the different steps, students can perform the experiment in the real lab more accurately and quicker.
  6. Students understand important safety precautions before using hazardous chemicals. 

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