Projection Experiment: Nitration of Aromatics

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Nitration of Aromatics

Objective: Electrophilic Aromatic Substitution

Peter Keusch

German version

Chemicals:
10 mmol of benzene / ethanol 96 % (0.9 mL / 10 mL)
10 mmol of phenol / ethanol 96 % (941 mg / 10 mL)
0.2 mmol of 2-naphthol / ethanol 96 % (29 mg / 10 mL)
HNO₃ 65 %

Safety precautions:

Benzene is known to be a human carcinogen on sufficient evidence of carcinogenicity in humans. Breathing benzene can cause drowsiness, dizziness, and unconsciousness; long-term benzene exposure causes effects on the bone marrow and can cause anemia and leukemia. Severe eye irritant. Skin and respiratory irritant.

Phenol is acute poisoning by ingestion, inhalation or skin contact may lead to death. Phenol is readily absorbed through the skin. Highly toxic by inhalation.

Nitric acid may be fatal if swallowed or inhaled. Extremely corrosive. Contact with skin or eyes may cause severe burns and permanent damage.

2-naphthol is harmful if swallowed or inhaled. May be harmful in contact with the skin. Eye, skin and respiratory irritant.

Ethanol is highly flammable.

Safety goggles, protective gloves and effective ventilation required. The ethanolic solutions of benzene, phenol and 2-naphthol should be made under a fume hood!

Experimental procedure:

Three test tubes are set up as shown in the following below.

Test tube 1 1 mL of ethanolic solution of benzene
Test tube 2 1 mL of ethanolic solution of phenol
Test tube 3 1 mL of ethanolic solution of 2-naphthol

15 drops of HNO₃ are added to the solutions of the aromatics.

Results:

Test tube 1 - remains colorless
Test tube 2 immediately
after 1/2 minute
after 1 minute yellow-orange
intensively orange
red
Test tube 3 immediately yellow

Photo 1

Photo 2

Photo 3

Discussion:

· In contrast to benzene, phenol is nitrated by HNO₃ (65 %) already at room temperature. The nitrating agent is the nitronium ion (1).

· At a low reaction temperature a mixture of o-nitrophenol (approx. 35 %) and p-nitrophenol (approx. 15 %) is formed (Photo 2: orange solution) (2).

equation

· The significantly high reactivity of phenol compared to that of benzene is caused by the strong activating OH group. The electron donating resonance effect of the hydroxyl group outweighs the weaker electron withdrawing inductive effect. The electron density is higher in the pi-system of phenol then it is in benzene. So the electrophilic aromatic substitution is facilitated.

· Like phenol, naphthol also bears one OH group causing an acceleration of the substitution rate. Electrophilic substitution of 1-naphthol occurs at the 2- and 4-position. The hydroxy group in 2-naphthol activates the 1 position (3).

equation

Reference:
Rod Beavon Electrophilic substitution - Animation

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