Synthesis of 7-hydroxy-4-methylcoumarin

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BACKGROUND

Principle:

General synthesis of coumarins involves the interaction of a phenol with a β-ketoester in presence of an acid condensing agent (Pechmann reaction).

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Concentrated sulphuric acid is usually used as the condensing agent for simple monohydric phenols and β-ketoesters, although phenol itself reacts better in the presence of aluminium chloride.

The mechanism of the reaction is thought to involve the initial formation of a β-hydroxy ester, which then cyclises and dehydrates to yield the coumarin.

Polyhydric phenols, particularly here the two hydroxyl groups are meta oriented, react with great ease and sulphuric acid, is used as the condensing agent with careful temperature control to ensure a good yield.¹

Aim: To prepare 7-hydroxy-4-methyl coumarin.

Reaction:

Mechanism:

Use:

It is used commercially as laser dye. It is used as the starting material for the production of the insecticide ‘hymecromone’.

REQUIREMENTS

Chemicals:

Conc. sulphuric acid, Resorcinol, Ethyl acetoacetate, Ice, Sodium hydroxide solution (5%), Sulphuric acid (2 M), Ethanol 95%)

Apparatus:

Three necked flask – 3 litres, Thermometer, Mechanical stirrer, A dropping funnel, Beaker, Buchner funnel, Measuring cylinder, Filter paper

PROCEDURE

Take 1 litre of concentrated sulphuric acid in a 3 litres capacity 3 necked flask fitted with a thermometer, mechanical stirrer and a dropping funnel. Immerse the flask in an ice bath.

When the temperature falls below 10 °C, add a solution of 100 g (0.91 mol) of resorcinol in 134g (130.5 ml, 1.03 mol) of redistilled ethyl acetoacetate drop wise and with stirring.

Maintain the temperature below 10 °C by means of an ice-salt bath during the addition (2 h). Keep the mixture at room temperature for 18 h and then pour it into a mixture of 2 kg of crushed ice with vigorous stirring and 3 litres of water.

Collect the precipitate by suction filtration and wash it with three 25 ml portions of cold water. Dissolve the solid in 1500 ml of 5 percent sodium hydroxide solution, filter and add dilute 2 M sulphuric acid (about 550 ml) with vigorous stirring until the solution is acid to litmus.

Filter the crude 4-methyl-7-hydroxycoumarin at the pump, wash with four 25 ml portions of cold water and dry at 100 °C. The yield is 155 g (97%).

Recrystallise from 95 percent ethanol: the pure compound separates in colourless needles, melting point: 185 °C.

Calculation:

Here limiting reagent is resorcinol; hence yield should be calculated from its amount taken.

Molecular formula of resorcinol = C₆H₆O₂

And molecular formula of 7-hydroxy-4-methyl coumarin = C₁₀H₈O₃

Molecular weight of resorcinol = 110 g/mole

And molecular weight of 7-hydroxy-4-methyl coumarin = 176 g/mole

Theoretical yield:

110 g resorcinol forms 176 g 7-hydroxy-4-methyl coumarin

Therefore, 100 g resorcinol will form …….? (X) g 7-hydroxy-4-methyl coumarin

= 160 g

Theoretical yield = 160 g

Practical yield = ————- g

% Yield = (Practical Yield)/(Theoretical Yield) × 100

CONCLUSION

7-Hydroxy-4-methyl coumarin was synthesized and the percentage yield was found to be………..%

REFERENCES

Vogel’s Textbook of Practical Organic Chemistry by Brian S. Furniss, Antony J. Hannaford, Peter W. G. Smith & Austin R. Tatchell; Fifth Edition; Page No. 1193.

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FAQs

Which catalyst is used in synthesis of 7-hydroxy-4-methylcoumarin?

Resorcinol in redistilled ethyl acetoacetate