Reference no: EM132370921

Experiment: Synthesis of a Chalcone and Determination of Extinction Coefficient

AIM - Preparation of substituted chalcone and determine its extinction coefficient using UV-VIS.

NOTE: Complete the risk assessment for all substituted benzaldehydes (methylbenzaldehyde, chlorobenzaldehyde) and substituted acetophenones (methyl acetophenone, chloroacetophenone) along with other chemicals used in this experiment in your lab book before the experiment session. In chemical list of risk assessment form you must work out the mass or volume equivalent to 5.0 mmol for these compounds.

INTRODUCTION - The preparation of a substituted chalcone is an example of the aldol reaction followed by dehydration. The reaction is a base-catalysed condensation reaction and proceeds via an enolate anion.

Scheme 1 The base-catalysed condensation reaction between a p-substituted benzaldehyde and a p-substituted acetophenone.

Further reading on aldol reactions: McMurry (7^th ed.), pp. 849-855.

EXPERIMENTAL PROCEDURE -

Synthesis of the chalcone

Select the substituted benzaldehyde (methylbenzaldehyde) and the substituted acetophenone (4′- Methylacetophenone) you are going to use and determine the amount of material required for 5.0 mmol (all of these reagents will be available).

measure volume the substituted benzaldehyde (0.6 ml) and the substituted acetophenone (0.5 ml) and the solid sodium hydroxide (>0.2 g, approx. 2 pellets) and combine them in a small porcelain mortar. Grind the mixture with a pestle for 10 minutes. The mixture should quickly turn slightly yellow and then solidify. Continue grinding with the pestle, but, about every 2 minutes, scrape the solid material off the sides of the mortar with a spatula. Add water (10 mL) and collect the solid by vacuum filtration. Add further water (5 mL) to the mortar and use this to transfer any remaining solid to the filter - repeat if necessary.

Recrystallise this solid from ethanol (10-15 ml hot ethanol) and determine the % yield, the melting point and the infrared spectrum of the product. (Heat on water bath to dissolve crystals if need be)

Electronic (ultraviolet-visible) absorption spectra of unsaturated and aromatic ketones

Solution A: chalcone in methanol, approx. 4 x 10^-4 M

Weigh out, to the nearest 0.0001 g (0.1 mg), approximately 0.005 g (that is, approximately 5 mg) of the chalcone. Use the analytical balance for this purpose (do not spill your product onto the balance - marks will be deducted if you do). Note that the quantity does not have to be exactly 0.005g. It could be any quantity between about 0.004 and 0.006 g. However, you must note the exact mass, to the nearest 0.0001 g, as this mass is used in the calculation of the extinction coefficient (ε). Dissolve this chalcone in 50 mL of methanol in a 50 mL volumetric flask. You can use some of the methanol to wash the crystals into the volumetric flask. The concentration of this solution is approx. 4 x 10^-4 M

Solution B: chalcone in methanol, approx. 4 x 10^-5 M

Place 5 mL of solution A (use a 5 mL volumetric pipette with a pumpette attached) into a clean 50 mL volumetric flask, and add methanol to bring the volume up to the 50 mL mark. Calculate the exact concentration of this solution from the mass of chalcone you weighed out and the dilution factor. The molecular masses can be obtained from the formulae.

Measuring spectra and extinction coefficients

Record the absorption spectrum in the ultraviolet range (380-230 nm; absorbance range 0-2) of solution B. Determine the wavelength of maximum absorption (λ_max), the maximum absorbance (A(λ_max)) and the extinction coefficient (ε) at that wavelength according to:

A(λ_max) = ε × C × l, where:

A(λ_max) = maximum absorption (no units)

ε = extinction coefficient (L·mol^-1·cm^-1)

C = concentration (mol·L^-1, or M)

l = path length (1 cm)

WASTE DISPOSAL: All waste from this experiment must go to acidic waste bottles in fumehood.

REPORT - A Lab report is required to be written for this experiment and must contain the below sections.

• Aim
• Introduction
• Method
• Results
• Discussion (reference mark and answers to additional question marks are included here)
• Conclusion

NOTE:

• All spectra (IR/NMR/UV-Vis/AAS...etc) must be attached in reports
• All graphs must be drawn in excel and pasted in the reports
• All chemical equations and reaction mechanisms must be drawn in Chemdraw

ADDITIONAL QUESTIONS -

1. Show your calculations for ε of the chalcone and complete the table below. Compare the λ_max and ε values determined for your product with those of diphenylpenta-dienone. What can you conclude regarding the effect of structure on the position of the major absorption band (π - π *)? Explain.

Attachment:- Experiment Assignment Files.rar