Interpretation of UV–Visible Spectrum and Calculation of λmax of 1,3,5-Hexatriene

 

Experiment: Interpretation of UV–Visible Spectrum and Calculation of λmax of 1,3,5-Hexatriene

Aim

To record and interpret the UV–Visible spectrum of 1,3,5-hexatriene and determine its maximum absorption wavelength (λmax).

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Principle

1,3,5-Hexatriene contains three conjugated double bonds. Conjugation allows π-electrons to become delocalized, lowering the energy required for electronic transitions.

When ultraviolet light is absorbed, electrons undergo π → π* transition. The absorption occurs at a specific wavelength where the absorbance becomes maximum, known as λmax.

As the number of conjugated double bonds increases, the absorption shifts to longer wavelength (bathochromic shift).

Absorbance follows Beer–Lambert Law:

$A = \varepsilon c l$

Where

• A = Absorbance

• ε = Molar absorptivity

• c = Concentration

• l = Path length of cuvette

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Apparatus

• UV–Visible spectrophotometer

• Quartz cuvette

• Volumetric flask

• Pipette

• Beaker

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Chemicals / Reagents

• 1,3,5-Hexatriene

• Ethanol or hexane (solvent)

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Procedure

1. Prepare a dilute solution of 1,3,5-hexatriene in hexane or ethanol.

2. Fill a quartz cuvette with solvent and set it as the blank in the spectrophotometer.

3. Fill another cuvette with the sample solution.

4. Scan the spectrum in the range 200–350 nm.

5. Record absorbance values at different wavelengths.

6. Plot a graph of Absorbance vs Wavelength.

7. Identify the wavelength corresponding to maximum absorbance (λmax).

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Observation Table

Sr. No.	Wavelength (nm)	Absorbance
1	230	0.18
2	240	0.36
3	250	0.64
4	255	0.82
5	258	0.94
6	260	0.88
7	270	0.60
8	280	0.35

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Calculation of λmax is the wavelength at which maximum absorbance occurs in the spectrum.

$${\lambda }_{max}=258nm$$

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Interpretation of Spectrum

• The absorption band in 1,3,5-hexatriene arises due to π → π* electronic transition.

• The presence of three conjugated double bonds causes absorption at longer wavelength compared to benzene.

• This shift toward longer wavelength is known as bathochromic shift due to conjugation.

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Result

The UV–Visible spectrum of 1,3,5-hexatriene was recorded and the maximum absorption wavelength (λmax) was found to be approximately 258 nm.