Interpretation of UV–Visible Spectrum and Calculation of λmax of Naphthalene

 

Experiment: Interpretation of UV–Visible Spectrum and Calculation of λmax of Naphthalene

Aim

To record and interpret the UV–Visible spectrum of naphthalene and determine its maximum absorption wavelength (λmax).

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Principle

Naphthalene is an aromatic hydrocarbon containing two fused benzene rings with a conjugated π-electron system. When UV radiation passes through the compound, the π-electrons absorb energy and undergo π → π* electronic transition.

The wavelength at which maximum absorption occurs is called λmax. Due to extended conjugation in naphthalene, absorption occurs at longer wavelength compared to benzene.

Absorbance obeys 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

• Naphthalene sample

• Ethanol or hexane (solvent)

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Procedure

1. Prepare a dilute solution of naphthalene in ethanol or hexane.

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

3. Fill another cuvette with the sample solution.

4. Scan the spectrum in the range of 200–350 nm using the spectrophotometer.

5. Record absorbance at different wavelengths.

6. Plot a graph of Absorbance vs Wavelength.

7. Identify the wavelength where maximum absorbance occurs (λmax).

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Observation Table

Sr. No.	Wavelength (nm)	Absorbance
1	220	0.42
2	240	0.55
3	260	0.72
4	270	0.85
5	275	0.93
6	280	0.81
7	290	0.60
8	300	0.38

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Calculation of λmax

$${\lambda }_{max}=\text{Wavelength at which absorbance is maximum}$$

From the observation table:

$${\lambda }_{max}=275nm$$

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Interpretation of Spectrum

• The absorption band of naphthalene is due to π → π* electronic transition.

• The presence of two fused aromatic rings increases conjugation, resulting in absorption at longer wavelength than benzene.

• The strong peak near 275 nm corresponds to the main electronic transition.

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Result

The UV–Visible spectrum of naphthalene was recorded and the maximum absorption wavelength (λmax) was found to be approximately 275 nm.