Advanced Organic Chemistry: UV-visible light absorption spectroscopy - PHENOLS

Scroll down and take time to study the content and/or follow links or [Use the website search box]

Doc Brown's Advanced Chemistry: PART 15.5 uv and visible light absorption spectroscopy - phenols

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination

SPECTROSCOPY INDEXES  *  All Advanced Organic Chemistry Notes  *  [WEBSITE SEARCH BOX]

15.5.1 The origin of colour, the wavelengths of visible light, our perception!

15.5.2 uv-visible spectroscopy theory, spectrometer, examples of absorption & reflectance spectra explained

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

email doc brown * Use mobile phone/ipad etc. in 'landscape' mode * This is a BIG website, take time to explore it


uv-visible absorption spectra of aromatic compounds - phenols

The uv and visible absorption spectrum of phenol and 3-nitrophenol

uv-visible light absorption spectrum of phenol wavelenghts of maximum absorbance spectra of aromatic phenols

(c) doc bImage adapted from https://www.nist.gov/ (solvent ?)

Phenol shows a λmax of 275 nm in the ultraviolet region.

Phenol shows no absorption in the visible region, so it is colourless (solid or in solution)

 

uv-visible absorption spectrum of 3-nitrophenol wavelengths of maximum absorbance example of aromatic spectra of substituted phenols

Image adapted from https://www.nist.gov/ (solvent ?)

diagram image of the visualisation of the wavelengths of visible light in nanometres nm

3-nitrophenol shows a λmax of 275 nm in the ultraviolet region (same as phenol)

But, unlike phenol, 3-nitrophenol shows a 2nd λmax at 340 nm, a peak still in the uv region, but this broad absorption band stretches to 450 nm into the visible region.

Therefore you are getting absorption of visible light photons in blue region, as a consequence 3-nitrophenol is a pale yellow as a solid or dissolved in a colourless organic solvent.

Note the addition of another different substituent group in the benzene ring alters the energy levels of the pi electrons, lowering them sufficiently to allow excitation with lower energy uv photons.

Methylbenzene (spectrum below) is colourless and the methyl group does not extend the conjugated system of pi electrons of the benzene ring.

However, in the case of 3-nitrophenol, the nitro group does extend the conjugated pi electron system.

The nitro group, O=N=O with its pi electrons, is directly attached to the benzene ring.

uv-visible absorption spectrum of methylbenzene C6H5CH3 example of aromatic spectra of arenes

 

need a more colourful phenol



Key words & phrases: interpreting the uv-visible absorption spectra of phenols aromatic compounds, identifying the maximum absorption peaks in the uv-visible absorption spectra of phenols aromatic compounds, explaining the uv-visible absorption spectra of phenols aromatic compounds, how to use the visible absorption spectra of phenols aromatic compounds to explain the different colours of phenols aromatic compounds, applications of the uv-visible absorption spectra of phenols aromatic compounds uv-visible light absorption spectrum of phenol, identifying the maximum absorption peaks in the uv-visible absorption spectrum of phenol, explaining the uv-visible absorption spectrum of phenol, how to use the visible absorption spectra of phenol to explain the colour of phenol, applications of the uv-visible absorption spectrum of phenol uv-visible light absorption spectrum of 2-nitrophenol, identifying the maximum absorption peaks in the uv-visible absorption spectrum of 2-nitrophenol, explaining the uv-visible absorption spectrum of 2-nitrophenol, how to use the visible absorption spectra of 2-nitrophenol to explain the colour of 2-nitrophenol, applications of the uv-visible absorption spectrum of 2-nitrophenol


Associated links

UV and visible spectroscopy index

SPECTROSCOPY INDEXES

INDEX of all AROMATIC COMPOUND chemistry revision notes

The physical and chemical properties of phenol and some of its derivatives and their uses

All Advanced Organic Chemistry Notes

Use My Google search site box

Website map buttons below

Website content Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

TOP OF PAGE