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Advanced Level Organic Chemistry: UV and visible light absorption spectroscopy - ALKENES

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Doc Brown's Advanced Chemistry: PART 15.5 uv and visible light absorption spectroscopy of ALKENES

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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

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The uv and visible absorption spectrum of alkenes

(a) The spectrum of buta-1,3-diene

uv-visible absorption spectrum of buta-1,3-diene electronic spectra of alkenes dienes spectra maximum ultraviolet absorption peak wavelength

Image adapted from https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/uv-vis/spectrum.htm

The uv absorption spectrum of buta-1,3-diene H2C=CH-CH=CH2, a molecule with a relatively small conjugated system i.e. the minimum alkene conjugated structure with two C=C double bonds separated by a C-C single bond.

For comparison with alkenes below, think of this molecule as R(CH=CH)nR, where n = 2 (R = H).

A conjugated system is where an alternating single and double bond system produces some overlap of the pi electrons with the sigma bonds. You can consider that the C-C bond has some double bond character and the double bond more sigma bond character - a sort of blurring of the C-C=C-C=C system - note that you must have at least two C=C double bonds separated by a C-C bond to get conjugation in alkenes. (see appendix for more details on conjugated systems)

With alkenes, uv-visible light absorption is due to pi electron excitation, ∆Eelec for π ==> π*

λmax 217 nm is in the ultraviolet region, so buta-1,3-diene does not absorb visible light, so appears colourless to us.

The conjugation lowers the energy needed to excite the molecule and absorb a uv photon in the process.

For comparison, non-conjugated 'monoenes' have higher ∆Eelec values, needing shorter wavelength photons for excitation e.g. for the uv absorption spectra of other alkenes we have:

ethene H2C=CH2 λmax = 162 nm, propene CH3CH=CH2 λmax = 170 nm,

but-2-ene CH3-CH=CH-CH3 λmax 179 nm (λmax for buta-1,3-diene was higher at 217 nm).

but, like buta-1,3-diene, the conjugated cyclohexa-1,3-diene has an even larger λmax of 259 nm.

(data from https://sites.science.oregonstate.edu/~gablek/CH335/Chapter10/bare_UV_vis.htm )

 

(b) Alkenes with more than two C=C double bonds (polyenes)

Having looked at the simplest conjugated alkene system compared to a single alkene bond molecule, you should predict that an even longer conjugated alkenes system should have greater λmax values and, will they eventually have a ∆Eelec value low enough for visible light photons to be absorbed, hence become coloured?

The answer is yes and look at a carrot in the larder!!!

We now look at series of molecules that have longer conjugated systems of alternating C=C and C-C bonds.

visible uv absorption spectrum of conjugated systems polyenes hexa-1,3,5-triene octa-1,3,5,7-tetraene deca-1,3,5,7,9-pentaene alkene spectra maximum ultraviolet absorption peak wavelength molecular structure

Image adapted from https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/uv-vis/spectrum.htm

The series R(CH=CH)nR, where n = 2 to 5.  If R = H, the molecules would correspond to:

H2C=CH2 ethene for n = 1 (see the first spectra data above), λmax = 162 nm (no conjugation possible)

H2C=CH-CH=CH2 buta-1,3-diene for n = 2 (see the first spectra above), λmax = 217 nm

H2C=CH-CH=CH-CH=CH2 hexa-1,3,5-triene for n = 3, λmax = 262 nm

H2C=CH-CH=CH-CH=CH-CH=CH2 octa-1,3,5,7-tetraene for n = 4,  λmax = 289 nm

H2C=CH-CH=CH-CH=CH-CH=CH-CH=CH2 deca-1,3,5,7,9-pentaene for n = 5, λmax =  340 nm

Non of the absorption peaks are in the visible region, so all of these are colourless compounds, but, when n = 5, the right-hand absorption band is almost in the beginning of the visible region (> 380 nm).

What you observe is a steady trend of increasing λmax/nm, of decreasing energy (∆Eelec), as the linear conjugated system increases in length by one extra C-C=C 'unit' at a time.

Dodeca-1,3,5,7,9,11-hexaene has a λmax of 362 nm, continuing the trend from above, maybe very pale yellow?

Looking at another series of polyenes, for the series CH3(CH=CH)nCH3, the highest absorbance peaks are, when :

n = 7, λmax peaks at 355, 375 and 398 nm

n = 8 λmax peaks at 375, 395, 420 nm

n = 9 λmax peaks at 393, 416 and 443 nm

n = 10 λmax peaks at 406, 431 and 460 nm

These will all be coloured compounds because there is some weakish absorbance in the visible light range of >380 nm because the conjugated polyene structure is now low enough in energy.

Note that the polyene structure, (-CH=CH)n, is the chromophore, the structural feature responsible for the molecule being coloured from electronic excitation by visible light photons.

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

visible absorption spectrum of beta carotene electronic spectra maximum absorbance peak wavelength molecular structure spectra of polyene carotenoids

Beta carotene is a polyene of 11 alternating C=C double and C-C single bonds, with λmax  of 450 (here, other values are quoted ~500 nm).

Strong absorption in the blue and violet region results in the characteristic orange 'carrot' colour.

Again you have a chromophore base on an extended conjugated polyene system, compared to the polyenes described in section (b), for the structural feature (-CH=CH)n n is effectively 11.

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

The beta-carotene molecule is an orange organic pigment found in carrots

Note that the 'monomer' Z-2-methylbuta-1,3-diene (isoprene), with a much shorter delocalised electron system, from which beta-carotene is biosynthesised, is colourless.

The extended conjugated alternate C-C single and C=C double bond system becomes the chromophore, sufficiently low in energy to allow pi electron excitation by visible light photons.

When dissolved in hexane solvent you observe three absorbance peaks 425, 450 and 480 nm.

Therefore the red, orange and yellow light is transmitted, giving carotene (hence carrots) an orange colour.

The visible absorption spectrum of beta carotene, the orange pigment in carrots (the molecular structure is the full E-isomer, all >C=C< in the E 'trans' configuration). The spectrum is obtained using a hexane solution - hexane does not absorb light in the visible region so it doesn't interfere with the spectrum.

The visible absorption spectrum of carotene has a λmax of ~450 nm with strong absorption in the blue region of the visible spectrum - note the λmax has now shifted well into the visible region - not surprising with a conjugated system consisting of 11 alternating C=C double bonds !!!

Compare carotene with the colourless polyenes described in section (b) with their shorter conjugated systems.

Extra notes on carotenoids (a few points re-edited from Wikipedia)

There are several molecular forms of carotenes (examples of carotenoids) which are important photosynthetic pigments for photosynthesis.

As already mentioned, they absorb ultraviolet, violet, and blue light and scatter yellow, orange and red light

Carotenes contribute to photosynthesis by transmitting the light energy they absorb to chlorophyll.

They also protect plant tissues by helping to absorb the energy from very reactive singlet oxygen, an excited form of the oxygen molecule O2 which is formed during photosynthesis.

Carotenes are also responsible for the orange colours of many other fruits, vegetables and fungi (for example, sweet potatoes, chanterelle and orange cantaloupe melon). Carotenes are also responsible for the orange (but not all of the yellow) colours in dry foliage. They also (in lower concentrations) impart the yellow coloration to milk-fat and butter.

 

(d) The colour of astaxanthin

molecular structure of astaxanthin skeletal formula structural formula conjugated alkene carotenoid molecule

Image adapted from https://www.researchgate.net/figure/Chemical-structure-of-b-carotene-and-of-the-xanthophylls-astaxanthin-and-lutein-main_fig1_6526730

Astaxanthin is a keto-carotenoid used as a dietary supplement and food dye.

As well as alkene groups it has two other organic functional groups, namely a secondary alcohol group and a ketone group, at both ends of the molecule.

The conjugated system (the chromophore) in astaxanthin, compared to carotene, is extended by the ketone group and the λmax is shifted to a longer wavelength and the colour is a stronger deeper red than 'paler' orange carotene - because the chromophore group is extended compared to carotene, giving a deeper colour.

It is a more reddish colour than beta carotene, because it shows stronger overall absorption in the blue-green regions of the visible spectrum.

visible absorption spectrum of astaxanthin in trichloromethane electronic spectra maximum absorbance peak wavelength molecular structure spectra of carotenoids

Image adapted from https://www.researchgate.net/figure/Absorbance-spectra-of-pure-astaxanthin-concentrations-of-2-and-4-mg-L-respectively-in_fig1_238383838

The λmax for astaxanthin in trichloromethane solvent occurs at ~492 nm.

 

The spectra of some organic compounds can be significantly different in different solvents e.g. for astaxanthin:

comparison of uv-visible absorption spectra of astaxanthin in different solvents water aqueous methanol

Image adapted from https://www.researchgate.net/figure/Normalized-absorbance-spectra-of-astaxanthin-and-astaxanthin-H-aggregates-The-UV-Vis_fig1_265608682

The λmax for astaxanthin in methanol occurs at ~460 nm in methanol (similar in CHCl3 solvent), but falls to ~380 nm in aqueous methanol, but still absorbing in the visible region.

One explanation is hydrogen bonding between the carbonyl group (part of conjugated system) and water molecules.

e.g. the hydrogen bond   >C=Oδ-llllδ+H-O-H  (there only seems to be a small effect with pure methanol)

Whatever the explanation, the pi electron excitation is raised by the presence of water, so there is more absorbance in the uv region and less in the visible light system.

So you would observe different colours for astaxanthin in trichloromethane and aqueous methanol solvents.

There is a blue spectral shift when water is added to the methanol solvent.

 

More about the very interesting astaxanthin molecule (thanks to https://en.wikipedia.org/wiki/Astaxanthin )

Astaxanthin is a blood-red pigment and is produced naturally in the freshwater microalgae Haematococcus pluvialis and the yeast fungus Xanthophyllomyces dendrorhous (also known as Phaffia).

When the algae is stressed by lack of nutrients, increased salinity, or excessive sunshine, it creates astaxanthin.

It is a strong absorber of harmful uv light in bright sunshine.

Animals who feed on the algae, such as salmon, red trout, red sea bream, flamingos, and crustaceans (i.e. shrimp, krill, crab, lobster, and crayfish), subsequently reflect the red-orange astaxanthin pigmentation to various degrees.


Appendix on explaining conjugation

The word "conjugation" is derived from a Latin word that means "to link together" and in organic chemistry it is used to describe the situation that occurs when π systems (e.g. double bonds) are "linked together".

An "isolated" π (pi) system exists only between a single pair of adjacent atoms (e.g. C=C as ethene, propene, but-1-ene and these are NOT conjugated molecules.

An "extended" π (pi) system exists over a longer series of atoms (e.g. C=C-C=C or C=C-C=O etc. as in buta-1,3-diene, which is one of the simplest conjugated systems.

An extended π (pi) system results in increased chemical reactivity and a lowering of the associated electronic energy levels.

To understand the fundamental requirement for the existence of a conjugated system you must considers the p orbitals involved in the bonding within the π system.

A conjugated system requires that there is a continuous array of "p" orbitals that can align to produce a π bonding overlap along the whole system.

This results in extra π bonding interactions between the adjacent π systems resulting in an overall stabilisation of the system.

Dienes (and all polyenes) can have theoretically have resonance hybrid structures that results in a double bond having a small amount of single bond character and single bonds have a little amount of double bond character

You can only have extended conjugation if double bonds alternate with single bonds e.g.

H2C=CH-CH2-CH=CH2 penta-1,4-diene cannot form a conjugated system, but,

H2C=CH-CH=CH-CH3 penta-1,3-diene can form a conjugated system.


Key words & phrases: interpreting the uv-visible absorption spectrum of astaxanthin, identifying the maximum absorption peaks in the uv-visible absorption spectrum of astaxanthin, explaining the uv-visible absorption spectrum of astaxanthin, how to use the visible absorption spectra of astaxanthin to explain the colour of astaxanthin, applications of the uv-visible absorption spectrum of astaxanthin interpreting the uv-visible absorption spectra of polyenes, identifying the maximum absorption peaks in the uv-visible absorption spectra of polyenes, explaining the uv-visible absorption spectra of polyenes, how to use the visible absorption spectra of polyenes to explain the different colours of polyenes, applications of the uv-visible absorption spectra of polyenes interpreting the uv-visible absorption spectra of beta carotene carotenes carotenoids, identifying the maximum absorption peaks in the uv-visible absorption spectra of beta carotene carotenes carotenoids, explaining the uv-visible absorption spectra of beta carotene carotenes carotenoids, how to use the visible absorption spectra of beta carotene carotenes carotenoids to explain the different colours of beta carotene carotenes carotenoids, applications of the uv-visible absorption spectra of beta carotene carotenes carotenoids


Associated links

Index of ALL advanced level revision notes on ALKENES

Revision notes: Structure and naming of ALKENES including cyclo- and many isomers

UV and visible spectroscopy index

SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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