Chemical+Structure+and+Color.unlocked

7/30/2019 Chemical+Structure+and+Color.unlocked

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Chemical structure and coloroption: food chemistryhigher level chemistry

international baccalaureate

Wednesday, December 12, 12


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review:

anthocyanins

carotenoids

chlorophyll

heme

delocalization ofelectrons



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colors are due to the

absorption of visible light.

what absorbs visible light?



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conjugated system



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We have seen that chemicals appear colored because they absorb visible

in this region of the spectrum can produce electronic transition in organic m

Chromophores are unsaturated groups which absorb UV and visible raThe part of the molecule responsible for absorbing the radiation is called the chromophore

includes unsaturated groups such as C C, C O, N N, NO2 and the benzene ringis more likely to absorb visible light and appear colored when it contains a conjugated syst

C C and CC bonds with the electrons delocalized over a larger area.

A conjugated system of sp2 hybridized carbon atoms. The p orbitals

can overlap to form an extended delocalized system of electrons.



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The sp2 hybridized carbon atoms each have a p orbital which overlap to form an extended sy

rings and other double bonds can also form part of a conjugate system. The wavelength whic

to maximum absorbance for some chromophores is shown in the table below.

Chromophore max/nm

C C 175

C O 190 and 280

C CC C 210

190 and 260

The relationship between the wavelength of the radiation absorbed and the length of the conj

can be explained using the wave model of the electron. The wavelength of the light absorbe

the same way, and for essentially the same reason, that a guitar string produces a lower no

increases.



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anthocyanins



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carotenoids

-carotene



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chlorophyll



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heme



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porphinsThe porphyrin ring is an extended

system and so absorbs visible light

Both chlorophyll and heme contain a planar

heterocyclic porphyrin ring whose structure contains

a cyclic system of conjugated double bonds.



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Exercises1. The structure and color of two related molecules are shown below.

Molecule A Molecule B

colorless blue

(a) Identify the hybridization of the carbon atoms * in both molecules.

(b) Identify the molecule which has the largest extended system of electrons.

(c) Identify the molecule that absorbs light of the longer wavelength.

(d) Use the color wheel to deduce which colors are absorbed by A and B.

(e) Explain the different colors of the two compounds.


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