Phenolphthalein is colorless in acidic medium whereas it is pink in color in basic medium. Have you ever wondered why?

The answer to this question lies in the way it interacts with Light. Although light can interact with matter in several ways, in this article we will restrict ourselves to transmission and reflection of light.

What happens when light strikes an object:

  • It absorbs all wavelengths of light incident on it – perfect Blackbody
  • It reflects all wavelengths of light incident on it – perfect Reflector
  • Reflects some part of light(some range of wavelength), while absorbing other parts

When light strikes an object, a portion of the visible spectrum may be absorbed by the object. Some portion that is not absorbed reflects off the object. If the reflected part of light lies in the visible region(Wavelength 400-700 nm), we perceive it as colored. For example, we know that chlorophyll is green in color. This is because chlorophyll reflects the light whose wavelength lies in the region corresponding to green color (500-570 nm), while absorbing other regions. Objects reflecting all light lying in visible range are colorless/white.

Why do organic compounds absorb some parts of light, while reflecting others?

The answer lies in the properties of the compounds. Most of the organic compounds having a less extent of conjugation or no conjugation reflect almost all light of the visible region, thus being colorless or white, while compounds having higher extent of conjugation usually exhibit colors. For multiple bonds to be conjugated, they must be in an alternating double bond – single bond – double bond, etc arrangement.

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When light is incident on molecules, the electrons of the molecules absorb the light, jump from their valence shell - Highest Occupied Molecular Orbital (HOMO) to higher energy level – Lowest Unoccupied Molecular Orbital (LUMO). If the difference in energies between HUMO and LUMO is high, then amount of energy required to excite the electrons is also high.

The Energy of various colors in the visible spectrum is as follows:

(High Energy) Violet > Indigo > Blue > Green > Yellow > Orange > Red (Low Energy)

When lower energy portion of light is absorbed, the reflected higher energy part of light is perceived as light. The color that is reflected is the complementary of the color that is absorbed.

If the difference in energies between the LUMO and HOMO is higher, the higher energy portion of light is absorbed, while reflecting its complementary color. For example if the high energy violet portion of the spectrum is absorbed, its complementary color of yellow is what is observed. If the lower energy blue or green colors are absorbed, the colors orange or red would be observed

In case of highly conjugated systems the difference in energies between the HOMO and LUMO orbitals is quite less than the less conjugated/non conjugated systems. The degree of conjugation determines the difference in energies between the orbitals. Thus molecules with higher extent of conjugation absorb light of lower energies, reflecting light of higher energies. If the reflected light lies in the visible region, then our eyes perceive them as colors. For Example, beta Carotene on account of its higher extent of conjugation absorbs energy range: blue and green- thus reflecting the other parts: red and orange, hence appears reddish orange in color. However in case of 1,3,5 hexatriene – the extent of conjugation is less, thus absorbs UV light (200-230 nm) thus appears colorless. Basically, the color of a compound depends on the wavelength of the reflected light.

How is this related to Phenolphthalein?

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In Acidic medium, Phenolphthalein is colorless on account of its “Non-Conjugation”, while in a basic medium it becomes an highly conjugated ion, hence pink in color.

When carbonyl group (aldehyde or ketone) is treated with 2,4 DNPH – Dinitrophenyl-hydrazine commonly known as 2,4 DNP Test gives yellow/orange/red precipitate, and the reason for this coloor is same as discussed above

Question To Ponder

When pure green light is incident on a red flower, what will be the color of the flower?