Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.
(a)
(b)
(c)

Verified step by step guidance

1. Step 1: Analyze the structure in (i). The molecule contains an extended conjugated system with alternating double bonds in a fused aromatic ring system. This extended conjugation allows for absorption of visible light, making the molecule likely to be colored. Mark the series of continuous sp2 hybridized atoms in the aromatic rings.
2. Step 2: Examine the structure in (ii). While there are conjugated double bonds present, the conjugation is not extensive enough to absorb visible light. Molecules with limited conjugation typically absorb in the UV region and are not colored. Conclude that this structure is unlikely to be colored.
3. Step 3: Evaluate the structure in (iii). The molecule contains a highly conjugated system with alternating double bonds in a fused aromatic ring system. This extended conjugation is sufficient to absorb visible light, making the molecule likely to be colored. Mark the series of continuous sp2 hybridized atoms in the aromatic rings.
4. Step 4: Highlight the importance of extended conjugation in determining whether a molecule is colored. Extended conjugation lowers the energy gap between the HOMO and LUMO, allowing absorption in the visible spectrum.
5. Step 5: Summarize the findings: (i) and (iii) are likely to be colored due to their extended conjugation, while (ii) is unlikely to be colored due to insufficient conjugation.

Key Concepts

Conjugation

Conjugation refers to the overlap of p-orbitals across adjacent sp2 hybridized atoms, allowing for delocalization of π electrons. This delocalization stabilizes the molecule and can lead to the absorption of visible light, resulting in color. The presence of extended conjugation typically correlates with a lower energy gap between the ground and excited states, which is crucial for color prediction.

Color in Organic Compounds

The color of organic compounds is often determined by their ability to absorb specific wavelengths of light, which is influenced by their electronic structure. Compounds with extensive conjugated systems tend to absorb light in the visible spectrum, leading to the perception of color. The specific color observed is complementary to the wavelength of light absorbed.

Hybridization and Molecular Geometry

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals, which influences the geometry and reactivity of molecules. In the context of color prediction, sp2 hybridization is significant as it allows for the formation of planar structures conducive to effective π overlap. This geometry is essential for establishing conjugated systems that can absorb light.