12 Questions
Which of the following compounds is an example of a compound with sufficient conjugation that absorbs in the visible region?
β-carotene
What effect does the length of the conjugated system generally have on the values of λmax?
It increases the values of λmax
What is an auxochrome?
A group that increases the value of λmax by extending the conjugation through resonance
Which of the following is NOT an example of an auxochrome group?
CO2
What is the general effect of bathochromic groups on wavelength?
They cause a red shift
What does ΔE = hν= hc/λ represent?
The relationship between energy, frequency, and wavelength
What type of transitions do chromophores with both π and non-bonding electrons undergo?
π → π* and n → π* transitions
What is the role of chromophores in relation to the color of compounds?
They increase the ability to absorb radiation
What happens to the absorption maximum when two or more conjugated chromophore groups are present?
It is shifted to a longer wavelength
Which compound shows a lower energy transition, 1,3-butadiene or ethylene?
1,3-butadiene
What type of molecular orbitals are formed in conjugated dienes, from the separate alkene groups?
2 bonding molecular orbitals and 2 antibonding molecular orbitals
Which compound shows a higher absorption maximum, ethylene or 1,3-butadiene?
1,3-butadiene
Study Notes
Conjugation and Absorption
- Compounds with sufficient conjugation absorb in the visible region.
- The length of the conjugated system generally increases the values of λmax.
Auxochrome Groups
- An auxochrome is a group that increases the intensity of color by extending the conjugation of a chromophore.
- The -OH group is not an example of an auxochrome group.
Bathochromic Groups
- Bathochromic groups shift the absorption maximum to a longer wavelength.
Energy Transitions
- ΔE = hν = hc/λ represents the energy of a transition.
Chromophores and Transitions
- Chromophores with both π and non-bonding electrons undergo n → π* and π → π* transitions.
- The role of chromophores is to determine the color of compounds by absorbing specific wavelengths of light.
Multiple Chromophore Groups
- When two or more conjugated chromophore groups are present, the absorption maximum increases.
Comparing Compounds
- 1,3-butadiene shows a lower energy transition than ethylene.
- 1,3-butadiene shows a higher absorption maximum than ethylene.
Molecular Orbitals
- In conjugated dienes, separate alkene groups form molecular orbitals that are delocalized and extended.
Test your knowledge on chromophores and their role in the absorption of light. Explore the factors affecting the wavelength and intensity of absorption in compounds containing chromophores.
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