Assignment 1 Answers.pdf for the elctron transfer

answers

1. Properties and Reactivity of Oxygen Species
1. Compare and contrast the electronic configurations, magnetic properties, and reactivities
of 3O2 and 1O2.
o Electronic Configuration: In 3O2 (triplet oxygen, ground state), there are two
unpaired electrons in separate π∗ antibonding orbitals. In 1O2 (singlet oxygen,
excited state), all electrons in the π∗ orbitals are paired.
o Magnetic Properties: 3O2 is paramagnetic due to its unpaired electrons. 1O2 is
diamagnetic because all of its electrons are paired.
o Reactivity: 3O2 is a relatively stable and less reactive species that often requires a
catalyst to react with organic molecules. 1O2 is a highly reactive species that readily
participates in reactions with organic substrates.
2. Why is it necessary to use photosensitizers or high-energy methods to generate singlet
oxygen? Singlet oxygen is a high-energy, excited state of oxygen. The direct transition from
the ground-state triplet oxygen (3O2) to singlet oxygen (1O2) is a "spin-forbidden" process,
meaning it has a very low probability of occurring. Photosensitizers absorb light energy and
then transfer that energy to triplet oxygen, overcoming the energetic and spin-related barrier
to produce singlet oxygen. High-energy methods, such as electrical discharge, provide a
similar energetic input to force the transition.
3. Explain why singlet oxygen reacts readily with organic substrates, while triplet oxygen
often requires catalysts (e.g., transition metals) to react. The key difference lies in their spin
states. Most organic substrates are in a ground-state singlet spin state (all electron spins are
paired). Singlet oxygen can react directly with these singlet substrates in a spin-allowed,
concerted reaction. Triplet oxygen, with its two unpaired electrons, must undergo a spin
inversion to react with a singlet substrate. This spin-forbidden process is slow and kinetically
hindered, so transition metals are often required as catalysts to provide an alternative,
lower-energy reaction pathway.
4. Describe two different methods for generating 1O2 in the lab.
o Photosensitization: This is the most common method. A photosensitizer, such as
Rose Bengal or Methylene Blue, is added to a solution containing triplet oxygen and
irradiated with light. The photosensitizer absorbs the light, reaches an excited state,
and then transfers its energy to the ground-state triplet oxygen, generating singlet
oxygen.
o Chemical Reaction: Singlet oxygen can be produced chemically by the reaction of
hydrogen peroxide (H2O2) with sodium hypochlorite (NaOCl).
5. How can singlet oxygen be detected spectroscopically? What is the characteristic
phosphorescence wavelength of 1O2? Singlet oxygen can be detected by its characteristic
phosphorescence emission. As it returns to the ground state, it releases energy in the form
of light. The characteristic phosphorescence wavelength of 1O2 is 1270 nm.
6. Name a reaction in which singlet oxygen reacts with an alkene and describe the product.
Singlet oxygen undergoes an ene reaction with an alkene that has an allylic hydrogen. In this
reaction, the singlet oxygen adds across the double bond and abstracts the allylic hydrogen.
The product of this reaction is an allylic hydroperoxide.

2. 7. What are the chemical quenchers of 1O2? Chemical quenchers are molecules that react
with and neutralize singlet oxygen, converting it back to its ground state. They are not
consumed in a stoichiometric reaction. Common examples of chemical quenchers include
amines (such as DABCO), phenols, and carotenoids like β-carotene.