Lecture 13: Gas-phase Oxidation Reactions

You will work in groups on the atmospheric oxidation of organic compounds by inorganic radicals. Before coming to class:
  1. Carefully read the information on this page.

  2. Watch the 15 minute video lecture Gas Phase Oxidation Reactions on the MediaSite page.

  3. Take a short quiz on WebBoard.

Photolysis

What happens when light is absorbed by a molecule? Let's look at the absorption of a photon of light by a hydrogen molecule. If the photon of light contains just enough energy to promote an electron from the lower energy level to the upper one (91 nm), the bond order decreases from 1 to 0. The bond is broken and two hydrogen atoms are formed.



If the photon is more energetic (wavelength < 91 nm), the hydrogen atoms will have excess kinetic energy.

What about molecular oxygen? There is a double bond. When the molecule absorbs a photon of light with a wavelength of 240 nm or less, an electron is promoted from a bonding orbital to an antibonding orbital. The bond order decreases from 2 to 1. There is still some bonding but the bond is weak and the excess energy from the photon is sufficient to break it.



Light breaks chemical bonds when the energy of the photon is greater than the bond dissociation energy.

Bond Dissociation Energy

The energy required to break a particular bond in a specified molecule is the bond dissociation energy. There is a table of average bond energies below. Be careful! Bond dissociation energy and bond energy are not the same. It takes 493 kJ/mol of bond dissociation energy to break the first O-H bond in water and 424 kJ/mol to cleave the remaining O-H bond. The average bond energy of the O-H bonds in water is 459 kJ/mol. The O-H bond energy will vary a little from molecule to molecule. The averge is about 464 kJ/mol.

Methane has 4 C-H bonds and the bond dissociating energies are 435 kJ/mole for D(CH3-H), 444 kJ/mole for D(CH2-H), 444 kJ/mole for D(CH-H) and 339 kJ/mole for D(C-H). The average bond energy is 414 kJ/mole. The C-H bond energy changes depending on the structure of the molecule. The average for many molecules is shown in the table.



Radical Stability

Bond homolysis leads to the formation of radicals, atoms or molecules with unpaired electrons. All radicals are electron-poor and reactive, but some radicals are more reactive than others.
  • When the unpaired electron is on an electronegative element (like oxygen), it is less stable than when the unpaired electron is on a less electronegative element (like carbon).



  • Electron-withdrawing groups make radicals less stable.



  • Radicals are stabilized by resonance, atoms with filled or half-filled p orbitals adjacent to the radical center.



  • Resonance stabilization is more important than inductive destabilization.

Oxidations with Ozone

In organic chemistry, you learned that ozone oxidized carbon-carbon double bonds:



In the atmosphere ozone is the precursor to the more important oxidant, hydroxyl radical.



  • The O-O bonds in ozone are weak and can be cleaved by light of 330 nm.

  • This produces a very reactive oxygen atom.

  • In the troposphere, there is a significant concentration of water vapor. Reaction of O with water forms 2 hydroxyl radicals.

  • Hydroxyl radicals form because they are more stable than oxygen atoms.

Hydroxyl Radicals

Radicals can react in several ways
  1. abstract an atom from another molecule, forming a more stable radical
  2. add to an unsaturated molecule, such as an alkene, and form a new, more stable radical
  3. react with molecular oxygen to form a peroxy-radical
  4. dimerize or react with another radical, forming a diamagnetic compound
From the photolysis reactions below, we can see that it takes less energy to break a C-H bond in methane, and form a methyl radical, than to cleave an O-H bond in water, and form a hydroxy radical.



The methyl radical must be more stable than the hydroxy radical. Because of this, a hydroxy radical will abstract a hydrogen atom from methane. The C-H bonds in most other organic molecules have lower bond dissociation energies than methane.

Hydrocarbon Oxidation



The concentration of molecular oxygen in the atmosphere is very high. This diradical reacts with other radicals formed in air, including the methyl radical. This forms a peroxy radical.

What happens next?
  1. The peroxy radical can react with more alkane. This is part of a radical chain process.
  2. It can react with NO2 to form NO3 and an alkoxy radical. The alkoxy radicals can lose a hydrogen atom or abstract one.

  3. The alcohol and aldehyde products are more reactive with radicals (HO, ROO, NO3) that abstract hydrogen atoms. Ultimately, they are converted to CO2.

Breaking C-C Bonds

When there is an alkoxy radical on the carbon adjacent to a carbonyl group, the C-C bond spontaneously breaks to give a more stable carbonyl radical and an aldehyde or ketone. Aldehydes can be photolysed to form form the relativley stable CHO radical and an alkyl radical.



Oxidations with NO3

Nitrogen trioxide is readily photolyzed to NO2 and oxygen atom when the sun shines, and we've seen that oxygen atom reacts with water to form hydroxyl radical.



At night, NO3 build up. It has the same reactivity as HO with hydrocarbons.