Many volatile organic compounds (VOC) are responsible for photochemical smog. Propanone is a VOC. It's produced in a great amount for industrial purposes. So, propanone can be a potential contributor to photochemical smog.

I have 2 questions:

  1. Is propanone really responsible for photochemical smog?

  2. If it is, then what kind of photochemical reactions happen with propanone? (Please describe the mechanism also)


Most of the information is taken from Wikipedia or is mentioned therein

First some definations.

  1. What is a VOC?

Volatile organic compounds (VOCs) are organic chemicals that have a high vapor pressure at ordinary room temperature. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and enter the surrounding air, a trait known as volatility. For example, formaldehyde, which evaporates from paint and releases from materials has a boiling point of only –19 °C (–2 °F).

  1. What is a photochemical smog?

Noxious haze of minute chemical droplets and ozone, resulting from sunlight induced chemical reaction between industrial and automobile pollutants (mainly nitrogen dioxide, hydrocarbons and aldehydes).

Indeed, propanone(acetone) is a VOC. Majority of acetone is released from industries origins. It indeed evaporates rapidly, from water and soil and has a 22-day half-life and degraded by UV light via photolysis primarily into methane and ethane or other alkyl, alkoxy or acyl radical.

As said, the main components of photochemical smog is nitrogen dioxide, hydrocarbons and aldehydes. The reaction include breakdown of hydrocarbons and aldehydes into alkyl and alkoxy radicals. These radicals react with nitrogen dioxide to form PAN(peroxyacetyl nitrate) which is the main cause of photochemical smog. The reaction mechanism is given here:

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Another mechanism is given here:

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What is observed is that the active component is acyl radical which is necessary in the PAN formation. Both aldehyde and acetone necessitate in acyl radical through their decomposition. From here:

The primary photochemical decomposition of aldehydes produces organic free radicals. For ex: acetaldehyde decomposes into methyl and formyl radicals.

$$\ce{CH3CHO + hv → •CH3 + HCO•}$$

Similarly, acetone decomposes into methyl and acetyl radicals.

$$\ce{(CH3)2CO + hv → •CH3 + CH3CO•}$$

But acetone contribution to photochemical smog is negligible as acetone is quite stable to irradiation and photolysis. From here:

[...]Acetone has shown little reactivity to irradiation. Its contribution to produce significant amount of PAN or ozone in ambient air is negligible.


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