The Smell of Freshness

Organic compounds are added to cleaning products and air fresheners to make the air smell fresh and clean. Scientists are taking a closer look at the chemistry behind the use of these compounds to determine whether they are hazardous to human health.

One of the organic compounds that is widely used to provide the "smell of freshness" is a molecule known as limonene (1-methyl-4-(1-methylethenyl)-cyclohexene). The 2-dimensional structural formula of limonene is shown on the right.

Limonene is a colourless liquid at room temperature and pressure and is found naturally in the rind of citrus fruits such as lemons. It is one of the compounds that contributes to the typical odour of citrus fruit.
Commercial quantities of limonene are produced from citrus fruits using centrifugal separation or steam distillation.

Scientists are studying the reactions of limonene closely because the chemical reactions of this molecule with the ozone in the air in your home are the same as the chemical reactions that occur in the atmosphere that produce secondary organic aerosols (SOAs), microscopic particles suspended in the air, which contribute to the visible haze known as smog in densely populated areas.

The researchers tested various different scenarios for the production of SOAs in the home from limonene and found that the concentration of SOAs produced was between 5μg/cm³ and 100μg/cm³. The acceptable level of aerosols in breathable air is about 12μg/cm³.

The researchers suggest that the best way to reduce SOAs in your home is to either use unscented cleaners or to keep windows open while cleaning. 

Reference:
Somayeh Youssefi, Michael S. Waring. Transient Secondary Organic Aerosol Formation from Limonene Ozonolysis in Indoor Environments: Impacts of Air Exchange Rates and Initial Concentration Ratios. Environmental Science & Technology, 2014; 48 (14): 7899 DOI: 10.1021/es5009906

Further Reading:
Empirical Formula
Molecular Formula
2-Dimensional Structural Formula
Condensed Structural Formula
Skeletal Structural Formula
Percentage Composition
Parts Per Million (ppm) Concentration
Molar Mass

Suggested Study Questions:

1. Write the molecular formula for limonene.
2. Give the empirical formula for limonene.
3. Calculate the percentage of
   • carbon in a molecule of limonene
   • hydrogen in limonene
4. Draw a skeletal structural formula for limonene.
5. Limonene has the IUPAC name 1-methyl-4-(1-methylethenyl)-cyclohexene. Draw the 2-dimensional structural formula for limonene and circle each of the following groups:
   • cyclohexene parent hydrocarbon in red
   • methylethenyl branch in blue
   • methyl branch in black
6. Would you classify limonene as a saturated or unsaturated hydrocarbon? Explain your answer.
7. Convert these concentrations in μg/cm³ to concentrations in parts per million (ppm)
• 5μg/cm³
• 12μg/cm³
• 100μg/cm³
8. Calculate the molar mass of limonene.
9. Calculate the mass of limonene in 100 L of air at 25^oC and 100 kPa for each of the following concentrations:
   • 5μg/cm³
   • 12μg/cm³
   • 100μg/cm³