1. Molecular Shape, Polarity, and Our Sense of Smell
Living By Chemistry
Molecular Shape, Polarity, and
Our Sense of Smell

2. How does our sense of smell work?
Scientists have proposed many theories about how smell works and created models corresponding to these theories.

3. Receptor Site Theory
Receptor site theory: The currently accepted model explaining how smells are detected in the nose. Molecules fit into receptor sites that correspond to the overall shape of the molecule. This stimulates a response in the body.

4. Why do some molecules smell while others do not?
Only molecules that land in receptor sites that fit the shape of the smell molecules will smell.

5. How does the nose detect and identify different smells?
In the receptor site model, each receptor site has a specific shape that corresponds to the shape of just a few smell molecules.

6. Inside the Nose Inside the nose is a watery mucous lining.
The intermolecular attractions of polar molecules cause them to dissolve easily in water.

7. What does polarity have to do with smell?
Polar molecules dissolve easily in other polar molecules, such as water.
Nonpolar molecules do not dissolve easily in polar molecules.
In order to smell, the molecule needs to dissolve in the water found in the mucus lining of the nose and be attracted to the receptor site.

8. Small Polar Molecules
If the overall shape of a molecule is asymmetrical and the molecule is made from more than one kind of atom, chances are it is a polar molecule and will have a smell.
Chlorotriflouromethane is polar because of the chlorine atom on one side.

9. Small Nonpolar Molecules
If the molecule is symmetrical, chances are it will be nonpolar and not have a smell.
Tetrafluoromethane is symmetrical and nonpolar.

10. Check-in Is this molecule polar? Does it have a smell?
This is one of the molecules that gives coffee its smell. Its name is 2-furylmethanethiol.

11. Key Question
What does phase have to do with smell?

12. Think about it.
1. If you place an open perfume bottle and a piece of paper in a sunny window, the aroma of the perfume will soon fill the air, but you won’t smell the paper at all. Explain what is going on.
2. What is the heat from the Sun doing to the perfume to increase the smell?

13. Molecules in the Gas Phase
In order to smell a substance, the molecules have to physically get into your nose.
Molecules in the gas phase can get into your nose the easiest.
Volatile liquids and solids tend to smell because they can vaporize easily.

14. Solids with No Smell
Ionic solids (salt) and metals do not have a smell because they have strong particle attractions and therefore, do not evaporate easily.

15. Cl- Ca2+ Cl- (repeating throughout the solid in three dimensions)
Check-in
Which of these will have a smell? Explain your reasoning.
Substance
Structure
Phase
CaCl2, calcium chloride
Cl- Ca2+ Cl- (repeating throughout the solid in three dimensions)
solid
C8H8O3, vanillin
liquid
HCN, hydrogen cyanide
gas