Living By Chemistry

Molecular Shape, Polarity, and

Our Sense of Smell

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

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.

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.

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.

Inside the Nose

Inside the nose is a watery mucous lining.

The intermolecular attractions of polar molecules cause them to dissolve easily in water.

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.

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.

Small Nonpolar Molecules

If the molecule is symmetrical, chances are it will be nonpolar and not have a smell.

Tetrafluoromethane is symmetrical and nonpolar.

Check-in

This is one of the molecules that gives coffee its smell. Its name is 2-furylmethanethiol.

Is this molecule polar? Does it have a smell?

Key Question

What does phase have to do with smell?

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?

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.

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.

Check-inWhich 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