Organic Smell Identification

Introduction

Aromatherapy—it’s organic chemistry! Students are invited to preview the distribution of organic compounds in nature in the form of natural products used in fragrances, flavorings, and perfumes. This kit introduces students to the wide variety of structural types in organic chemistry and the uses of organic chemicals in their everyday lives. Students will learn about the role of chemical structure recognition in the biological function of natural products and that chemistry makes scents!

Concepts

• Organic chemistry
• Natural product chemistry
• Fragrance and flavor agents
• Odor receptor theory

Materials

Safety Precautions

Chemical scents A, C, D, E, G, H, K, M, O, Q and S are body tissue irritants (irritating to skin, eyes, and mucous membranes). Very small samples of each scent have been provided. Because the quantity of each scent is so small, proper wafting of the scents is considered safe. Avoid direct and prolonged inhalation. Handle the vials with care to avoid spilling the chemicals. Instruct students beforehand on the proper method for carefully smelling (wafting) chemical odors. The correct method involves holding the sample vial in one hand and using a hand to waft the vapors toward your nose. Do not allow students to place the sample vial directly in front of their face or nose.

Procedure

1. Twenty volatile organic compounds with familiar pleasant odors are found in the enclosed vials. Glass wool has been added to each vial except K and L to increase the surface area of the liquid scent and to provide a more recognizable odor.
2. Students are instructed to smell each compound carefully and to attempt to identify the odor. The proper wafting motion to smell each scent sample is illustrated in Figure 1.
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3. The molecular formula of each compound and its scent identification letter are found on the side of the vial. Ask students to compare their identification of each chemical scent with its natural origin given on the back of the matching scent identification card.
4. The vial samples and cards should be retained and stored for future use.

Student Worksheet PDF

13351_Teacher1.pdf

Lab Hints

• Due to revisions in the Hazard Communication Standard, it is not permissible for GHS labels to identify chemicals by using a letter or number code. To perform the activity as written, the end user may transfer contents to another vial, cover the chemical name with a sticker containing the letter code or write the letter code on a Post-it^® and cover the chemical name.
• Allow students to compare the chemical scents provided in this kit versus natural sources of the same scent. This can be done conveniently with the ester scent compounds contained in vials E, M, N and O. Place orange, banana, pear, and strawberry slices in plastic bags. Ask students to identify matching pairs of natural and chemical scents and to compare their fragrance intensity and purity.
• It may be convenient to divide students into groups and have each group attempt to classify one set of scents (for example, scents may be divided into fruity, floral, and other categories). This will help to avoid sensory overload. Have a box of unscented tissues on hand to allow students to blow their noses and thus “rest” their sense of smell at periodic intervals.
• In comparing natural fruit scents versus their chemical scent partners, students may observe that the high concentration of a chemical scent tends to be much more intense and to lack subtlety. The naturally occurring fruit odor is due to a complex mixture of chemicals and is diluted by the presence of inert ingredients (water). The minor components of a mixture blend the chemical scents and give the natural scent its distinctive nuances.

Teacher Tips

• This kit provides a great introduction to organic chemistry! Consider some of these recommended follow-up activities.
  1. Divide students into teams and have each team build a model of one of the organic scent compounds. Students can compare structures and look for patterns.
  2. Introduce students to the idea of functional groups in organic chemistry. Give them a table of functional group structures and have them identify the functional groups present in each of the scent molecules. Ask the students to classify the scent compounds into categories based on their functional group identification (e.g., esters, alcohols, aromatic compounds).
  3. Prepare an organic ester in the lab and identify it based on its scent. Flinn Scientific sells several ester preparation kits.
• Is it time for review? Students can calculate molecular weights for the chemical scents based on their chemical formulas. Use the structural information to review the rules of Lewis dot structures and the shapes of molecules (VSEPR theory).
• Comparison of the chemical structures and scents of the two carvone samples (vials F and G) provides an enrichment activity for highly motivated students. (R)- and (S)-carvone exhibit a unique property of organic compounds called chirality. Interested students can research the definition of chirality and the importance of chirality in biology, pharmacology, and natural product chemistry.
• Take advantage of the current popular interest in aromatherapy to encourage students to learn more about how fragrances are described and formulated.

Discussion

All of the scents included in this kit are volatile (relatively low-boiling) organic compounds used as fragrance and flavor additives in perfumes, foods, cosmetics, and pharmaceutical products. Table 1 in the Teacher PDF summarizes the following information for each compound: chemical name and molecular formula, chemical structure, scent and its application as a fragrance and flavor ingredient.

Although many of these compounds are manufactured in bulk and used as “artificial” additives, they are also produced naturally by living organisms. Many of these natural products have been identified as individual components of the complex organic mixtures (“essential oils”) that are responsible for the distinctive scents of flowers, fruits and vegetables. Essential oils are obtained by steam distillation or solvent extraction from flowers, leaves, seeds, and stems. As an example of the composition of essential oils, the odor of natural banana extract has been traced to the presence of at least 27 volatile organic compounds. More than 100 compounds have been identified as constituents of the natural flavor of ripe strawberries!

Although the sense of smell is an intricate physiological response, it can be understood on a molecular level in terms of a fairly simple sequence of events. Fragrance compounds must be small enough so that a sufficient number of molecules can evaporate and reach the nose. The sense of smell originates with olfactory nerve cells located in the the nasal passages. The cell membranes of these nerve cells contain many different protein molecules, called odor receptors, that are able to bind small organic fragrance molecules. Odor receptor proteins bind organic molecules that have the proper combination of molecular shape and physical properties. Receptor binding triggers a cascade of events on a cellular level that culminates in opening up membrane channels for calcium ions to flow into the nerve cells. The flow of calcium ions sets up an electrical impulse potential that is responsible for nerve transmission. An electrical nerve impulse travels from the olfactory cells to the olfactory region in the brain, the part of the brain that interprets the chemical and electrical signals and translates it as a smell response.

The question of how the nose can detect a variety of different odors is very interesting. The olfactory system takes advantage of a combination of multiple receptors: each protein receptor can bind to several different odor molecules, and a single odor molecule can be recognized by multiple specific receptors. As a consequence, different odor molecules are recognized by different combinations of receptors, and it is the unique and subtle pattern of bound receptor sites that encodes the identity of an odor. This has been compared to playing unique combinations of musical “fragrance” notes that blend together and give rise to distinctive musical fragrance compositions.

As noted in the table of structural information for the organic scent compounds, many of these fragrance chemicals are also used as flavor additives for foods. The flavor of food depends on an intimate combination of chemical responses to both taste and odor. This student activity kit provides an interesting introduction to key concepts in both organic chemistry and biochemistry.