CHEMISTRY NOW: Introduction to Enantiomers and Handedness (Grades 9-12) Print

Objective:

Students will understand the three-dimensional configuration of enantiomers. Students will be able to identify chiral and achiral molecules.


Introduction Notes:

CHEMISTRY NOW: Introduction to Enantiomers and Handedness

 

Subject Area: Chemistry

Grade Level:  High School Chemistry

Lesson Title: Introduction to Enantiomers and Handedness

National Science Education Standards:
Science as Inquiry: 9-12

Physical Science Standards:

Structure of Atoms: 9-12

Structure and Properties of Matter: 9-12

Chemical Reactions: 9-12

 

Suggested Prior Knowledge:  Isomers

 

Purpose: To give students an understanding of enantiomers and chirality, and their influences on chemical reactivity.

Key Vocabulary: 

Stereoisomers: isomers that have the same connections among the atoms but have different arrangement of the atoms in space

geometricor cis-trans isomersor diastereoisomers: non-mirror-image stereoisomers

enantiomer: stereoisomers that are non-identical mirror images of one another or are non-superimposable mirror images of each other

chiral: molecule that has handedness and can exist as an enantiomer

achiral or nonchiral: lacking handedness – in chemistry, a molecule that is identical to its mirror image, has a plane of symmetry, and can be superimposed on its mirror image

symmetry plane (plane of symmetry): an imaginary plane that divides a molecule in half, where one half is the mirror image of the other

Objective:

  1. Students will understand the three-dimensional configuration of enantiomers.
  2. Students will be able to identify chiral and achiral molecules.

Materials:

Student worksheet, right-hand glove, goggles

Optional: molecular stick-model kits with goggles and mirror’ additional examples of chiral and achiral molecules. For visually-impaired students, use molecular models that use shapes instead spheres to represent the atoms.

Procedure:

Begin with an introduction to what the students will be learning about in the course of this lesson. Today we will be learning about the three-dimensional structure of enantiomers.     Before we get started, let's recap what isomers are and their significance to chemistry (should be taught as prior lesson). Use the following leading question and follow-up:

 

         Who can draw an isomer of the normal butane molecule C4H10 on the board? Only one answer is possible as the example below demonstrates.

 

 

Follow up with an open-ended question that initiates a conversation about the reactivity and properties of the isomers, for example:

         Are these molecules different even though they share the same formula? Students may answer shape, structure, or arrangement of atoms or groups of atoms.

         As an open-ended question, ask: is there anything else that can or may be different about these isomers? Students may suggest that something other than the shape of the molecule changes as well.  Answers suggesting reactivity should be encouraged for elaboration by the student.

 

Differences in chemical and physical properties of isomerism may have not been discussed in previous lessons. It is important that students understand that a seemingly minor change to the molecular shape of the molecule changes the reactivity in many cases. For these isomers, here are some differences and applications to point out to the class:

  • n-butane – commonly used as fuel for lighters; highly flammable; boiling point of            -0.5°C.  Has a flat shape as compared to methylpropane.
  • methylpropane – used as refrigerant and as a propellant in aerosol cans; flammable; boiling point  -11.7°C. Has a spherical shape.

 

Once students agree that the shape and the configuration play a role, continue with an introduction of how isomers are classified. This lesson will focus on the branch leading to enantiomers. Use the chart below to discuss the definitions isomers, stereoisomers, and enantiomers:

 

 

Explain to students that stereoisomers can be broken into two groups, enantiomers and diastereoisomers. Enantiomers are mirror-image isomers and diastereoisomers are non-mirror- image isomers. In this lesson, we will be examining only the enantiomers. For a molecule to exist as an enantiomer, is must be chiral. The human hand is chiral because it has no symmetry plane and cannot be superimposed on its mirror image. Review the following photos with the class. (If possible, have students replicate these photos with molecular stick models and a mirror.)

 

 

Review the following photo with the class. For a molecule to exist as an enantiomer, it must be chiral.  (If possible, have students replicate these photos with molecular stick models and a mirror.)

 

 

Qualifications (tips) of Molecular Handedness:

The following are some general insights that should be discussed with students to "qualify" molecules for chirality. Utilize the student worksheet, diagrams in this lesson plan or, when possible, molecular models and audio/video presentations.

 

achiral molecules

chiral molecules

  • In organic compounds when carbon is bonded to two or more of the same group.
  • Has a plane of symmetry
  • Identical to its mirror image
  • Can be superimposed on mirror image
  • In organic compounds whenever carbon is bonded to four different atoms or groups of atoms, chirality is possible. (Be careful: they may be identical and not qualify as enantiomers.)
  • Lacks a plane of symmetry
  • Not identical to its mirror image
  • Cannot be superimposed on mirror image
  • Can exist as an enantiomer
  • Shows optical activity

 

Try this quick experiment to smell the two carvone isomers:

Procedure:

 

1.      Give each group a handful of caraway seeds and some mint leaves.

2.      In one bowl or plate, crush the caraway seeds; in the other bowl, crush up the mint leaves.

3.      Smell the crushed items from each sample and notice that the isomers have distinct smells.

 

Ask students:

  •  Describe the smells of the two items. Do you think that these molecules are the same?   Why or why not?
  •  As an interactive to this experiment, have students see if they can figure out the similarity between the carvone molecule and fitting their right hand into a left-hand glove.
  •  Show the students the molecule of carvone. This is what they have been smelling. Have students discover how the isomers are the same and different. Carvone has an isomer and by examining the drawing, they will notice that the molecules are mirror images of each other, just as their hands were – so one molecule is essentially left handed (S) and the other is right handed (R).

Isomers of Carvone Molecule

(S) isomer "left-handed" is found in caraway seed

(R) isomer "right-handed"  is found in spearmint

 

Both caraway and mint molecules look the same, but because they are mirror images they are not exactly the same – they are isomers of carvone. The human nose has specific receptors for each kind of isomer. Imagine receptors in the nose shaped like gloves: each isomer of carvone can only fit in the corresponding glove.

 

Take some time to discuss how chirality is relevant to modern medicine and industry. The three- dimensional conformation of molecules plays an important role in the world of medicine. How medicines act in the body are often specific to the shape of the molecule. In the binding of a substrate (reactant) to the catalytic (active site) of an enzyme, the two molecules must fit each other closely and in a complementary fashion. Have students try putting a left hand in a right- hand glove as a demonstration of this concept. Students can also attempt to shake hands using only their right hands: even though they are both hands in their functionality and construction, shaking hands with others only works one way.

 

Additional Resources:

  • http://www.youtube.com/watch?v=3WZZXPOsPNI
  • http://pubs.acs.org/cen/coverstory/8023/8023chiral.html
  • http://pubs.acs.org/cen/coverstory/7843/7843scit1.html

Student Worksheet for: Enantiomers and Handedness         

 

Name:  ________________________________________________ Date: ______________

 

1. Draw an isomer of the following molecule:

 

 

2. What else is likely to change among isomers other than the position of atoms or groups of atoms?

 

 _________________________________________________________________________

 

Isomer Classification Key:

 

 

Define the following:

 

1. Isomers:

 _____________________________________________________________________

 

2. Stereoisomers:

 _____________________________________________________________________

 

3. Enantiomers:

______________________________________________________________________

 

4. Chiral:

 

______________________________________________________________________

 

5. Non-Chiral:

 

_______________________________________________________________________

 

Image Source: Principles of Biochemistry 2nd ed., Lehninger, Nelson, Cox, 1993

 

 

4. Recreate these images depicting enantiomers utilizing molecular stick models and your hands:

 

 

 

 

 

 

5. Which of the following objects are chiral?

  1. a glove
  2. a baseball
  3. a chair
  4. a foot
  5. a pencil
  6. a mug

 

6. Using your molecular stick models to recreate the following pairs of molecules. Indicate if they are identical or enantiomers. (A. identical / B. enantiomer / C. enantiomer)

 

 

7. Using your molecular stick model, create a new enantiomer and draw your result below:

 

 

 

 

 

 

 

 

 

 

8. Research how enantiomers may play a role in real-world applications, such as in medicine and the drug industry. Discuss specific issues and specific molecules where possible.

 

 

 

 

 

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