Models of Organic Compounds
General, Organic and Biological Chemistry Kit
Materials Included In Kit
Organic model sets with at least six carbon atoms, 6*
*See Catalog No. AP5453 for additional model sets.
- In general, guided-inquiry activities are most successful if students understand that the activity replaces lecture. Students are more likely to take responsibility for learning when they are actively engaged in the process of “constructing knowledge.” Guided-inquiry activities simulate the scientific method—students look at data, search for patterns or relationships and try to identify guiding principles that will explain the data.
- The instructor’s role in guided-inquiry activities is very important. The instructor facilitates independent student learning by keeping them on track and reviewing progress at key junctures. In this activity, for example, the instructor may want to call a time-out after Question 4, the definition of isomers. Ask several groups to give their definitions, then ask students to explain or defend their definition, or to modify their definition based on new information.
- The following topics and concepts should be reviewed prior to scheduling this activity: Lewis structures, VSEPR theory, multiple (double and triple) bonds, including the difference between sigma and pi bonds and hybrid orbitals.
- Protocols for teaching Lewis structures that work well in general chemistry (e.g., count up all the valence electrons, add single bonds, followed by lone pairs) are not as useful when teaching organic chemistry. An alternative strategy is to teach students the typical number of bonds that an atom will form when it has zero formal charge. Carbon forms four bonds, nitrogen three bonds, oxygen two bonds, fluorine one bond.
- Students need good spatial reasoning skills to visualize molecules in three dimensions. All students will benefit from the opportunity to hold models in their hands, rotate them, turn them upside down, etc.
- This activity may be combined with an optional tutorial or cooperative class activity on naming organic compounds.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Engaging in argument from evidence
Obtaining, evaluation, and communicating information
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Scale, proportion, and quantity
Systems and system models
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
HS-PS1-1. Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
Answers to Prelab Questions
- What is the maximum number of covalent bonds each of the following elements will form when it is neutral?
- The structural formula of a molecule shows all of the atoms in the structure and the order in which they are connected by covalent bonds. Add hydrogen atoms as needed to each atom in the following structural formulas so that each atom has a closed-shell electron configuration and zero charge.
- Methane, CH4, is the chief component of natural gas.
- Describe the molecular geometry and bond angles in methane.
Methane has a tetrahedral geometry around the central carbon atom. The ideal tetrahedral H—C—H bond angle is 109.5°.
- Draw a diagram that illustrates the three-dimensional shape of methane.
- Recall the Lewis structure and molecular geometry of carbon dioxide, CO2.
- Sketch the overlap of atomic orbitals in the C—O sigma and pi bonds.
- Carbon dioxide is a linear molecule. Explain.
The Lewis structure of CO2 has two C=O double bonds. The electrons in each double bond are counted as one set of electrons when using VSEPR to predict molecular geometry. Thus, the C atom has two sets of electron density that must be arranged 180° apart to minimize electron-electron repulsion.
Answers to Questions
Part A. Structures of Organic Compounds
Obtain a set of molecular models to answer the questions in Part A.
Part B. Functional Groups
- Build models of ethane, C2H6, and propane, C3H8, and write out their structural formulas.
- Do the C—C single bonds in ethane and propane rotate freely? Explain.
Yes, the hydrogen atoms on adjacent carbon atoms in ethane can “slide” past each other as the C—C bond turns or rotates.
- There are two possible structures for butane, C4H10. Build models of both structures and draw their structural formulas.
- The two possible structural formulas for butane are called isomers. Write a general definition of isomers that describes the relationship between the two structures.
Isomers have the same molecular formula but different structural formulas.
- Without building models, draw out the possible structural formulas for three isomers of pentane, C5H12.
- Alkanes are hydrocarbons—compounds containing only carbon and hydrogen—in which all of the C—C bonds are single bonds. What is the general formula for an alkane, where n is the number of carbon atoms?
- Alkenes are hydrocarbons that contain at least one C═C double bond in their structure. Build models of ethene (C2H4) and propene (C3H6) and draw their structural formulas.
- Describe the molecular geometry around the C═C double bond in an alkene. What is the H—C—H bond angle in ethene?
The molecular geometry about the C═C bond is planar—the two carbon atoms and atoms attached to them lie in a plane. The H—C—H bond angle is 120°.
- Unlike C—C single bonds, C═C double bonds do not rotate. Draw diagrams showing the overlap of the orbitals responsible for the sigma and pi bonds, respectively, in a C═C double bond. Use the orbital diagram to explain why the C C double bond does not freely rotate.
“Turning” the C═C bond would destroy the overlap of the p orbitals in the pi bond.
- Butene (C4H8) has one C═C double bond in its structure. Draw structures for three possible structural isomers of butene.
- The structural formula for 2-butene can be abbreviated CH3—CH═CH—CH3. Because of the lack of free rotation about the C═C double bond (see Question 9), there are two possible structures for this compound. Build models and draw structural formulas for two three-dimensional structures of 2-butene.
- The two forms of 2-butene shown in Question 11 are called geometric isomers. What is the same and what is different about geometric isomers?
Geometric isomers have the same molecular formula and the same structural formula, but different arrangements of atoms in space.
- What is the general formula of an alkene, where n is the number of carbon atoms? Why do you think alkenes are called unsaturated and alkanes are called saturated hydrocarbons?
Alkenes are “unsaturated” because they contain fewer than the maximum number of hydrogen atoms possible for the number of carbon atoms. Alkanes are “saturated” because they cannot “add” any more hydrogen atoms.
- Benzene, C6H6, is the parent compound of a class of compounds called aromatic compounds that are very common in nature. The carbon skeleton for benzene is shown below. Add hydrogen atoms and double bonds, as necessary, to complete the structure of benzene.
- Build a model of benzene and describe its molecular geometry (e.g., planar, tetrahedral).
Benzene is a planar molecule—all of the atoms lie in a single plane.
- The structural formula of benzene in Question 14 shows alternating single (C—C) and double (C═C) bonds. It has been found, however, that all of the carbon–carbon bonds in benzene are identical. This fact may be explained in terms of resonance. Define resonance and draw two resonance forms for benzene.
Resonance occurs in a molecule when it is possible to write two or more valid Lewis structures for the molecule. The actual structure of benzene is the average of the two possible Lewis structures.
- Alcohols are organic compounds containing an –OH group attached to a carbon atom. Draw the structural formula of ethanol, C2H5OH.
- Low-molecular weight alcohols such as ethanol are polar compounds and are miscible with water. As the number of carbon atoms in an alcohol increases, the solubility of the alcohol in water decreases. Thus, octanol, C8H17OH, is practically insoluble in water. Explain.
In octanol, the polar –OH group “competes” with the nonpolar chain of eight carbon atoms. It is insoluble in water because the long nonpolar chain predominates.
- Compounds containing at least one carbon atom that is attached to four different groups give rise to a special class of isomers called enantiomers. Enantiomers are defined as non-superimposable mirror images of each other. Build models and complete the following diagrams to show the enantiomers of alanine, an amino acid.
- What does it mean to say that the enantiomers shown in Question 19 are non-superimposable? Why do you think this property of molecules is sometimes called “handedness?”
The enantiomers cannot be superimposed on one another by rotating or turning the molecules. Our hands are an example of nonsuperimposable mirror images.
Organic compounds are classified into functional group classes based on their structure and properties. A functional group is defined as a specific arrangement of atoms, such as –OH or –NH2
, that undergoes characteristic chemical reactions and gives organic compounds similar physical properties. Table 1 shows the structures of common organic functional groups. The symbol R is used to represent rings or chains of carbon and hydrogen atoms. Table 1. Organic Functional Groups
Circle and label the functional groups in the following natural and consumer organic products. The first one has been done for you as an example.