Model Building of Molecular Compounds Accelerated Chemistry I Most of our learning is in two-dimensions. We often draw representations of molecules on flat paper as structural formulas. Two-dimensional representations include electron dot structures and structural formulas. In electron dot structures, a pair of dots is used to represent a pair of electrons in a single covalent bond. A hydrogen molecule is represented by H:H and an oxygen molecule with a double bond is represented as O::O. In structural formulas, instead of dots a pair of electrons in a covalent bond is represented by a dash ( ). The same hydrogen and oxygen molecules are represented as H H and O=O. Although such models help us understand the structure of molecules, flat models do not give us the three-dimensional view necessary for an accurate mental picture of most molecules. In covalent molecules, single, double and even triple bonds can form between atoms. In some cases the atoms are arranged in a single continuous chains. At other times the chains are branched. Still other molecules can be found as a ring of atoms. Sometimes a given molecular formula might represent more than one compound. For example, C 2 H 6 O represents both ethanol and dimethyl ether, compounds with both different structural formulas and different chemical and physical properties. Substances with the same molecular formulas but different structural formulas are called structural isomers. Scientists who determine the structure of molecules often start with molecular model kits similar to the ones you will use in this experiment. The most famous picture in modern science history is that of Watson and Crick next to their very large model of DNA. With these models, we can better understand how the atoms of the molecule interact. O B J E C T I V E S 1. Construct models of some simple and more complicated covalent molecules. 2. Draw structural formulas showing the shapes of different isomers. 3. Construct more than one structure for some of the molecules represented. Materials 1 model kit
Procedure 1. Before doing anything else, complete the pre-lab questions. 2. Obtain a molecular model and open the box. In most kits, different colors are meant to represent different elements. Record in the table below the color corresponding to each element. 3. Make sure you make each molecule before you try to draw it. There is only one structural formula for each. When you are satisfied you have the correct structure, sketch it on the report sheet. Try to represent in your sketch the three-dimensional nature of the model. Note: All of these molecule only have single bonds. a. water, H 2 O b. carbon tetrachloride, CCl 4 c. methane, CH 4 d. methanol, CH 3 OH e. ethanol, C 2 H 5 OH 4. Make sure you make each molecule before you try to draw it. For the following molecules there can be more than one structural isomer of the molecular formula. For each one find as many different structural isomers as you can. Sketch a carbon skeleton (not a three dimensional figure - include all carbons and functional groups) for each structural isomer on the report sheet. Note: All of these molecule only have single bonds. Finally, write the correct name of the different isomer. a. butane, C 4 H 10 c. hexane, C 6 H 14 b. butanol, C 4 H 9 OH d. dichloroethane, C 2 H 4 Cl 2 5. Make sure you make each molecule before you try to draw it. All of the molecules so far have had only single bonds. The following group of molecules has double or triple bonds and there can be more than one structural isomer of the molecular formula for each. For each one find as many different structural isomers as you can. Sketch a complete structural formula for each structural isomer on the report sheet. Finally, write the correct name of the different isomer where necessary. a. carbon dioxide, CO 2 c. butene, C 4 H 8 b. Nitrogen, N 2 d. butyne, C 4 H 6 6. Make sure you make each molecule before you try to draw it. The following molecules are biologically relevant and contain only single bonds. There is only one structure for each molecule. Sketch a structural formula for each molecule on the report sheet. a. glucose, C 6 H 12 O 6, glucose has ring structure with 5 carbons and 1 oxygen in the ring; the sixth Carbon is attached to the carbon next to the oxygen. There are also five hydroxyl groups ( OH) with one on every carbon except for the branch carbon b. 2 - deoxyribose, C 5 H 10 O 4, deoxyribose has ring structure with 4 carbons and 1 oxygen in the ring; Again the extra carbon is attached to the carbon next to the oxygen. Starting with carbon on the opposite side of the oxygen from the branch, the carbons are numbered 1, 2, 3, 4 and 5. There are three hydroxyl groups ( OH) with one attached to each carbon except the branch carbon and the 2 carbon.
Date Model Building of Molecular Compounds Accelerated Chemistry I Report Sheet Pre-lab Questions 1. Draw an electron dot structure for water. 2. Draw the structural formula for the following molecules: a. hydrogen gas b. oxygen gas c. methane gas 3. What is meant by the term structural isomers? 4. Explain why the model for each of the following atoms has a specific number of bonding sites: a. hydrogen, with one site b. oxygen, with two sites c. nitrogen, with three sites d. carbon, with four sites Data and Observations Data Table 1 Atom Hydrogen Chlorine Oxygen Nitrogen carbon Number of Bonding Sites (number of holes) one one two three or five four Color
Three Dimensional sketches of the models from step 3 of the procedure a. water, H 2 O b. carbon tetrachloride, CCl 4 c. methane, CH 4 d. methanol, CH 3 OH e. ethanol, C 2 H 5 OH Structural Isomers of the models from step 4 of the procedure Molecule Isomers Isomers a. Butane, C 4 H 10 Two Isomers b. C 4 H 9 OH Four Isomers
c. hexane, C 6 H 14 Five Isomers d. dichloroethane, C 2 H 4 Cl 2 Two Isomers Structural Isomers of the models from step 5 of the procedure a. Carbon Dioxide, CO 2 b. Nitrogen, N 2
c. butene, C 4 H 8 three Isomers c. butyne, C 4 H 6 two Isomers Structural Isomers of the models from step 6 of the procedure: Check the procedure for directions on how to assemble these molecules a. glucose, C 6 H 12 O 6 b. 2 - deoxyribose, C 5 H 10 O 4
Analysis and Conclusions 1. A model of the molecule Silicon tetrachloride might look similar to the model you constructed for methane but the substances are very different. What are some limitations of models for representing molecules? 2. Cyclobutane has the formula C 4 H 8. The prefix cyclo- indicates a structure where the carbon atoms are connected in a ring. Draw the structural formula of this compound. 3. How many structural isomers are there for propene? Justify your answer. 4. Determine the shape of the following molecule: a. water b. methane c. ethyne d. carbon dioxide e. ammonia 5. Explain why DNA must be made from 2 deoxyribose and not 3 deoxyribose (Hint: think back to Bio I and remember how DNA is replicated). 6. Octane has 18 different isomers. Draw and name them all. Use the back side of this page for more room. (Hint: Draw all the octanes, then all the heptanes, then all the hexanes and so on.