H H. An Introduction to Organic Structures

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1 An Introduction to rganic Structures Introduction: In rganic Chemistry, understanding the structures of the molecules is an important concept. In this activity, you will have the opportunity to write out structures on paper and compare them to their three-dimensional models with the use of a molecular model kit. Background: To this point, all of the organic molecules have been written out on paper or the chalkboard as a two-dimensional representation. owever, these and many other molecules are three-dimensional. To represent a three-dimensional structure on a piece of paper or on a chalkboard requires either a talent for drawing or some conventions. ne method is to sketch the actual shape of the molecule the way that it appears. Too often, we use straight lines at right angles in place of how it really appears. A second method involves a convention for showing atoms that are coming towards and ones that are going away. For methane, the Lewis Structure looks like image A while the actual appearance looks like image B. Note that in B, the triangle is used to indicate an atom towards you while the dashed line indicates an atom away from you. C C A B A second aspect of using the models is to see how long chain hydrocarbon molecules can rotate and twist into a variety of shapes. For alkanes, all of the C-C bonds are single bonds. With these bonds, we say that they have free rotation, that is they can (and do) rotate constantly with respect to each other. The model kits will allow you to see this particular aspect. For cycloalkanes and alkenes, which are hydrocarbons with a double bond, there is no free rotation possible. Third, the model kits are an excellent way to see the different structural isomers for some of the smaller alkanes. By merely pulling apart and exchanging two parts of many existing structures, you will be able to create a structural isomer. Additionally, the chirality, or handedness, of C molecules can be illustrated by showing that two molecules with the same formula and connection can be made to look different. Lastly, a number of functional groups will be introduced with the model kits. A functional group is any special group that is not made up of just carbon and hydrogen. These functional groups can greatly affect both the structure and the properties of the parent hydrocarbon chain. ne of the atoms prevalent in many of these functional groups is oxygen. xygen is found in the functional groups of ethers, alcohols, aldehydes, ketones, carboxylic acids, and esters.

2 Procedure: 1. btain a model kit for your group. Using black to represent Carbon atoms, white for ydrogen, and the SRT STICKS, makes C 4. Turn the structure on its side. Answer the questions about methane on the Questions Page. 2. Remove one of the s and add another C atom. Then add s until you have made C 2 6. Rotate the two C s with respect to each other. Then answer the questions about ethane. 3. Remove one of the s again and add a third C atom. Add s until you have made C 3 8. Rotate the all three C s with respect to each other. Then answer the questions about propane. 4. Remove one of the end s and add a fourth C atom. Add s until you have made C You should have made butane. If you made 2-methyl propane, then rearrange your C s. Rotate all four C s into as many distinct ways as possible. Then answer the questions about butane. 5. From the end C s, remove one each. Then, with a short bond, connect the two end C s to make cyclobutane. Can you rotate the C s with respect to each other? Answer the questions about cyclobutane. 6. Return to the propane model by removing one of the C atoms. Replace one of the s with a Cl atom (use a green color ball). Answer the questions about the chloropropane molecule. 7. Return to the butane molecule in #4 by adding back one more C atom and add the s. Replace one of the s on the chain with a Cl. Answer the questions about the chlorobutane. 8. Return to the cyclobutane molecule in #5. Replace an with a Cl on the cyclo molecule. Answer the questions about the chlorocyclobutane. 9. Disassemble your butane molecule and make the molecule ethene, C 2 4, where two s are on each C atom. Note that the C s do not have four bonds. Reconnect the two C s with two of the longer, flexible sticks to create a double bond between the two C s. This is called an alkene. Attempt to rotate the two C s. Then answer the questions about ethene. 10. Replace two of the s from the ethene molecule above with two Cl s. Answer the questions about your dichloroethene. 11. Make two forms (mirror images) of the molecule containing a C with a, Cl,, and C 3 group attached. ave your instructor inspect your molecules to make sure that you have both optical isomers and not the same molecule! 12. Six molecules containing an oxygen (red) functional group will be set out for you to look at. Draw structural formulas for each of these and decide which functional group is present from the list on the next page.

3 Functional Group Generic Identification Description Ether R - - R an atom between two C chains Alcohol R - - an group attached to a C chain Aldehyde R - C - a C group on the end of a C chain Ketone R (C=) R a C = in between two C chains Carboxylic Acid Ester R - C - - R - C - - R a C group on the end of a C chain a C group in between two C chains Questions: Methane: When the molecule is turned over and over, does it ever change its appearance? Ethane: Using the pictures A and B from the first page as a guide, draw a complete structure and a sketch that shows the bonds as they really appear. Propane: ne other method for showing the structure using a condensed structure is to use a zigzag drawing pictured below. Based on your constructed model, is this an accurate picture? C 2 C 3 C 3 or Butane: Draw a picture of butane using the same method (zig-zag) as for the propane molecule above.

4 Cyclobutane: Draw the geometric representation (shape) for this molecule. Chloropropane: ow many different chloropropanes with the formula C 3 7 Cl are possible? Give condensed formulas and names for all of them. Clorobutane: ow many different chlorobutanes are possible? ow many different molecules can have the formula C 4 9 Cl? (The answer is different!) Give the names and structures of all of these molecules that have the formula C 4 9 Cl. int: Two are chlorobutanes and two are chloromethylpropanes. Chlorocyclobutane: ow many different chlorocyclobutanes are possible? Give the name(s) and structure(s) of all of these molecules. Ethene: Draw the molecule C 2 4 using the projections like you did with Ethane.

5 Dichloroethene: ow many different arrangements can you come up with for this formula? Draw all of them below. Draw the two mirror images of the C 3 CCl() molecules below using the 3D method. Functional Groups: For the molecules built for you, write out complete structures for each below and decide which functional group is present. General Questions: 1. Are butane and cyclobutane structural isomers? Why or why not? 2. ne of the functional group molecules is one you have seen in class what is its name? What is its inorganic formula? int: it is an acid from Ch. 15.