"Formula Writing of Organic Compounds - PDF"
78 ORGANIC CHEMISTRY I. Rationale Organic compounds are compounds of carbon. There are more organic compounds than all other compounds combined. Many important industrial chemicals and products are organic compounds - plastics, dyes, drugs, synthetic fabrics, etc. Our important fuels are organic compounds. The compounds that make up our bodies and all other living things are almost entirely organic compounds. We can simplify our study of this vast array of compounds if we realize that all are made up of a few types of simple arrangements of atoms: functional groups. Their properties depend on which of these functional groups they contain, in what proportions, and in what arrangements. In this lab we want to look at some of these functional groups and see how the atoms within them are arranged. II. Prerequisites 1. As required In GENERAL INFORMATION - LAB, be ready to turn in a COPY of your: A. Equipment list B. Flow Chart 2. You should be able to answer the following questions: A. What is the color and how many legs are there for a carbon atom in the molecular model kit? B. What is a functional group? C. The full structural formula is different than a condensed structural formula because: D. A carboxylic acid has what functional group? E. What are isomers? III. Objectives When you are finished with this exercise you should be able to: 1. Construct a model and draw full structural formulas of compounds containing the following functional groups: 05/09/95 79 A. alcohol B. aldehyde C. carboxylic acid D. amine E. alkane F. alkene G. benzene ring 2. Construct models and draw the full and condensed structural formulas of isomers. IV. Lab Exercise 1. Condensed vs. Full Structural formulas A structural formula shows not only how many atoms of each element is present in a compound (which is what a molecular formula does), but also how these atoms are arranged or grouped in the molecule. In organic compounds the arrangement of the atoms in a molecule plays a major role in determining the compound's chemical and physical properties. Structural formulas can show the arrangement in a variety of ways. All are more or less pictorial representations of the molecule's appearance. In some cases, structural formulas look only slightly different than molecular formulas. For example, for the compound whose molecular formula is C3H8 (propane), the structural formula might be written: CH3CH2CH3 This is intended to show two CH3 groups and a CH2 group between them. The structural formula for a compound with the molecular formula C4H10 (n-butane) might be written: CH3CH2CH2CH3 Another compound with the same molecular formula but with a different structural formula is iso-butane: CH3CHCH3 or CH3-CH-CH3 * * CH3 CH3 In these examples some of the bonds are also shown. Structural formulas which show groups of atoms, but few, if any, bonds, are called condensed structural formulas. Condensed structural formulas are compact and require you to have enough experience to recognize what things are part of a group and which are not. 80 Another way to write/draw a structural formula is the full structural formula. In the full structural formula all bonds are shown. For example: H H H H H H H * * * * * * * H-C-C-C-H H-C-C-C-C-H * * * * * * * H H H H H H H propane n-butane H H H * * * H-C-C-C-H * * * H * H H-C-H * H iso-butane Full structural formulas are two-dimensional representations of molecules which are actually three-dimensional. However, just as an artist can represent reality on a flat surface by drawing or painting in perspective, full structural formulas can also be drawn in perspective to give them a more realistic appearance. An alternative to drawing in perspective is to build models of the molecules. The models contain all of the atoms and show all of the bonds of the molecule, but give you a better chance to actually see the shape of the molecule. For the remainder of the exercise you will be building molecules which illustrate various "functional groups" or various "isomers". After you building the required molecules, draw their full structural formulas. Use the following directions for building each molecule: NOTE: DO NOT EVEN THINK ABOUT drawing the structural formulas until AFTER you have made the model. THEN, based on your model, draw the formula. A. Make models by assembling the proper components of the molecular model set. Note that we may have more than one brand of molecular models, and that different elements may be represented differently in each. The following lists should help you decide which atoms and bonds to use for your models: 81 Lab-Aids molecular models: CARBON: black, FOUR (4) legs HYDROGEN: white, one (1) leg OXYGEN: blue, TWO (2) legs NITROGEN: red, four (4) legs - but you use only THREE legs CHLORINE: green, one (1) leg SINGLE BONDS: Use one (1) flexible white tube DOUBLE BONDS: use two (2) flexible white tubes Minit Molecular Building System: CARBON: black, FOUR (4) legs HYDROGEN: white, one (1) leg OXYGEN: red, two (2) legs NITROGEN: blue, three (3) legs CHLORINE: green, one (1) leg SINGLE BONDS: use one (1) stiff green straw for each DOUBLE BONDS: use two (2) flexible white straws for each B. Gather up the correct number of each type of atom as indicated in the molecular or structural formula given for the molecule. For CH3CH2CH3 (propane) you should have 3 carbons and 8 hydrogens. C. From these atoms, assemble each functional group shown in the formula. For the above molecule, there are two CH3 groups and a CH2 functional group. We will discuss functional groups more fully below. D. Assemble the functional groups in the arrangement shown by the condensed structural formula. E. Draw the full structural formula WHILE YOU HAVE THE MODEL TO LOOK AT. 2. Functional groups Specific groups of atoms (pieces of molecules) which tend to give a molecule particular chemical and/or physical properties are called functional groups. A. Alkanes In the compounds used as examples in Section 1 we saw three functional groups: CH3-, -CH2- and -CH-. Alkanes are functional groups which contain carbon and hydrogen joined only by single bonds. The compounds n-butane, propane, and iso-butane, whose condensed and full structural formulas were given above, contain alkane functional groups. Make a model and then draw the full structural formula for pentane, CH3CH2CH2CH2CH3: 82 B. Alcohol An alcohol functional group is written in the condensed form as -OH or HO- (depending on the direction). The full form would be -O-H. The condensed formula of ethyl alcohol is CH3CH2OH. Build a model of this compound and draw its full structural formula: C. Aldehyde An aldehyde functional group is normally written in condensed form as -CHO. The full structural formula would be O 5 -C-H If not written to show each bond in the functional group, the formula could be written either as -CHO or -COH. The distinction is completely arbitrary. Why is the condensed formula for an aldehyde always written -CHO instead of -COH? The simplest aldehyde containing compound is formaldehyde, a compound used in embalming. Its condensed formula is HCHO. Make a model of this compound and draw its full structural formula: D. Carboxylic acid A carboxylic acid functional group is -COOH. The full structural formula would be O 5 -C-O-H In this functional group the -OH contained is not an alcohol. Vinegar contains the carboxylic acid functional group in a compound called acetic acid, CH3COOH. Make a model of this compound and draw its full structural formula: 83 E. Amine Amines, -NH2, are functional groups related to ammonia. The odor of fish and decaying matter is often due to certain amine-containing compounds which result from the breakdown of proteins. Guess where cadaverine got its name. Make a model of cadaverine, NH2CH2CH2CH2CH2CH2CH2NH2, and draw its full structural formula: F. Alkene Alkenes contain only carbon and hydrogen, but they also contain at least one double bond. The simplest compound containing an alkene functional group is ethylene, CH2=CH2, the starting compound for the synthesis of polyethylene. Make a model of ethylene and draw its full structural formula: G. Benzene Ring Aromatic compounds contain a functional group called the benzene ring or phenyl group: Toluene is compound which is a solvent for glues. Make a model of toluene and draw its full structural formula. Its condensed formula is 3. Isomers Combining different functional groups is one way to make different compounds. Another way is to take the same functional groups and combine them in different ways. 84 Compound which contain the same atoms (and so have the same molecular formulas) but in which the atoms are arranged in different ways are called isomers. The compounds n-butane and iso-butane from above were isomers. One way to draw the condensed structural formula of a compound whose molecular formula is C4H10O is CH3CH2CH2CH2OH Build a model of this compound and draw its full structural formula. Then (without disassembling this model) build two more models which still contain the same number and types of atoms, and which contain only alkane and alcohol functional groups. Make sure that the second and third models are really different from the first -- move them around and twist groups around the bonds connecting them. Sometimes you will find that what you thought were two "different" models are actually alike. (HINT: First try swapping oxygens and hydrogens; then try carbon-hydrogen swaps.) Only after you are sure that the models are really different should you then draw the full structural formulas of all three molecules -- while looking at the models. Build two models which are isomers of a compound which has the condensed formula of CHCl=CHCl. (This means that you should end up with two models, not three!) Make sure that each isomer retains the double bond, and each has an H and a Cl on each carbon. Draw the full structural formula for each: 85 Build two isomers of molecules with the structural formula of H * CH3-C-NH2 * Cl Each model should have a CH3, a NH2, a H and a Cl attached to the central C. (Note: the structure shown may be considered one of the two isomers that you are to build.) Draw full structural formulas: