Drawing, Representation, and Projection of Chemical Formulae.

Transcription

1 Drawing, Representation, and Projection of hemical Formulae. In order to learn and communicate organic chemistry you must learn the language of organic chemistry. Pictures play an important role in this language, particularly pictures that convey the 3- dimensional properties of an object. You will have to practice drawing on your own but the primer below gives you some simple conventions (tricks) that chemists use to make pictorial communication easier. Line Segment Drawings. Freshman hemistry presents chemical formulae to us in the following manner: O This is useful for learning the basics and for expressing the details of a small structure, but can be quite cumbersome for expressing ideas about larger molecular structures. For example, this steroidal fragment shown in freshman format is almost unintelligible; it also requires a lot of time to draw. Many of the structures of importance to organic chemistry are large and as such the old freshman format is no longer useful. Fortunately, organic chemistry deals with compounds containing a high percentage of carbon and hydrogen, and the valencies of the two, 4 and 1 respectively, remain constant for most compounds. Therefore, we can use a different drawing convention, one where the carbons are represented as the vertices at the end of each line segment (bond), and the hydrogens are omitted from the drawing but are assumed to be there in the number necessary to give carbon its full valence of four. Take for example hexane. On the left is the freshman format and on the right is the line-segment format. 1

2 In lecture I will use the line segment format most of the time and add other drawings to emphasize details. ere again is the structure of the steroidal fragment from the first example but now shown in the line-segment format. Bond-wedge onvention. Often you must convey the three dimension nature of a structure. One simple way to do this is through the bond-wedge convention. This is called a convention because you can't actually show 3-dimensions on a flat piece of paper so we must agree to some convention of what certain shapes are going to tell us. In this convention, a solid wedge means that the bond is coming out of the plane of the paper toward us; a dashed wedge means that the bond is going into the plane of the paper away from us; finally a simple line means that the bond is in the plane of the paper. Let us take the example of 2,2-dimethylpropane (neopentane) and look at the freshman format the simple line-segment drawing and the line-segment with bond-wedge convention. Freshman Format Line-segment Bond-wedge 2

3 an you imagine the 3-dimensional character of the molecule? an you see how much more information there is in the last drawing and how much simpler it is to view? Let's again look at our steroid example. What new information has been included? Newman Projections Another problem we address in organic chemistry is the placement of groups around a bond. There can be as many as six groups distributed about a - bond and it can be difficult to see where all the groups lie with respect to one another. In these cases the Newman projection is most helpful. This is called a projection because that is exactly what it is, but in order to understand the Newman projection we must know the convention that dictates what the picture means. b f b Front atom Back atom f b f Let us start with ethane. On the left is the bond-wedge diagram and on the right is the Newman projection. In a Newman projection one views the molecules directly down the axis of a bond. A big circle is used to represent the atom at the back and a dot is used to represent the atom in front. The attached groups span out from their respective atoms. Newman projections are useful to depict the stereochemistry around any bond not just - bonds. ere are some examples: 1) The 2-3 bond in butane; 2) The 2- bond in R-2-bromopentane; 3) The N- bond in methylamine; 4) The 1-2 and 4-5 in cyclohexane. 3

4 Butane onformers Me Me Anti-periplanar Me Me Syn-clinal R 2-bromobutane Br Looking down the - bond; is in back Methylamine.. N Looking down the N- bond; N is in front yclohexane Looking down two bonds at the same time Fisher Projections Viewing the tetrahedron as shown we see that the atoms make a cross when projected into the plane. In the absence of a convention the 3-dimensional information about the structure is lost on going from A to B. owever, if we say that all bonds that point north-south are going into the plane of the paper away from us, and all bonds going east-west are coming out of the plane of the paper toward us, then we can retain the spatial information. 4

5 A N B N North-South point in W E W E S S East-West point out Bond-wedge Fischer Projection Such a projection is called a Fisher projection and is useful for describing the orientation of groups around a tetrahedral atom. This representation has had the greatest impact in sugar chemistry. Some examples are included: 1) At 2 of R-2-bromopentane; 2) At 2 of S-lactic acid; 3) At 2 through 5 of glucose. 3 Br Br 2 5 R bromobutane O OO O OO 3 S lactic acid O O O 2 O O O O O O O O 2 O D glucose When using Fischer projections it is important to remember that the 3-dimensional information is 5

6 encoded in the structure by convention according to the direction of the bond, is North-South or East- West. Therefore, rotating a Fischer projection by 90 degrees exchanges North-South for East-West and gives you the mirror image structure. Rotating by 180 retains the original 3-dimensional structure. O OO O OO 3 O = = 3 OO O OO Mirror Images (Enantiomers) Another important thing to keep in mind is that because the information in the Fischer projection is dictated by convention the structure is viewed from its eclipsed conformation. This is not usually the most stable conformation. So use Fischer projections to compare and name structures quickly but not for making rigorous arguments about the most stable structure of the molecule. 6