If stereochanges are involved in the reaction, you must work with clear, accurate representations of the molecules. Models may be used as an aid, but rarely can all of the necessary elements be modeled in plastic.
Models cannot be handed in with an examination or submitted to an editor along with a journal article. Thus, you must learn to draw.
Two general kinds of representations are used in organic chemistry to show stereostructure:
- The major portion of the molecule is drawn as planar, and stereocenters are shown with wedged bonds for groups projecting above, and dashed bonds for groups projecting below, the plane of the board or paper. Problem (5) uses this type of representation for the reactant.
- A perspective view is drawn. Problem (5) illustrates this kind of representation for the the product.
Although both kinds of drawing may be used in a single equation, they generally are not both used within the same structure.
To make good drawings, you must keep in mind both a set of drawing conventions and some facts about molecular structure. The conventions are simple:
- In making a perspective drawing, molecules are displayed as if viewed from slightly above and slightly to the right of the position of the molecule.
- Additional three-dimensional character is added by breaking bond lines where they pass behind others; frontmost bonds may be drawn slightly thicker, as well.
- When employing the wedge and dash convention, bonds lying in the plane of the paper are drawn with normal lines.
- Wedges, for bonds projecting out of the plane, generally are drawn tapering toward the plane; dashed bonds, which project behind the plane, may be drawn tapering away from the plane.
- Bonds behind the plane are dashed (||||||) not dotted (.....). Dots are reserved for partial bonds, as in transition states.
The necessary facts of molecular structure you have been accumulating since general chemistry must be kept in mind. For example:
- Four-valent carbon is tetrahedral; this means that the carbon and any two bonds define a plane, while the carbon and the other two bonds define a perpendicular plane.
- Three-valent carbon is trigonal, the carbon and all three bonds lying in a plane. A corollary of this is that both atoms of a double bond, and each of the four directly attached atoms, must lie in the same plane.
- Two-valent carbon is linear, the carbon and its two bonds defining a straight line. A corollary of this is that both atoms of a triple bond, and each of the two directly attached atoms, must lie on a straight line.
These and other facts of structure are so familiar to you that you often do not even think about them. But to make your drawing an accurate rendering of a molecule, one that will help you write a correct mechanism, requires that you recall all of these things while you are drawing.
You may find it helpful in the following exercises to construct a model of the structure. Your eventual goal, however, is to be able to work with pencil and paper, or chalk and blackboard, alone, and to be able to manipulate the drawing just as efficiently as the physical object.
- Draw the structures described below using the wedge-dash-solid line notation only:
- 2-(R), 3-(S)-dibromopentane with the bromines eclipsed;
- Perform an internal rotation of (a) so that the bromines are anti-
- Rotate the whole structure in (b):
- 90o about an axis perpendicular to the paper;
- 180o about a vertical axis in the plane of the paper;
- 180o about a horizontal axis in the plane of the paper;
- Repeat (a) and (b) for the enantiomer of the given structure;
- D-Galactose is 2-(R), 3-(S), 4-(S), 5-(R), 6-pentahydroxyhexanal. Draw the all-staggered conformation.
- Make perspective drawings corresponding to the following descriptions:
- Methyl cyclopentane in the envelope conformation;
- Methylcyclopentane in the twist conformation;
- Both chair conformations of methylcyclohexane;
- Both twist-boat conformations of methylcyclohexane;
- 3-Methylcyclohexene;
- trans-decalin;
- cis-decalin (both all-chair conformers);
- trans-, trans-perhydrophenanthrene;
- trans-, cis- and cis-, trans-perhydrophenanthrene;
- cholesterol;
- cholestanol (the A-B cis-, saturated relative of cholesterol).
- Many reactions of open-chain molecules involve transition states in which the reactant is folded into a chair-like or boat-like conformation. Draw the following:
- Both chair conformers of D-galactose (above) in the pyranose form;
- A chair-like conformation of allyl vinyl ether, such as might be the transition state for the Claisen rearrangement;
- A boat-like conformation of (b);
- Each double bond of allyl vinyl ether becomes stereogenic when
substituted with a terminal methyl group. Repeat (b) and (c) for all
possible stereoisomers when both double bonds are so substituted.