Wedge-and-dash representation

Example 10.8 identify chiral and achiral molecules in each of the following pair of compounds. (Wedge and Dash representations according to Class XI, Fig 12.1). 

A-6. A wedge-and-dash representation of a form of ribose (called /3-D-ribopyranose) is shown here. Draw the most stable chair conformation of this substance. 

Figure 8.4 The wedge-and-dash representation of the groups around a chiral carbon indicates where each group points in space. The Fischer projection is used to orient a carbohydrate molecule such as a sugar. 

Convert the following representation of ethane, C2H6/ into a conventional drawing that uses solid, wedged, and dashed lines to indicate tetrahedral geometry around each carbon (gray = C, ivory = H). 

Draw a three-dimensional representation of the oxygen-bearing carbon atom in ethanol, CH3CH2OH, using the standard convention of solid, wedged, and dashed lines. 

Draw a three-dimensional representation (using wedges and dashed lines) of the structure. 

Draw two different three-dimensional representations for CH2CI2 (dichloromethane) using wedges and dashes. 

Convert the Fischer projection formula to a representation with wedges and dashes. 

Enantiomers can be drawn using wedges and dashes to show the tetrahedral geometry or by using Fischer projections in which the tetrahedral nature is assumed. In both representations, horizontal bonds are coming out of the paper and vertical bonds are behind the paper. 

The ability to determine if a molecule and its mirror image are superimposable is key for recognition of enantiomers. Because a model is not always available, the way the molecule is drawn on paper is important. This book uses line notation with wedges and dashes to indicate stereochemistry (see Chapter 1, Section 1.1, and Chapter 4, Section 4.1), as with 4A. Occasionally, other representations are useful. Emil Fischer (Germany 1825-1919 Nobel laureate, 1902) was a remarkable chemist whose work influences chemistry even today. His name is used for a particular method for drawing stereoisomers. 

Based on your model, draw a bond-line representation with as many atoms as possible in the plane of the paper. Use wedge and dash bonds to represent any atoms that do not lie in the plane of the paper. 

Draw wedge and dash skeletal representations of cis and trans 1,3-dimethylcyclopentane. 

Use wedge-and-dash bonds to draw a 3-D representation of ammonia. 

Figure 1. The structure of strychnine. A strychnine molecule shown (A) in line form and (B) as a space-filling representation. In this line diagram and others in this chapter, wedged bonds project above the plane of the paper and dashed bonds project into it.

Draw a three-dimensional perspective representation for the compound showing approximate bond angles as clearly as possible. Use ordinary lines to indicate bonds in the plane of the paper, solid wedges for bonds in front of the pap>er, and dashed wedges for bonds behind the paper. 

The difficulty of representing the real, dynamic, three-dimensional structure of ethane should be more apparent to you now. In the real world, there is not often the time to draw out carefully a good representation of even as simple a molecule as ethane. The solid and dashed wedges of Rgure 2.25 are the traditional attempts at adding a three-dimensional feel to the two-dimensional drawing. More complicated molecules can raise horrendous problems. Adequate codes are needed, and you must learn to see past the coded structures to the real molecules both easily and quickly. It s worth considering here, at this very early point, some of the pitfalls of the various schemes. 

All representations have a carbon with two bonds in the plane of the page, one in front of the page (solid wedge) and one behind the page (dashed line). Four possibilities ... 

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