Wedge/hash bonds

The stereochemistry is usually expressed in structure diagrams by wedged and hashed bonds. A wedge indicates that the substituent is in ont of a reference plane and a hashed bond indicates that the substituent is pointing away om the viewer (behind the reference plane). This projection is applied both to tetrahe-... 

A molecule editor can draw a chemical structure and save it, for example as a Molfile. Although it is possible to include stereochemical properties in the drawing as wedges and hashed bonds, or even to assign a stereocenter/stereogroup with its identifiers R/S or E/Z), the connection table of the Molfile only represents the constitution (topology) of the molecule. 

This is often the trickiest part. Just remember that you are trying to make each of the carbon atoms look tetrahedral. (Note that we don t normally use wedged and hashed bonds otherwise things get really messy.)... 

There are several ways of doing this. In structure 2, the bold, wedged bond suggests a perspective view of a bond coming towards you, while the hashed bond suggests a bond fading away from you. The other two normal bonds are in the plane of the paper. 

Change the wedge/hash appearance of stereo bonds. 

The creation of a graphical representation of a tetrahedral stereocenter poses a choice as to which substituents should receive the wedge or hash bond. Shelley found that the same atom prioritization used to avert overlap served well. When the stereo bond is assigned to the lowest priority substituent, the stereo bond tends to be directed away from rings and toward hydrogens, as expected. 

Epoxides are excellent electrophiles and substrates for 5 2 reactions the release of strain in the three-membered ring makes them particularly reactive. They react with all the nucleophiles we have discussed, with reliable inversion of stereochemistry (Figure 9.74). Note that the stereochemistry in the products is relative, not absolute— since the starting material is not chiral, racemic mixtures would be obtained (hence, the plain rather than the tapered wedge and hash bonds). The one unfamiliar example here is the reaction with lithium aluminum deuteride, LilAlDJ. For the present, think of this as essentially a deuteride anion (or hydride from lithium aluminum hydride), but this reactive molecule will quickly become a good friend to you in reductions and devising syntheses. 

The addition of halogens to alkenes presents some unusual observations. The addition of bromine to most alkenes is trans and stereospecific (Figure 11.12 the product is, however, racemic, hence the straight wedge and hash bonds), but additions of the other halogens give rise to mixtures of products from syn and a tf-addition. How and why does this tra s-addition occur It must certainly be a stepwise reaction, as any synchronous process would give rise to ds-addition. 

The plain lines represent bonds that lie in the plane of the paper. The full wedge represents a bond coming out of the paper and the hashed wedge indicates a bond pointing behind the paper. 

Chemists often want to draw these two conformations quickly and two different methods are commonly used, each with its own merits. In the first method, we simply draw the side view of the molecule and use wedged and hashed lines to show bonds not in the plane of the paper (as you saw in Chapter 16). Particular attention must be paid to which of the bonds are in the plane and which go into and out of the plane. 

The first is in conversion of CTs which represent 2-D pictures with chirality indicated by hash/wedge bonds (bond.inout). In these cases, a Z co-ordinate is artifically added, and this representation is turned into a 3-D representation for conversion to local chirality. 

In Summary Determination of organic structures relies on the use of several experimental techniques, including elemental analysis and various forms of spectroscopy. Molecular models are useful aids for the visualization of the spatial arrangements of the atoms in structures. Condensed and bond-line notations are useful shorthand approaches to drawing two-dimensional representations of molecules, whereas hashed-wedged fine formulas provide a means of depicting the atoms and bonds in three dimensions. 

Condensed and bond-line formulas are abbreviated representations of molecules. Hashed-wedged line drawings illustrate molecular structures in three dimensions. 

A Fischer (Real Life 5-2) projection is a simphfied way of depicting tetrahedral carbon atoms and their substituents in two dimensions. With this method, the molecule is drawn in the form of a cross, the central carbon being at the point of intersection. The horizontal lines signify bonds directed toward the viewer the vertical lines are pointing away. Hashed-wedged line structures have to be arranged in this way to facilitate their conversion into Fischer projections. 

Draw the hashed-wedged line structures corresponding to Fischer projections A and B, above. Is it possible to transform A into B by means of a rotation about a single bond If so, identify the bond and the degree of rotation required. Use models if necessary. 

A molecular model can help you see that the aU-ecUpsed form actually possesses a roughly circular shape, as illustrated by the scrolled rendition above (center picture). In the subsequent all-eclipsed hashed-wedged line structure, notice that the groups on the right of the carbon chain in the original Fischer projection now project upward (wedged bonds). From this conformer, we can reach the aU-staggered form by 180° rotations of C3 and C5. 

---