MODEL EXERCISE 3.1 The molecule CH2BrCl

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MODEL EXERCISE 3.1 The molecule CH2BrCl Powered By Docstoc
					Molecular shape

It should be stressed that the representations of dichloromethane shown in
Structures 3.4 and 3.5, and the one shown in Figure 3.1, all show exactly the
same molecule with exactly the same bond lengths and bond angles; they simply
represent three different views of the same molecule.

    MODEL EXERCISE 3.1 The molecule CH2BrCl
    Make a model of bromochloromethane, CH2BrCl, and use this to help you to
    draw flying-wedge representations of the molecule from a number of different
    viewpoints. You will probably be surprised at the number of different represen-
    tations that can be made of the one molecule!

Of course, it is not necessary to draw 18 (or more) representations of the one mol-
ecule. Each figure contains exactly the same information, so one figure is sufficient
to define the molecule. It is important to be able to draw the figures from a number
of views, however, because with more complex molecules one needs to be able to
choose a view that conveys the structural information with the greatest clarity.

    Now try to draw a flying-wedge representation of the two conformations of the
    ethane molecule shown in Figure 3.2.

                                                                                        Figure 3.2
                                                                                        The two models of ethane used in
                                                                                        Model Exercise 2.1 (in model (a),
                                                                                        corresponding C\H bonds attached
                                                                                        to the front and back carbon atoms
      (a)                                            (b)                                are in the same plane, as exemplified
                                                                                        by the yellow-shaded area).

    The closest representations are shown in Structures 3.12 and 3.13.

             H          H             H          HH

            H C     C H            H C       C
             H       H              H            H
                 3.12                     3.13

The form shown in Structure 3.14 is sometimes described as the ‘sawhorse’
conformation (although it has only a passing resemblance to this carpenter’s tool,                 H
even when drawn without atom symbols, as in Structure 3.15). The different                  H
representations 3.12–3.15 show clearly that even in two dimensions we can readily                  C
distinguish different conformations. With a little practice you will find that you        C H          H
can easily draw, and interpret, these flying-wedge representations. You will also       H   H
discover that they are a great help in understanding organic chemistry in three                 3.14               3.15

                                                                                                 The representation of molecules

       Drawing and manipulating computer models
       In this Computer Activity you will use ISIS/Draw and WebLab ViewerLite
       to reproduce the two different conformations corresponding to the models of
       ethane shown in Figure 3.2, and Structures 3.12 and 3.13.
       The Activity in Exploring the third dimension (on one of the CD-ROMs) should
       take you approximately 20 minutes to complete.

Another way of representing conformations is the Newman projection. These have
a more restricted use, but they are an invaluable complement to the flying-wedge
notation. A Newman projection shows a view of a molecule looking along the line
of one bond from the left, for example the C\C bond in ethane. The nearer carbon                          nearer carbon atom
                                                                                                        in Newman projection
atom of that bond is represented by a point, and the bonds to the three groups
attached to it by straight lines at 120° to each other (Structure 3.16). The further                              3.16
atom of the C\C bond is shown as a circle, with the bonds to its three attached
groups as lines to the circumference (Structure 3.17). The two are then superim-
posed to give the Newman projection.
Newman projections and flying-wedge representations of the two conformations of                          further carbon atom
ethane in Figure 3.2 are shown in Figure 3.3.                                                           in Newman projection

                                       HH                                                H
               H      H                                 H          HH                H       H
            H C       C H     ≡                     H C        C             ≡
             H         H           H          H      H             H                 H       H
                                             H                                           H
 (a)               3.12                           (b)       3.13

Figure 3.3 Flying-wedge representations, and the corresponding Newman projections, of
the ethane molecule in the conformations shown in Figure 3.2a and b.

It is not so easy to draw the Newman projection in Figure 3.3a, in which the hydro-
gen atoms of one methyl group are ‘hiding behind’ the hydrogen atoms of the other
methyl group, as viewed along the C\C bond (see also Figure 3.2a). This is called
the eclipsed conformation. The conformation that shows the hydrogen atoms
attached to the further carbon atom positioned exactly between the hydrogen atoms
attached to the nearer carbon atom (as in Figures 3.2b and 3.3b), is called the
staggered conformation.

       QUESTION 3.1
                                                                                                             Cl          H Cl
       Which of the following Newman projections (a)–(c) represents the identical
       conformation to the flying-wedge representation of 1,2-dichloroethane in                           H C        C
       Structure 3.18?                                                                                     H             H
                 Cl                   Cl                       Cl
                                                                                                       3.18 1,2-dichloroethane
           H          H           H         H             H         Cl

           H              H       H          Cl              H                   H
                Cl                     H                               H
                (a)                    (b)                             (c)