Symbolism in drawing structure

For drawings in which we are trying to present a perspective of control concepts (configurations), we use very simplified symbols. In drawings where we are trying to illustrate concepts of structure (overrides, feedforward, etc.), we use a more detailed symbolism that we have found useful in our design work. 

In the previous chapter, the structures of many molecules and ions were described by drawing structures showing how the electrons are distributed. However, there is another way in which the structures of molecules are described. That way uses different language and symbols to convey information about the structures in an efficient, unambiguous way. In this way, the structures of molecules and ions are described in terms of their symmetry. Symmetry has to do with the spatial arrangement of objects and the ways in which they are interrelated. For example, the letter T" has a plane that bisects it along the "post," giving two halves that are identical in relationship to that plane. However, the letter "R" does not have such a plane that divides it into two identical parts. This simple example illustrates a symmetry characteristic that is known as a plane of symmetry. There is much more that can be done with symmetry in terms of molecular structure so this chapter is devoted to this important topic. 

Draw structures for the folllowing molecules, and then show them again using at least one organic element symbol in each. 

Using the four symbols shown to represent four different amino acids, draw structures four possible peptides for a four-member chain that can be made by linking them together in different orders. 

Single, double, triple, and aromatic bonds are represented by the symbols —, =,, and However, the bond type may be omitted when suggestive, and sp2-hybridized bonded atoms can be written in lower case letters. So C=C—C=C means 1,3-butadiene, which is equivalent to cccc. Branches are indicated by parentheses, e.g., CC(C)C(=0)0 is isobutyric acid. To specify rings, the atom that closes the ring is numbered and specified, as ClCCCCCl for cyclohexane and clcc2ccccc2ccl for naphthalene. The SMILES notation can be used in chemical structure drawing programs for quick input of a structure by the skilled user. 

The first thing that coach would draw would be a little symbol for each of the players in the game. Similar to our coach s Xs and Os, most electrons in Lewis structures are drawn with an x or a dot ( ). In this book, I use dots. 

A final note about nomenclature is in order for benzene. Drawing a benzene ring occupies a lot of space, so a shorthand representation is used in many structures. In 96, the shorthand symbol Ph is used to represent a phenyl substituent. In the older chemical literature, the Greek symbol ( ) (phi) was sometimes used. The Ph representation will be used often in this book, and compound 96 is named 5-chloro-2,6-diphenyloct-2-ene. 

Organic chemists have found a way to draw complex molecular structures in a very simple way, by not showing the C and H atoms explicitly. A line structure represents a chain of carbon atoms by a zigzag line, where each short line indicates a bond and the end of each line represents a carbon atom. Atoms other than C and H are shown by their symbols. Double bonds are represented by a double line and triple bonds by a triple line. Because carbon almost always forms four bonds in organic compounds, there is no need to show the C—FI bonds explicitly. We just fill in the correct number of hydrogen atoms mentally compare the line structure of 2-chlorobutane, QT3C1TC1CF12C]T3 (3a), with its structural form (3b). Line... 

The three representations that are referred to in this study are (1) macroscopic representations that describe the bulk observable properties of matter, for example, heat energy, pH and colour changes, and the formation of gases and precipitates, (2) submicroscopic (or molecular) representations that provide explanations at the particulate level in which matter is described as being composed of atoms, molecules and ions, and (3) symbolic (or iconic) representations that involve the use of chemical symbols, formulas and equations, as well as molecular structure drawings, models and computer simulations that symbolise matter (Andersson, 1986 Boo, 1998 Johnstone, 1991, 1993 Nakhleh Krajcik, 1994 Treagust Chittleborough, 2001). 

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