2 - Chapter 2 The structure of organic molecules The...

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1 Chapter 2 – The structure of organic molecules The ability to analyze chemical reactions always begins with a description of the molecular structure of each reactant and product. With the structure in hand, one can generate a list of bonding changes that take place during the transformation. The most valuable model for this purpose is one that tracks the changes in the valence electrons, since only valence electrons participate in covalent bonding. The electron configuration model – or Lewis Dot Model – provides a representation of the arrangement of valence electrons in a molecule. This representation corresponds to a simple, graphical notation of chemical bonding, and is exceedingly useful for analyzing chemical reactions involving organic molecules. Shared pairs of electrons (i.e., covalent bonds) are represented as lines connecting two atoms; a localized, non-bonded pair of electrons (i.e., an electron lone pair) is represented as a pair of dots next to an atom. Generating error-free Lewis structures is an essential skill for organic chemistry. There are many ways to accomplish the rapid and accurate generation of structures, and with practice and experience, you will undoubtedly develop an approach that works best for you. A formal, step-by-step approach to generating Lewis structures is described below. This process will prove useful for starters, but may or may not be useful to you in the long run. A more practical approach is based on the concept of electron pair domains, or EPDs. The EPDs are building blocks that can be pieced together to construct Lewis structures. Handout 03 will cover the topic of EPDs in great detail.
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2 A stepwise approach to drawing Lewis structures Draw the Lewis structure of CH 3 NHOH. Begin by drawing the molecular skeleton according to the clues given in the formula. This is a trial structure, and as the name implies, it is only a guess. H C H H N H O H A trial structure based on the molecular formula. Next we need to count the total number of valence electrons that are present in CH 3 NHOH. Grouping like atoms, we re-write the molecular formula of CH 3 NHOH as CH 5 NO. The chart below, comprised of select sections of the periodic table, reminds us about the number of valence electrons each different atom type contributes. This chart should be memorized. H Li Na B Al C Si N P F Cl Br I 1 3 45 6 7 S O A portion of the periodic table depicting the number of valence electrons associated with each atom. Next we use the chart above to determine the number of valence electrons contributed by a compound with the molecular formula of CH 5 NO. An example of this calculation is illustrated by the table below. Because the sum of valence electrons contributed by each individual atom of our molecular formula totals 20, the final Lewis structure that we draw must depict a total of exactly 20 valence electrons in order to be valid.
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2 - Chapter 2 The structure of organic molecules The...

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