chapter_1 - Chapter 1 Valence Bond Theory 1.1 The Covalent...

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1 Chapter 1 Valence Bond Theory 1.1 The Covalent Bond Organic compounds are covalent compounds; and with only a very few exceptions, all the bonds in organic compounds are covalent bonds. The emphasis in our picture of the covalent bond is on the sharing of electron pairs between two neutral atoms. When two atoms share one pair of electrons, they are attached to each other by a single bond. When two atoms share two pairs of electrons, they are attached to each other by a double bond, and when they share three pairs of electrons by a triple bond. Single bonds are by far the most common type of covalent bond. The sharing of electrons in a covalent bond by two atoms is energetically favorable under ordinary conditions on the Earth. Energy is released when a bond forms; energy is required to break a bond. This simple statement is a very important general principle in organic chemistry. Stated another way, bond making is good, bond breaking is bad . Some very basic aspects of our picture of the covalent bond are important to keep in mind. 1. The shared electron pair lies between the two bonded nuclei because the two electrons can be attracted by both nuclei and can offset the repulsion between the nuclei themselves. 2. The two electrons of an electron pair in a covalent bond have opposite spins. This observation is a consequence of the Pauli Exclusion Principle, which states that no two electrons can have the same four quantum numbers.
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2 3. Two atoms that form a covalent bond keep approaching each other until the energy of the system reaches a minimum. If they get any closer the energy increases because of repulsions between the nuclei. So there is an optimum distance between the two nuclei that are bonded. This distance is called the bond length. 1.2 Lewis Structures Perhaps the simplest and most frequently used representations of the structures of organic molecules # are Lewis structures, usually modified or simplified for convenience. As we shall see it is especially easy to simplify Lewis structures in organic chemistry because we are dealing with relatively few and relatively simple elements. A correctly drawn Lewis structure indicates how all the atoms of a molecule are attached to one another and accounts for all the valence electrons of all the atoms in the molecule. The valence electrons of an atom of a nonmetal are those of highest principal quantum number We can formulate a set of rules for drawing Lewis structures and then introduce some simplifications. 1. Only valence electrons are shown in a Lewis structure. In the elements we will encounter the number of valence electrons is equal to the group number, which is the number of the element’s column in the periodic table. These numbers are summarized in the table below and should be learned. Element
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This note was uploaded on 11/17/2011 for the course ORGANIC CH 307 taught by Professor Boikes during the Fall '09 term at Rutgers.

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chapter_1 - Chapter 1 Valence Bond Theory 1.1 The Covalent...

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