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Organic Chemistry 1


 
 

Drawing Organic Molecules

  1. Lewis structure: A molecular representation that depicts covalent bonds and nonbonding valence electrons.

    1. Resonance structures: Some Lewis structures have multiple, distinct, but equivalent ways of arranging multiple bonds and electrons, while still obeying the Octet Rule and maintaining the connectivity of atoms in the molecule. Each equivalent arrangement is called a resonance structure.

      • In reality, molecules exist as the average of all their different resonance structures. Individual resonance structures cannot be isolated because electrons in a molecule are actually delocalized and multiple bond characteristics can be spread out over the entire molecule.

    2. Formal charge: The formal charge on an atom equals the number of valence electrons in the unbonded atom minus both the number of lone-pair electrons the atom has and the number of covalent bonds to the atom.

      • A nonzero formal charge indicates that an atom has a different number of valence electrons than it would if it were unbonded and neutral.

      • The sum of the individual formal charges in a molecule equals the overall charge of the molecule; formal charges give a sense of where charge is localized within a molecule.

      • Formal charges on the atoms in carbon monoxide. The net charge on the molecule is –1 + 1 = 0.

        Lewis structure. Dots represent electrons and lines represent covalent bonds.

        Resonance structures of nitrate ion and benzene. The actual molecules are an average of their individual resonance structures.

  2. Condensed structure: Shorthand method of representing organic molecules in which some or all bonds are not explicitly drawn.

  3. Skeletal structure: Representation of an organic molecule in which:

    • carbon atoms and hydrogen atoms bonded to carbon are not explicitly drawn.

    • carbon-carbon bonds are represented by lines, with carbon atoms assumed to be present at the intersection of any two lines and at the ends of lines.

    • any carbon not explicitly making four covalent bonds is implicitly bonded to however many hydrogen atoms are necessary to give that carbon four bonds.

  4. Sawhorse structure: Shows the three-dimensional arrangement of atoms in a molecule. Dotted wedges indicate bonds pointing into the plane of the paper; solid wedges indicate bonds rising from the plane of the paper; and solid lines indicate bonds that are in the plane of the paper.