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zumdahl_chemprin_6e_csm_ch21 - CHAPTER 21 ORGANIC CHEMISTRY...

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773 CHAPTER 21 ORGANIC CHEMISTRY Hydrocarbons 1. A hydrocarbon is a compound composed of only carbon and hydrogen. A saturated hydro- carbon has only carbon-carbon single bonds in the molecule. An unsaturated hydrocarbon has one or more carbon-carbon multiple bonds but may also contain carbon-carbon single bonds. A normal hydrocarbon has one chain of consecutively bonded carbon atoms. A branched hydrocarbon has at least one carbon atom not bonded to the end carbon of a chain of consecutively bonded carbon atoms. Instead, at least one carbon atom forms a bond to an inner carbon atom in the chain of consecutively bonded carbon atoms. 2. To determine the number of hydrogens bonded to the carbons in cyclic alkanes (or any alkane where they may have been omitted), just remember that each carbon has four bonds. In cycloalkanes, only the C C bonds are shown. It is assumed you know that the remaining bonds on each carbon are C H bonds. The number of C H bonds is that number required to give the carbon four total bonds. 3. In order to form, cyclopropane and cyclobutane are forced to form bond angles much smaller than the preferred 109.5 ° bond angles. Cyclopropane and cyclobutane easily react in order to obtain the preferred 109.5 ° bond angles. 4. Aromatic hydrocarbons are a special class of unsaturated hydrocarbons based on the benzene ring. Benzene has the formula C 6 H 6 . It is a planar molecule (all atoms are in the same plane). Each carbon in benzene is attached to three other atoms; it exhibits trigonal planar geometry with 120° bond angles. Each carbon is sp 2 hybridized. The sp 2 hybrid orbitals go to form the three sigma bonds to each carbon. The unhybridized p atomic orbitals on each carbon overlap side to side with unhybridized p orbitals on adjacent carbons to form the π bonds. All six of the carbons in the six-membered ring have one unhybridized p atomic orbital. All six of the unhybridized p orbitals ovlerlap side to side to give a ring of electron density above and below the planar six-membered ring of benzene. The six π electrons in the π bonds in benzene can roam about above and below the entire ring surface; these π electrons are delocalized. This is important because all six carbon-carbon bonds in benzene are equivalent in length and strength. The Lewis structures say something different (three of the bonds are single, and three of the bonds are double). This is not correct. To explain the equivalent bonds, the π bonds can’t be situated between two carbon atoms, as is the case in simple alkenes and alkynes; that is, the π bonds can’t be localized. Instead, the
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CHAPTER 21 ORGANIC CHEMISTRY 774 six π electrons can roam about over a much larger area; they are delocalized over the entire surface of the molecule.
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