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Chemistry 0310 - Organic Chemistry 1 Chapter 3

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Chemistry 0310 - Organic Chemistry 1 Chapter 3 Dr. Peter Wipf Chemistry 0310 - Organic Chemistry 1 Chapter 3. Reactions of Alkanes The heterolysis of covalent bonds yields anions and cations, whereas the homolysis creates radicals. Radicals are species with unpaired electrons that react mostly as electrophiles, seeking a single electron to complete their octet. Free radicals are important reaction intermediates and are formed in initiation reactions under conditions that cause the homolytic cleavage of bonds. In propagation steps, radicals abstract hydrogen or halogen atoms to create new radicals. Combinations of radicals are rare due to the low concentration of these reactive intermediates and result in termination of the radical chain. !CHAIN REACTION SUMMARY reactant product initiation PhCH3 HCl Cl 2 h DH = -16 kcal/mol chain-carrying intermediates n o r D (low concentrations) PhCH2 . Cl . propagation PhCH2 . or Cl . PhCH . 2 DH = -15 kcal/mol or Cl . PhCH2Cl or PhCH CH Ph PhCH2Cl Cl2 2 2 PhCH2Cl or Cl2 termination product reactant termination Alkanes are converted to alkyl halides by free radical halogenation reactions. The relative stability of radicals is increased by conjugation and hyperconjugation: R H H H . CH2 > R C . > R C . > H C . > H C . R R R H Oxygen is a diradical. In the presence of free-radical initiators such as metal salts, organic compounds and oxygen react to give hydroperoxides. These autoxidation reactions are responsible for the degradation reactions of oils, fatty acids, and other biological substances when exposed to air. Antioxidants such as hindered phenols are important food additives. Vitamins E and C are biological antioxidants. Radical chain reactions of chlorinated fluorocarbons in the stratosphere are responsible for the "ozone hole". The Hammond postulate states that the structure of the transition state of organic reactions is related to the ground state (starting material or product) that is closest in energy. A catalyst lowers the energy of the transition state and thus speeds up the establishment of an equilibrium. Some nomenclature rules: The underlying principle for nomenclature is that each different compound should have a different name. The rules for nomenclature are organized by the International Union of Pure and Applied Chemistry (IUPAC). The following list summarizes the rules for the nomenclature of alkanes: 1. The longest contiguous chain of carbon atoms determines the parent name of the compound. 2. All substituents that branch off the main chain are named and numbered so that the lowest possible numbers result. 3. Prefixes di-, tri-, tetra-, penta-, etc. are used for substituents of the same kind. 4. In naming the compound, all compounds are listed in alphabetical order, ignoring all prefixes. 5. Numbers of substituents that are grouped together are separated by commas. The name of the last substituent is merged with the name of the straight chain alkane that forms the parent name of the compound. In bicycloalkanes, we name the compound according to the total number of carbons encompassed by the two rings. Numbering starts at one of the bridgehead carbons and proceeds along the longest bridge to the other bridgehead, then along the next longest bridge back to the fist bridgehead. The shortest bridge is numbered last. Interposed in the name in brackets is an expression that denotes the number of carbon atoms in each bridge, in order of decreasing length and separated by periods. The systematic names of alkyl halides and alcohols are assigned in the same way. The prefixes fluoro-, chloro-, bromo-, and iodo- are used to indicate the presence of halogens. Alcohols are named by changing the e ending of the name of the alkane to ol and using a number to indicate the position of the hydroxyl group. In naming an alcohol, the carbon chain is numbered so that the carbon atom bearing the hydroxyl group has the lowest possible number. Examples: OH Cl 4-chloro-1-butanol 2-methylbicyclo[2.2.1]heptane 5-sec-butyl-2,7-dimethylnonane.
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