UT Arlington CHEM 2321 - chap.9 notes (20 pages)

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chap.9 notes

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Pages:
20
School:
University of Texas at Arlington
Course:
Chem 2321 - Organic Chemistry
Organic Chemistry Documents

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Chap 9 Notes Intro to Addition Reactions Addition reactions characterized by the addition of two groups across a double bond in which pi bonds are broken Special names that indicate the identity of the two groups that were added Alkenes are usually the ones associated with addition reactions enabling them to serve as synthetic precursors for a wide variety of functional groups As shown pi bonds can readily be protonated and can attack electrophile centers Addition Vs Elimination A Thermodynamic perspective Both the reactions are temperature dependent where addition is favored under low temperature and elimination is favored under high temperatures Recall G determines whether the equilibrium favors reactants or products G must be negative for the equilibrium to favor products When it comes to the sign and magnitude of H many factors come into play in terms of affecting the sign and magnitude however in this chapter it is mainly bond strength In addition reactions one pi bond and a sigma bond is broken in order to form two sigma bonds Since sigma bonds are stronger than that of pi bonds the bonds broken will always be of less energy than that of bonds formed In other words Addition reactions are always exothermic with H being negative An example would be Aside from the enthalpy terms the entropy terms T S will always be positive due to the fact that 2 molecules are joining into to one molecule resulting in low entropy and as for temperature it is always positive Regioselectivity of Hydrohalogenation The treatment of alkenes with HX where X Cl Br or I results in an addition reaction called Hydrohalogenation In symmetrical alkenes it is easy to determine where to place the halogen however in cases where the alkene is non symmetrical the ultimate placement of the halogen must be considered Example of a symmetrical molecule Example of an Asymmetrical molecule Due to the issue of regiochemistry A Russian chemist Vladimir Markovnikov noticed that H is generally placed at



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