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

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chap.9 notes

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20
School:
University of Texas at Arlington
Course:
Chem 2321 - Organic Chemistry
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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 the position that is already bearing a large number of hydrogen atoms Similarly one can say that the halogen is generally placed at the more substituted position this regiochemical preference is called Markovnikov addition Regioselective reactions that proceed with a regiochemical preference There are conditions when the halogen is installed in the less substituted carbon and this is called the Anti Markovnikov addition This only occurs if the reagent used is impure and it was further noticed that even the identity of the impurity had played a part such as peroxides Trace amounts of peroxides would cause the halogen to across an alkene in an anti Markovnikov fashion A mechanism for Hydrohalogenation 1 The first step pi bond of the alkene is protonated generating a carbocation this is the rate determining step since it has a higher transition state energy than the second step 2 Second step intermediate is attacked by a halogen ion Protonation can occur with either resulting in a less substituted secondary carbocation or a more substituted carbocation Recall the Hammond postulate suggests that each of their transition states has significant carbocationic character Hence the transition state of the tertiary carbocation will be lower than the transition state for formation of secondary carbocation Similarly the energy barrier for the formation of the tertiary carbocation will be smaller than that of the secondary carbocation Finally the reaction will proceed via the most stable intermediate Regioselectivity of an ionic addition reaction is determined by the preference for the reaction to proceed through the more stable carbocation intermediate Stereochemistry of Hydrohalogenation Hydrohalogenation involves the formation of a chirality center such as The two enantiomers are produced in equal amounts racemic mixture since the carbocation consists of an empty p orbital both faces of the plane can be attacked with equal likelihood hence the racemic mixture Hydrohalogenation with Carbocation Rearrangement Halogen additions are subjected to carbocation rearrangements Recall from chap 6 The entire molecule can undergo a carbocation rearrangement before the halogen attacks but keep in mind that it is also possible for the halogen to attack the molecule before the rearrangement Hence a mixture of products is UNAVOIDABLE Bottom line when carbocation rearrangement CAN occur they do occur Hydration addition of water across a double bond There are three methods two of which are under Markovnikov addition and the third is via Anti Markovnikov addition 1 Acid catalyzed hydration 2 Oxymercuration Demercuration 3 Hydroboration Oxidation Acid catalyzed Hydration Acid Catalyzed hydration addition of water across a double bond in the presence of an acid This reaction proceeds via Markovnikov addition and it is an addition of H and OH across pi bonds with the OH group positioned at the more substituted carbon This example represents the presence of water and sulfuric acid in which the sulfuric acid is not consumed as it is a catalyst It is also represented by the reagent H 3O The rate of an acid catalyzed hydration is very much dependent on the structure of the starting alkene With additional alkyl group the reaction rate increases by many orders of magnitude Also if you notice the OH is place at the more substituted position Mechanism and source of Regioselectivity 1 the alkene is protonated to generate a carbocation 2 Nucleophilic attack the attacking nucleophile is neutral H 2O therefore a charged intermediate is formed called an oxonium ion since it bears an oxygen atom with a positive charge 3 Proton transfer in order to form an electrically neutral product The base used to deprotonate the oxonium ion is H2O since in acidic conditions the concentration of hydroxide ions is extremely low but the concentration of H 2O is quite large If you pay attention to the double arrows you will notice that this reaction goes both ways and has an actual equilibrium point The reverse reaction is called Acid catalyzed dehydration where the molecules undergoes an E1 elimination process Recall Le Ch telier s Principle which states that a system at equilibrium will adjust in order to minimize any stress placed on the system The equilibrium can be easily manipulated with the factor of concentration of water By controlling the amount of water that is present one can control which side the equilibrium will favor As for the stereochemical outcome of an Acid


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