7-1 Reactions of Alkenes Unit 7

7-2 Characteristic Reactions

7-3

7-4 Reaction Mechanisms  A reaction mechanism describes how a reaction occurs which bonds are broken and which new ones are formed the order and relative rates of the various bond- breaking and bond-forming steps if in solution, the role of the solvent if there is a catalyst, the role of a catalyst the position of all atoms and energy of the entire system during the reaction

7-5 Developing a Reaction Mechanism  How it is done design experiments to reveal details of a particular chemical reaction propose a set or sets of steps that might account for the overall transformation a mechanism becomes established when it is shown to be consistent with every test that can be devised this does mean that the mechanism is correct, only that it is the best explanation we are able to devise

7-6 Why Mechanisms? they are the framework within which to organize descriptive chemistry they provide an intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems they are tools with which to search for new information and new understanding

7-7 Electrophilic Additions hydrohalogenation using HCl, HBr, HI hydration using H 2 O in the presence of H 2 SO 4 halogenation using Cl 2, Br 2 halohydrination using HOCl, HOBr oxymercuration using Hg(OAc) 2, H 2 O followed by reduction

7-8 Addition of HX  Carried out with pure reagents or in a polar solvent such as acetic acid or  It is a two-step mechanism: The first step is the slow, rate-determining step. The second step is fast.

7-9 Addition of HX  Addition is regioselective regioselective reaction:regioselective reaction: an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed Markovnikov’s rule:Markovnikov’s rule: in the addition of HX, H 2 O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens

7-10 Addition of H 2 O addition of water is called hydration acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond HOH adds in accordance with Markovnikov’s rule

7-11 Addition of Cl 2 and Br 2 carried out with either the pure reagents or in an inert solvent such as CH 2 Cl 2 addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane anti stereoselectivityaddition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond we will discuss this selectivity in detail in Section 6.7

7-12 Addition of HOCl and HOBr  Treatment of an alkene with Br 2 or Cl 2 in water forms a halohydrin  Halohydrin:  Halohydrin: a compound containing -OH and -X on adjacent carbons

7-13 Oxymercuration/Reduction  Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond oxymercuration reduction

7-14 Hydroboration/Oxidation  Hydroboration:  Hydroboration: the addition of borane, BH 3, to an alkene to form a trialkylborane  Borane dimerizes to diborane, B 2 H 6 Borane HB H H 3CH 2 =CH 2 CH 3 CH 2 B CH 2 CH 3 CH 2 CH 3 Triethylborane (a trialkylborane) + BoraneDiborane 2BH 3 B 2 H 6

7-15 Oxidation/Reduction  Oxidation:  Oxidation: the loss of electrons alternatively, the loss of H, the gain of O, or both  Reduction:  Reduction: the gain of electrons alternatively, the gain of H, the loss of O, or both  Recognize using a balanced half-reaction 1. write a half-reaction showing one reactant and its product(s) 2. complete a material balance; use H 2 O and H + in acid solution, use H 2 O and OH - in basic solution 3. complete a charge balance using electrons, e -

7-16 Oxidation/Reduction three balanced half-reactions

7-17 Reduction of Alkenes  Most alkenes react with H 2 in the presence of a transition metal catalyst to give alkanes commonly used catalysts are Pt, Pd, Ru, and Ni catalytic reduction catalytic hydrogenationthe process is called catalytic reduction or, alternatively, catalytic hydrogenation + H 2 Pd CyclohexeneCyclohexane 25°C, 3 atm