Organic Chemistry 1 Exam 3 Chapter 6 Addition Reactions of Alkenes 6 1 Hydrogenation of Alkenes Hydrogenation the addition of H2 Hydrogenation of ethane H is the heat of hydrogenation and for all alkenes hydrogenation is o exothermic or H Metal catalyzed addition of hydrogen is normally rapid at room temperature and the alkane is produced in high yield usually as the only product 6 2 Heats of Hydrogenation The pattern for alkene stability determined from heats of hydrogenation parallels exactly the pattern deduced from heats of combustion Hydrogenation of Alkenes Step Process o Step 1 Hydrogen molecule react with metal atoms at the catalyst surface The relatively strong hydrogen hydrogen bond is broken and replaced by two weak metal hydrogen bonds o Step 2 The alkene reacts with the metal catalyst The component of the double bond between the two carbons is replaced by two relatively weak carbon metal bonds o Step 3 A hydrogen atom is transferred from the catalyst surface to one of the carbons of the double bond o Step 4 The second hydrogen atom is transferred forming an alkane The sites on the catalyst surface at which the reaction occurred are free to accept additional hydrogen and alkene molecules 6 3 Stereochemistry of Alkene Hydrogenation Although two hydrogens are not transferred simultaneously they both add to the same face of the double bond Syn addition when two atoms or groups add to the same face of a double bond Anti addition when atoms or groups add to opposite faces of the double bond o Stereoselectivity o Alkene hydrogenation can give us several products from one reactant o Reactions such as a catalytic hydrogenation that takes pace at the less hindered side of a reactant are examples of steric effects on reactivity 6 4 Electrophilic Addition of Hydrogen Halides to Alkenes Increasing reactivity of hydrogen halides in addition to alkenes o HF HCl HBr HI Electrophilic Addition of Hydrogen Halide to an Alkene Step Process o Step 1 Protonation of the carbon carbon double bond by the hydrogen halide o Step 2 Carbocation anion combination Electrophilic addition triggered by the acid acting as an electrophile toward the electrons of the double bond The characteristic chemical property of a C C structural unit is susceptibility 6 5 Regioselectivity of Hydrogen Halide Addition Markovnikov s Rule to attack by electrophiles Markovnikov s rule o When an unsymmetrically substituted alkene reacts with a hydrogen halide the hydrogen adds to the carbon that has the greater number of hydrogens and the halogen adds to the carbon having fewer hydrogens o Hydrogens want more hydrogens o Halogens want less hydrogens 6 6 Mechanistic Basis for Markovnikov s Rule According to Hammond s postulate the transition state for protonation of the double bond has much of the character of a carbocation and the activation energy for formation of the more stable carbocation secondary is less than that for formation of the less stable one primary 6 7 Carbocation Rearrangement in Hydrogen Halide Addition to Alkenes 6 8 Addition of Sulfuric Acid to Alkenes Acids other than hydrogen halides also add to the carbon carbon bond of alkenes in the same way Markovnikov s rule is obeyed because the mechanism of sulfuric acid addition to alkenes is analogous to that described earlier for the electrophilic addition of hydrogen halides Hydration of alkenes by this method is limited to monosubstituted alkenes and disubstituted alkenes of the type RCH CHR 6 9 Acid Catalyzed Hydration of Alkenes Acid cataylist o Acid Catalyzed Hydration Step Process o Step 1 Protonation of the carbon carbon double bond in the direction that leads to more stable carbocation o Step 2 Water acts as a nucleophile to capture tert butyl cation o Step 3 Deprotonation of tert butyloxonium ion Water acts as a Br nsted base Microscopic reversibility in any equilibrium the sequence of intermediates and transition states encountered as reactants proceed to products in one direction must also be encountered and in precisely the reverse oder in the opposite direction 6 10 Thermodynamics of Addition Elimination Equilibria Le Ch tlier s principle o A system at equilibrium adjusts so as to minimize any stress applied to it o Helps us predict qualitatively how an equilibrium will respond to changes in experimental conditions At a constant temperature and pressure the direction in which a reaction proceeds that is the direction in which it is spontaneous is the one that leads to a decrease in free energy G o G Gproducts Greactants spontaneous when G 0 o G is always positive G can be positive or negative o G G o When G is negative the reaction is exergonic When G is positive R 8 314 the reaction is endergonic o o 6 11 Hydroboration Oxidation of Alkenes Acid catalyzed hydration converts alkenes to alcohols according to Markovnikov s rule Hydroboration oxidation the synthetic method used to accomplish conversions from alkenes to alcohols Hydrogenboration o A reaction in which a boron hydride adds to a carbon carbon bond o 6 12 Stereochemistry of Hydroboration Oxidation 6 13 Mechanism of Hydroboration Oxidation Hydroboration Step Process o Step 1 A molecule of borane BH3 attacks the alkene Electrons flow from the orbital of the alkene to the 2p orbital of boron A complex is formed o Step 2 The complex rearranges the organoborane Hydrogen migrates from boron to carbon carrying with it the two electrons in its bond to boron Retention of configuration migration of the alkyl group from boron to oxygen Oxidation of an Organoborane Step Process o Step 1 Hydrogen peroxide is converted to its anion in basic solution o Step 2 Anion of hydrogen peroxide acts as a nucleophile attacking boron and forming an oxygen boron bond o Step 3 Carbon migrates from boron to oxygen displacing hydroxide ions Carbon migrates with the pair of electrons in the carbon boron bond these become the electrons in the carbon oxygen bond o Step 4 Hydrolysis cleave the boron oxygen bond yielding an alcohol 6 14 Addition of Halogens to Alkenes The products of these reactions are called vicinal dihalides The halogen is either chlorine or bromine Rearrangements occur if the carbocations are not intermediates or if they are they are captured by a nucleophile faster than they rearrange 6 15 Stereochemistry of Halogen Addition The reaction of chlorine and bromine with cycloalkenes illustrates an important stereochemical feature of halogen addition Anti addition is observed the two bromine atoms or