Addition to Carbon Carbon Pi Bonds H2O H2C CH2 H3C catalyst OH Need a catalyst This is one way ethanol is manufactured How does this reaction proceed Pi Bonds H H HC H CH H Alkene Two election clouds Alkyne Many mechanisms that apply alkenes also apply to alkynes Generic Addition Mechanism R elec R electrophile R R RDS R C R R nucleophile elec R nuc R R R R Carbocation Intermediate Which fate Rearrangement nothing better than tertiary Lose proton no protons to lose Capture nucleophile 1 Addition of H X to a Pi Bond X Cl Br I H3C H H3C Br CH2 or C CH2 H3C H H3C H3C H3C Primary less stable carbocation Tertiary 3 is more stable carbocation Which fate Rearrangement won t stabilize charge Lose proton reaction will go backwards Capture nucleophile Br H3C H Br Major Product Formed from tertiary carbocation intermediate H3C CH3 C H3C H3C H3C CH2 CH4 Br H3C Br H3C H3C Minor Product Formed from primary carbocation intermediate Br This noted addition of Br to the more substituted end of the alkene was discovered by Markovnikov Markovnikov s Rule When a hydrogen halide adds to an alkene the hydrogen is added to the carbon bearing the most hydrogen substituents least substituted carbon and the halide is added to the end of the alkene with the least number of hydrogens X CH2 H X H H goes to the least substituted carbon X goes to the more substituted carbon 2 Addition of H Br to a Triple Bond H H Br H CH3 H2C C C CH3 CH3 1 Minor Product 2 Major Product Major Product H2C C CH3 Carbocation fate Lose a proton unproductive would move reaction backwards Rearrangement Vinyl carbocations CANNOT rearrange H H2C X C x H2C H C H H Capture a nucleophile H2C Br Br C H H Br C H2C CH3 H CH3 Br Br H3C H3C CH3 2 with resonance One contributor with complete octets Br C CH3 1 without resonance CH3 Br Br Br H3C CH3 FINAL PRODUCT 3 Addition of H2O H2C CH2 H H3C OH OH Ethylene Example No H OH C HO H OH is not a leaving group unless you have a very good acid Mechanism H OH2 C H2O Carbocation Intermediate Rearrangement is unproductive Lose proton reaction goes backwards Capture nucleophile H2O H H2O H O H2O HO C H3O This reaction is reversible It is in EQUILIBRIUM HO H2O Favors alkene just a little How do you control the direction of equilibrium Think of la Chatelier s Principle increase the concentration on one side reactants or products to shift the equilibrium to the other side 4 Addition of H2O OH H2SO4 aq H3O Ph Phenyl C6H5 H H OH2 Ph H2O Ph C C Ph H O H2O Ph H Carbocation intermediate Resonance but no extra stability Rearrange to 3 with resonance HO H3O Ph Alkynes and H3O H H3O Ph HO Ph Alkene and H3O Alkyne and H3O CH3 CH2 H3O Ph H OH Ph CH CH2 Enol 5 Mechanism H H OH2 Ph Ph O OH2 C CH H OH Or C Ph C CH3 H H 2 with resonance O OH2 CH2 CH2 OH H Ph Ph CH2 Ph OH OH2 1 without resonance O OH2 H Ph Ph CH3 Resonance contributor with complete octets H3O CH3 Major Product The intermediate labeled 1 without resonance can proceed to deprotonation and formation of an enol The keto product is favored greatly OH O Ph Ph H CH3 H Enol Keto These products are tautomers which are created through tautomerization a process where a hydrogen atom migrates to another carbon and a double bond shifts to an adjacent bond Why not OH Ph OH2 C CH3 H2O OH This product will go backwards to the happiest stage keto form Ph CH3 6 Miscellaneous Additions a Catalytic Hydrogenation H H2 H Pt Syn addition new things are added on the same face Markovnikov s rule does not apply No nucleophile or electrophile Catalyst is a transition metal Think of it as removing the pi bond b Addition of Br2 Br Br Br Br Anti addition Markovnikov s rule does not apply Electrophile Br2 induced when Br2 approaches the pi bond Nucleophile alkene c Hydroboration Oxidation H 1 BH3 2 H2O2 HO OH Syn addition Anti Markovnikov Electrophile BH3 Borane because of the open octet Nucleophile alkene Addition of water on the alkene converts it into an alcohol 7 d Ozonolysis 1 O3 2 CH3 2S H O O H Syn addition No nucleophile or electrophile Ozone O3 reacts with an alkene and forms carbonyls o Imagine the pi bond splitting in half and each end bonding to oxygen 8