9. Addition of X2 (X = Cl, Br) – ANTI addition X X2 X So: Let’s look at bond connections first: Step 1: the e- rich alkene approaches the X-X bond, causing the X-X to polarize, creating an electrophile for the nucleophilic alkene to react with: + δ δ- X-X X In the next step, the halide attacks: X X X X Now – go back and add in stereochemistry… No, seriously – go back and add in the wedges/dashes… The Side View: top face blocked! Br Br Br Br Br H H H H H H Br 10. Halohydrin Formation – Addn of OH, X to alkene Markovnikov, ANTI Ex. X X2 H O 2 OH Alternatively, if you substitute an alcohol for the water and make an ether instead: Ex. X X2 CH3OH OCH3 1 Draw the Products: Ex. Br2 H O 2 Ex. Cl2 H O 2 Ex. Br2 CH3OH Mechanism: OH Br2 H O 2 Br Top View: H H O O Br Br H H Br Br Br H O Br Side View: Look at that ANTI addition occurring… 2 Br Br Br Br H-O-H H CH H 3 CH3 H CH3 O H H Br Br H CH3 O H So – Why “ANTI”? One face is blocked by halonium ion, just like in the addition of just X2: Br H CH 3 . And Now - Why “Markovnikov”? First, recall than any positive charge, full or partial, is more stable on a more substituted carbon. Markovnikov means the reaction proceeds through the most stable intermediate, full or partial charge. Which end is more substituted? Br H CH3 The more substituted end of the halonium ion breaks the C-X bond more easily because that carbon atom forms a partial positive charge that would be more stable (lower in energy, lower activation barrier), and thus the addition of water occurs there, faster, just like in oxymercuration, with a mercurinium ion. Br H CH3 Nuc The more substituted side would form a more stable partial charge. More stable intermediate = Markovnikov And now the last reaction type: 11. Oxidative Cleavage of Alkenes – requires the breaking of both the pi AND sigma bonds of alkenes, thus fracturing the carbon skeleton into pieces. 3 R1 R3 R1 R3 O + O R R R2 R4 2 4 There are two sets of reagents you must know how to work with: a. Ozonolysis – (with reductive work-up) -Conversion of alkenes to aldehydes and/or ketones -Uses Ozone (O3) – formed from oxygen in an ozonator e- O2 O3 O O O O O O Reagent: 1. O3 + 2. Zn, H3O Aldehydes and Ketones form with ozonolysis: R1 H R1 H O + O R R R R 2 4 2 4 R1 H R1 H O + O R H R2 H 2 Draw the products (BOTH): Ex. 1. O3 2. Zn, H O+ 3 Ex. 1. O3 2. Zn, H O+ 3 4 Ex. 1. O3 2. Zn, H O+ 3 + b. KMnO4, H2O or H3O • Conversion of alkenes to ketones/carboxylic acids/CO2 (H2CO3) • pH dependent reaction – if the reaction pH becomes too HIGH, dihydroxylation occurs! General Equations: R1 R3 R1 R3 O + O R R R R 2 4 2 4 R1 R3 R1 R3 O + O R2 H R2 H R3 O OH R1 H R1 H O + O R2 H R2 H OH O OH Draw the products: Ex. KMnO4, H2O 5 Ex. + KMnO4, H3O Ex. + KMnO4, H3O Now: Both of them… Ex. 1. O3 2. Zn, H O+ 3 Ex. + KMnO4, H3O Ex. KMnO4,NaOH, H2O Finally…What about: 1. O3 2. Zn, H O+ 3 KMnO4, H2O 6 .