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 R2 R4 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, H3O
+ 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 R2 R4 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, H3O
KMnO4, H2O
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