Reações de Halogenação e Estrutura Molecular

Prévia do material em texto

268 CHAPTER 8 
 
the OH group that is tethered to the bromonium group. 
The resulting intramolecular nucleophilic attack (two 
curved arrows) generates an oxonium ion, which then 
loses a proton (two curved arrows) to give the product. 
 
 
 
 
 
 
 
8.82. When the alkene is treated with molecular iodine 
(I2), the  bond functions as a nucleophilic center and 
attacks I2 (three curved arrows), resulting in an 
intermediate iodonium ion. The iodonium ion is then 
subject to attack by a nucleophile, such as the 
nucleophilic center that is tethered to the iodonium 
group. The resulting intramolecular nucleophilic attack 
(two curved arrows) generates an intermediate which 
then loses a proton (two curved arrows) to give the 
product. 
 
 
 
 
 
 
8.83. The cis-dibromide is not obtained, suggesting that 
the reaction proceeds via an anti addition process. This 
can be explained if we argue that the carbocation 
(formed upon protonation of the  bond) is converted 
into a bromonium ion, as shown here. The incoming 
nucleophile (bromide) would have to attack from the 
backside of the bromonium bridge, giving a trans 
dibromide: 
 
 
 
 
8.84. Inspection of the molecular formula reveals that 
compound A contains the same number of carbon atoms 
as the product of ozonolysis, compound B (21 carbon 
atoms). Thus, we can predict the structure of compound 
A by choosing any two carbonyl (C=O) groups in the 
product, removing the oxygen atoms, and connecting the 
sp2 hybridized carbon atoms with a double bond. There 
are three carbonyl groups, so there are three possible 
alkenes that should be considered. 
 
 
 
Attachment of carbons 1 and 2 would result in a complex 
ring system with far too much strain to possibly exist. 
Carbons 1 and 2 are remotely located on opposite ends of 
the molecule, pointing in opposite directions. Indeed, we 
will later learn that this polycyclic skeletal structure is 
very rigid, and the hypothetical connection of carbons 1 
and 2 is impossible. 
 
Similarly, if carbons 1 and 3 were attached it would also 
result in significant strain (below). Though these 
carbons are closer than 1 and 2, and they are linked by a 
slightly more flexible chain, the trans relationship of the 
two connecting groups creates too much angle strain: 
 
 
 
www.MyEbookNiche.eCrater.com