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This problem was in the problem set at the end of some chapter.


A little bit of background: the intersections of the lines in the drawing are known by organic chemists to be carbon atoms.
Also, O chemists know that carbon is tetra valent, which means if a vertex has two lines connected to it, then two hydrogens are understood to be there to complete the valence of 4.

The simplest carbon compound is methane, CH4     The next compound in a series created by adding another carbon atom and the requisite number of hydrogens to fill the valences is ethane.     the next example shows a double bond, this compound is ethylene     Ethylene undergoes this same reaction with bromine and CCl4 to produce 1,2-dibromoethane.     Based on this information one would expect the reaction product of the bromination of norbornene to be as shown below:  
  This is where the puzzle lies.
  Here we have a view of ethylene showing electronic structure. Remove two hydrogens from ethane and two electrons and the remaining electrons form a bond that has an electron cloud above and below the plane defined by all the atoms in ethylene. A bromine molecule approaching ethylene will polarize its electrons to allow one bromine atom of the bromine molecule to be slightly positive and the other slightly negative. The bromine molecule will split apart, leaving a negatively charged bromine atom as the other bromine atom binds to the electrons of the ethylene double bond. Thus a positively charged complex results. The negatively charged bromine ion attacks from the other side of the molecule from the bromide to form 1,2-dibromoethane.


Thinking In Moving Pictures
Sal I Restriction Enzyme Recognition Sequence