Because the molecular energy is higher in the 13-cis than 7
& 11-cis.
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cis
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3-Hexene is an alkene reactant that can either be trans or cis.
Br2 is a ragent and stands for Bromine. In order to know the
reaction between 3-hexene and Br2 we have to know whether it is
trans-3-hexene or cis-3-hexene.
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those are defined by position of double bonds,
the possible one,s are cis-cis,cis-trans,trans-trans.