| MadSci Network: Chemistry |
When a double or triple bond is attacked by an electrophilic (electron seeking) species, the carbon atoms in the multiple bond must bear some electron deficiency. In the case of the addition of HX, one carbon must bear all of the deficiency; in the case of adding Br2, the carbons share the deficiency. It is the initial attack of the Br2 that determines the rate (speed) of the reaction. A major difference between the double bond and the triple bond lies in the hybridization of the bonding orbitals on the carbons. In the double bond the hybridization is sp2, whereas it is sp for the triple bond. The sp hybridized carbon atom does not handle the electron deficiency as well as the sp2 hybridized carbon. Stated another way, the electron deficient sp carbon is of higher energy than the sp2 counterpart. The higher energy of the sp intermediate makes its attainment more difficult and slows the reaction in comparison to that for the sp2 carbon, since the attack by the electrophile is the "rate-determining step." If you have studied about this reaction enough to understand the "bromonium ion" description, this argument fits well with that description also, because, even though we show all of the electron deficiency on the bromine with a + charge in that description, it certainly must pass this deficiency on to the less electronegative carbons. Dr. Jerry Franzen Chemistry Department Thomas More College 859-344-3377 franzenj@thomasmore.edu
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