25.3. Organic Reactions http://www.ck12.org
Addition of Halogens
Halogenationis analogous to hydrogenation, except that instead of adding a hydrogen atom to each side of the
double bond, we add a halogen atom instead. InFigure25.23, we see that adding molecular bromine to an alkene
results in the formation of a dibrominated product. Unlike hydrogenation, no catalyst is required for this reaction to
occur. Again, using these same conditions on a C=O double bond willnotproduce the analogous addition product
due to the different reactivity of these two functional groups.
FIGURE 25.23
Halogenation of an alkene.Formation of Double Bonds
Formation of C=C bonds
FIGURE 25.24
Dehydrohalogenationof an alkane.FIGURE 25.25
Dehydrationof an alcohol.Formation of C=O bonds
Recall that one definition of oxidation is the loss of hydrogen atoms. Oxidation of a primary alcohol will first lead to
the formation of an aldehyde. This process is the net removal of H 2 (one from oxygen and one from carbon), leaving
behind a double bond. In other words, this oxidation is the opposite of the hydrogenation (reduction) reaction that we
looked at previously. If water is present, or if we use a strong enough oxidizing agent, the aldehyde will be further
oxidized to the carboxylic acid. This fits with the idea that some oxidation reactions involve the literal addition of
oxygen atoms to the molecule.