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The 4 Essential Symmetrical Additions

Explore the 4 crucial symmetrical additions to alkenes: X2 anti-addition, H2 catalytic hydrogenation, 1,2-Diols oxidation, and epoxidation with peroxyacids.

4. X2 (8.2A)

Addition of X2 (e.g. Br2) puts one Br on each carbon of the alkene.   What is also important to know is that this is always an anti addition (i.e. the 2 bromines add to opposite sides of the double bond).  So in a cyclic alkene you get the trans product.   Part of the mechanism has the first bromine add across both carbons of the double bond to form a cyclic bromonium.   I like to call this “capping” of the double bond.   Bromide then comes in and attacks from the backside to open it up and form the anti dibromide.

5. H2: Catalytic Hydrogenation (8.2D)

Hydrogenation of alkenes to alkanes is a reaction you likely ran into in an earlier classes.  A heterogeneous solid metal catalyst (e.g. metallic Pt, Pd or Ni) is necessary to lower the huge energy barriers of direct addition.   In a cyclic molecule you get only the cis product because the mechanism requires that the 2 Hs add syn (i.e. from the same side).

6. Oxidation to 1,2-Diols (8.2C)

Diols, like alcohols, are important molecules to make and use. There are 2 sets of reagents commonly used for this: cold basic KMnO4 (an old-fashioned reagent) or OsO4 with a workup in step 2. The way MnO4 and OsO4 work is to approach the alkene and deliver both oxygens at once from the same side which results in syn product. 

7. Epoxidation (8.2D)

Epoxides are useful as intermediates molecules between alkenes and alcohols.  A peroxyacid Is a carboxylic acid with an extra oxygen connected to the -OH oxygen.  That extra oxygen makes the molecule unstable and prone to donating oxygen.  mCPBA, a common peroxyacid delivers its extra oxygen to the alkene and “caps” it to form the epoxide.  This structure is reminiscent of the bromonium in reaction 4 above.

If you want to learn more about these reactions so you can remember them better or if you want to try some good practice problems, go to Unit 8 and the particular sections indicated. 

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Dr. B Tips

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