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The 3 Essential Unsymmetrical Additions to Alkenes

Delve into 3 key unsymmetrical alkene additions: HX addition, acid-catalyzed hydration, and anti-Markovnikov hydroboration/oxidation in organic chemistry.

1. HX (8.1B)

THE most important reaction to understand in undergraduate organic chemistry (I would argue) is this one. If you have ever learned one organic chemistry reaction, for instance in high school, this is the one. THE most fundamental organic chemistry mechanism is for this reaction so I will briefly discuss it. The π electrons of the double bond attack the partially positive hydrogen of HBr to give a carbocation. In unsymmetrical alkenes the reaction goes with Markovnikov selectivity because the H goes to the carbon to give the most stable carbocation intermediate (in this case 2o). The Br ends up on the carbocation carbon.  A quick way to remember this selectivity is H goes to the carbon that already has the most Hs.  If you understand this mechanism and its electron pushing (6.1a) you are off to a very good start.

2. H2O/ Catalytic Acid: Hydration (8.1C)

Hydration of alkenes (i.e. adding H2O using acid catalyst to make alcohols) is probably the 2nd most important reaction of alkenes. Alcohols are the gateway functional group to many important reactions and they are ubiquitous in biochemistry. Acid catalyst is necessary because H2O itself is far too unreactive to add directly to alkenes.  The reaction is again Markovnikov with H going to the carbon that already has the most Hs.

3. H20: Hydroboration/Oxidation (8.2E)

Anti-Markovnikov hydration (i.e. adding H2O with opposite selectivity to the simple hydration) is an important tool in synthesis. This is done with a 2 step reaction called hydroboration/oxidation.  It has a completely different mechanism than reactions 1 or 2.   Not only does the H end up on the carbon with the least amount of Hs already but, as an added bonus, the H-OH always adds from the same side of the double bond which is called a syn addition.

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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