The Generalized Electrophilic Addition

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Electrophilic addition is a reaction between an electrophile and nucleophile, adding to double or triple bonds. An electrophile is defined by a molecule with a tendency to react with other molecules containing a donatable pair of electrons. Thus, it is an "electron lover." A nucleophile is one that possesses a lone pair of electrons that can be easily shared. In essence, all nucleophiles are Lewis bases that attack nonhydrogen atoms (Lewis acids).

Introduction

In a general electrophilic addition reaction, one of the pi bonds is removed and creates two new sigma bonds. Another electrophilic addition reaction known as halohydrination includes bromoalcohol, more commonly known as bromohydrin. There are a variety of electrophilic reactions, and therefore a variety of different products that are very useful. It simply depends on which reagents are used to determine the final product.

Step 1

In the first step there is an electrophilic addition of bromine to the cyclopentene, forming a cyclic bromonium ion or also known as an open-chained carbenium ion (Troll, T). For more information about the cyclic Bromonium Ion please look at Electrophilic Addition of Halogens to Alkenes.

Step 2

Bromohydrin #2.jpg

Now the nucleophilic water molecule attacks the back of the more substituted carbon and pushes the bromonium ion onto the less substituted carbon. In general the regiochemistry of this reaction follows Markovnikov’s rule (Troll, T). The stereochemistry of the reaction is anti-addition because of better orbital overlap from backside attack, which means that the Br and the H₂O are on opposite sides of the double bond. The electrophilic bromide in the product becomes linked to the less substituted carbon. The nucleophile attacks the more substituted carbon, because the carbon is more positively polarized than the other carbon.

Step 3

Bromohydrin #3.jpg

The negatively charged bromonium ion that was not used in the reaction is still floating freely in the water. The bromine atom attacks one of the H’s located on the water molecule. The Hydrogen drops off its electrons on the oxygen molecule making the oxygen neutral.

Final Product

Finally we are left with the trans-2-Bromocyclopentanol and hydrobromination. Here's the entire mechanism in gumdrop form. (Orange = C, Yellow = H, Red = Br, White = O)

References

Vollhardt, Peter, and Neil E. Schore. Organic Chemistry: Structure and Function. 5th ed. New York: W. H. Freeman & Company, 2007
Troll, T. "Hydroxy-and Alkoxybromination of Alkenes." Science of Synthesis 35 (2006): 471.