What Are the Mechanisms of Electrophilic Additions to Alkenes

Electrophile An electron-poor species that can accept a pair of electrons to form a new covalent bond alternatively, a Lewis acid (Section 2.6). 

A reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions. 

We begin our introduction to the chemistry of alkenes with an examination of three types of addition reactions the addition of hydrogen halides (HCl, HBr, and HI), water (HgO), and halogens (Clg, Brg). We first study some of the experimental observations about each addition reaction and then its mechanism. By examining these particular reactions, we develop a general understanding of how alkenes undergo addition reactions. 

The hydrogen halides HCl, HBr, and HI add to alkenes to give haloalkanes (alkyl halides). These additions may be carried out either with the pure reagents or in the presence of a polar solvent such as acetic acid. The addition of HCl to ethylene gives chloroethane (ethyl chloride)  

The addition of HCl to propene gives 2-chloropropane (isopropyl chloride) hydrogen adds to carbon 1 of propene and chlorine adds to carbon 2. If the orientation of addition were reversed, 1-chloropropane (propyl chloride) would be formed. The observed result is that 2-chloropropane is formed to the virtual exclusion of 1-chloropropane  

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We can see from these examples that, if the carbon atoms participating in the double bond are not equally substituted, the proton from the hydrogen halide attaches itself to the less substituted carbon. As a consequence, the halogen ends np at the more snbstituted carbon. This phenomenon, referred to as the Markovnikov rule, can be explained by what we know about the mechanism of electrophile additions of protons to alkenes. The key is the relative stability of the resulting carbocation intermediates. 

In Chapter 4 we saw that aUcenes undergo electrophilic addition reactions, and we looked at the different kinds of reagents that add to alkenes. We also examined the step-by-step process by which each reaction occurs (the mechanism of the reaction), and we determined what products are formed. However, we did not consider the stereochemistry of the reactions. 

An addition reaction involves the combination of two molecules to form a product containing atoms from both reactants. All addition reactions occur on unsaturated organic structures—namely, alkenes, alkynes, carbonyls, and arenes. Not surprisingly, three distinct mechanisms are observed—electrophilic, nucleophilic, and radical. In the electrophilic addition pathway, the unsaturated system acts as the nucleophile supplying electrons to the electrophilic reagent. The opposite is true in the nucleophilic pathway. In either of these two cases, simple electrostatic considerations assist in predicting what will occur. 

Most reactions of carbonyl groups occur by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, am carbonyl condensation. These mechanisms have many variations, just a alkene electrophilic addition reactions and 8 2 reactions do, but the varia tions are much easier to learn when the fundamental features of the mechanisms are understood. Let s see what the four mechanisms are and what kinds of chemistry carbonyl groups undergo. 

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